The bacterial enhancer-binding protein NTRC is a molecular machine: ATP hydrolysis is coupled to transcriptional activation.

NTRC is a prokaryotic enhancer-binding protein that activates transcription by sigma 54-holoenzyme. NTRC has an ATPase activity that is required for transcriptional activation, specifically for isomerization of closed complexes between sigma 54-holoenzyme and a promoter to open complexes. In the absence of ATP hydrolysis, there is known to be a kinetic barrier to open complex formation (i.e., the reaction proceeds so slowly that the polymerase synthesizes essentially no transcripts even from a supercoiled template). We show here that open complex formation is also thermodynamically unfavorable. In the absence of ATP hydrolysis the position of equilibrium between closed and open complexes favors the closed ones. Use of linear templates with a region of heteroduplex around the transcriptional start site--"preopened" templates--does not bypass the requirement for either NTRC or ATP hydrolysis, providing evidence that the rate-limiting step in open complex formation does not lie in DNA strand denaturation per se. These results are in contrast to recent findings regarding the ATP requirement for initiation of transcription by eukaryotic RNA polymerase II; in the latter case, the ATP requirement is circumvented by use of a supercoiled plasmid template or a preopened linear template.

crS4-Holoenzyme can bind to the glnA promoter in a closed recognition complex in the absence of other factors but requires the presence of NTRC and ATP to isomerize into the open complex, in which the DNA around the transcriptional start site is denatured locally 1Present address: Max-Planck-Institut fuer Biochemie, 82152 Martinsreid, Germany.
( Fig. 1; Kustu et al. 1991). Neither ATP~S nor ADP can substitute for ATP in allowing formation of open complexes although both inhibit the ATPase activity of NTRC, indicating that they bind to the protein . NTRC binds to an enhancer consisting of two tandem DNA-binding sites centered at -140 and -108 with respect to the transcriptional start site and contacts the polymerase by means of a DNA loop. To hydrolyze ATP and activate transcription, NTRC must be phosphorylated on an aspartic acid residue within its aminoterminal regulatory ("receiver")domain (Weiss and Magasanik 1988;Sanders et al. 1992). This phosphorylation, which is mediated by the NTRB protein, occurs under nitrogen-limiting conditions and is labile; the half-life of phosphorylated NTRC is ---4 min at 37°C.
A number of mutant forms of NTRC--so-called NTRC c°nstitutive proteinsmhave some ability to activate transcription without being phosphorylated (Popham et al. 1989;Weglenski et al. 1989;Dixon et al. 1991;Klose et al. 1993;Flashner et al. 1995). All of these forms also have the ability to hydrolyze ATP without being phosphorylated and depend on nucleotide hydrolysis to catalyze the formation of open complexes by ¢54-holoenzyme in vitro. The latter findings distinguish the nucleotide requirement for open complex formation from that  Kustu et al. 1991;Porter et al. 1995). crS4-Holoenzyme can bind to the glnA promoter in a closed recognition complex in the absence of other factors. Conserved promoter sequences lie in the -24 and -12 regions with respect to the start site of transcription. Dimers of NTRC bind to the enhancer, which is composed of two dyad-symmetrical sites centered at -140 and -108. NTRC contacts the polymerase by means of a DNA loop (not shown). When phosphorylated, NTRC catalyzes the isomerization of a closed complex between crS4-holoenzyme and the promoter to a transcriptionally competent open complex, in which the DNA around the transcriptional start site is denatured. To catalyze open complex formation, NTRC must hydrolyze ATP. In open complexes, the conformation of polymerase is changed such that its DNase I footprint is elongated and extends downstream of the transcriptional start site ).
for phosphorylation. Unlike phosphorylated wild-type NTRC, NTRC c°nstitutive proteins provide a homogenous population of active molecules.
There are two striking features that distinguish cr s4 from other cr factors. First, cx s4 bears no detectable sequence homology to other cr factors (Merrick 1993), all of which are homologous to one another (Hellman and Chamberlin 1988;Lonetto et al. 1992). Second, conserved dinucleotides in crsn-dependent promoters must be exactly one turn of the DNA helix apart (10 bp), whereas conserved promoter elements for other holoenzyme forms of polymerase are 2 to 2 '/2 turns apart. Moreover, cr s4 alone can bind to "strong" (high-affinity)promoters in the absence of core polymerase (Buck and Cannon 1992;Cannon et al. 1994).
In this paper we show that the barrier to open complex formation by orS4-holoenzyme is thermodynamic, as well as kinetic. Because NTRC acts catalytically in the for-mation of open complexes, this indicates that the energy of ATP hydrolysis must be coupled to open complex formation to drive the reaction and hence that NTRC is a simple molecular machine (Alberts and Miake-Lye 1992). We also provide evidence that the rate-limiting step in open complex formation is not DNA strand denaturation per se.

The barrier to open complex formation is both kinetic and thermodynamic
When an excess of crS4-holoenzyme, a supercoiled template bearing the glnA promoter, and an NTRC ¢°nstitutive protein (NTRC D54E' s16oF) were incubated together in the absence of ATP, no detectable open complexes accumulated even after 60 min ( Fig. 2A, lanes 1,2; assessed in a single-cycle transcription assay, see Materials and methods). However, when ATP was added at 60 rain, half of the promoters carried an open complex within 2 min ( Fig. 2A, lane 3 and legend). If NTRC was subsequently prevented from hydrolyzing ATP by addition of the inhibitor ADP at 62 min , the preformed open complexes slowly decayed ( Fig must be a kinetic barrier between the two forms. Therefore, the failure to detect formation of open complexes in the absence of ATP hydrolysis cannot be attributable to their lack of stability but must be attributable to their failure to form at all. Experiments in which open complexes were separated from ATP by sieving or were diluted to decrease the concentration of this nucleotide far below that required for open complex formation indicated that a slow rate of decay of open complexes did not depend on the presence of ATP or ADP and that the rate of decay was not affected by these nucleotides Wedel 1993 Even when an excess of cr -holoenzyme was incubated with the template in the presence of nucleotides to allow immediate synthesis of transcripts, essentially no transcripts were seen ( Fig. 2A, Popham 1989). Additional experiments indicated that crS4-holoenzyme could not transcribe even in the presence of NTRC and all four nucleotides if the activator was prevented from hydrolyzing the f~-~/bond of purine nucleotides Popham 1989;Wedel 1993). Hydrolysis of the ~-~/ bond was blocked by the inhibitors ADP or ATP~/S or, alternatively, by providing ~, ~/-methylene-ATP instead of ATP and using a low concentration of GTP.
As noted above ( Fig. 2A, lanes 3-5; Fig. 2B), after inhibition of the ATPase activity of NTRC, the number of  Klose et al. 1993) were incubated with a supercoiled template (pJES 463; 1 nM= 20 fmoles/20 ~xl) bearing the wild-type glnA promoter-regulatory region at 3 7°C in a final volume of 120 ~xl. At 10 and 60 min 20-~xl samples were taken (lanes 1,2) to which ADP and heparin were added to 4 mM and 100 gg/ml, respectively, to stop the subsequent formation of open complexes {see Results; Weiss et al. 1991); after 2 min at 37°C, an elongation mix was added containing ATP, GTP, CTP, and [o~-32p]CTP to allow any polymerase molecules in open complexes to synthesize single transcripts. Immediately after the second sample was taken at 60 min, ATP was added to the remainder of the reaction mixture to 0.4 raM. After incubation for 2 min at 37°C, formation of open complexes was stopped by addition of ADP to 4 mM and a third sample of 20 gl was taken to which the elongation mix was added (lane 3)(because the reaction mixture already contained ADP, only heparin was added before ribonucleotides). Under similar conditions, the formation of open complexes has been shown to cease within 2 rain after the addition of ATP (Popham 1989  Because closed complexes/active crS4-holoenzyme have a measurable activity~that is, the ability to form open complexes~the models described above can be differentiated by asking whether the product formed when open complexes decay can be made to reform open complexes. To determine this, we first formed open complexes and separated them from free crS4-holoenzyme by passage through a sieving column (diagram in Fig. 3A). The isolated open complexes were then allowed to decay, and the crS4-holoenzyme released from them was given an opportunity to reform open complexes either on the same template or on a second template that yielded a transcript of different length. As detailed below, open complexes reformed in both cases (Fig. 3A,(9)(10)(11)(12)quantified in Fig. 3B,C), indicating that decay was at least in part to active crS4-holoenzyme.
The template used for the initial formation of open complexes was a linear fragment carrying a variant of the glnA promoter with four A/T to G/C transitions in the melting region; the half-life of open complexes on this template is 5-10 min at 37°C and -~30 min at 4°C. (We sought a short half-life at 37°C to minimize polymerase inactivation during the course of the experiment.) After rapid purification away from free crS4-holoenzyme over a sieving column at 4°C, open complexes were split into three portions. ATP was added back to all three, whereas NTRC and NTRB were added back to portions 2 and 3 only (diagram in Fig. 3A). To aliquot 3 was also added a 17-fold excess of a second template that yields a shorter transcript. The three samples were then placed at 37°C to allow simultaneous decay and reformation of open complexes. In the absence of NTRC, open complexes decayed from the first template with a half-life of --~5 min ( Figure 3B to the zero time point shows that portion 2, to which NTRC was added, had no more open complexes than portion 1. Similarly, extrapolation of the line for the "Sum" in C to the zero time point and comparison with the extrapolation of either line in B indicates that there were no excess open complexes in portion 3, in spite of the fact that it contained an additional template with a very high apparent affinity for polymerase (see Materials and methods and below ;Wedel 1993). Results similar to those in Figure 3 were obtained in a second experiment in which template and/or NTRC and NTRB were added back after allowing different times for decay of open complexes from the first template (not shown). Because decay of open complexes was to reactantnactive closed complexes or free crSa-holoenzymenthe barrier to open complex formation is thermodynamic as well as kinetic {Fig. 2C, model I ).

Melting of the DNA around the transcriptional start site does not circumvent the requirement for either NTR C or A TP hydrolysis
Because the barrier to open complex formation lies between closed complexes and localized DNA strand denaturation around the transcriptional start site, it is possible that crSa-holoenzyme alone cannot carry out the melting reaction, and that NTRC catalyzes open complex formation by acting as a site-specific helicase. To ask whether strand denaturation was, in fact, the ratelimiting step in open complex formation, we constructed and tested several templates carrying glnA promoter sequences in which the DNA duplex around the transcriptional start site was destabilized by base mispairing {i.e., "preopened" templates} {Fig. 4A). The region that is denatured in natural open complexes at crSa-dependent promoters extends from approximately position -7 to position +3 or +4 with respect to the start site of transcription (Sasse-Dwight and Gralla 1988; Whitehall et al. 1992). The three templates that we tested had regions of heteroduplex from -10 to -2, -1 to +4, and -10 to + 4, respectively; note that the denatured region in the third heteroduplex is longer than that in natural open complexes.
In the presence of NTRC and ATP, all of the promoters carrying regions of heteroduplex were used with comparable efficiency to one another and to the wild-type glnA promoter {Fig. 4B, cf. lanes 3, 5, and 7 with lane 1), whereas in the absence of NTRC they were used poorly {lanes 4, 6, and 8). However, in contrast to the glnA promoter, each of the promoters carrying a region of heteroduplex did support a small amount of transcription in the absence of NTRC. For the promoters carrying heteroduplex from -1 to + 4 and -10 to + 4, transcription in the absence of NTRC was 1%-2% of that obtained with NTRC (not visible in Fig. 4B), whereas for the promoter carrying heteroduplex from -10 to -2, this value was 5%-10%. We have obtained three lines of evidence that transcription in the absence of NTRC results from the fact that crS4-holoenzyme can initiate transcription without going through the normal open complex intermediate when presented with the unusual DNA struc-  template labeled homoduplex carries the wild-type glnA promotet-regulatory region, whereas the templates labeled heteroduplex contain regions of heteroduplex between the promoter and the transcriptional start site (-10 to -2) or between the promoter and a position just downstream of the transcriptional start site {-1 to +4 and -10 to +41. Heteroduplexes were constructed by purifying individual single strands from phagemid particles, followed by renaturation and digestion with a restriction enzyme that releases the promoter-bearing fragment (see Materials and methods). Because the heteroduplexes were reconstituted from purified single strands, they are free of the corresponding alternative heteroduplexes. In all cases, transcripts lack U residues, which minimizes background transcription by core polymerase originating at the fragment ends. (B1 Transcription. Transcription was carried out using the above templates {1 riM) and 50 nM ~S4-holoenzyme (50 nM core polymerase and 200 nM ors4}. Reaction mixtures yielding transcripts shown in the odd lanes also contained wild-type NTRC and NTRB (100 and 20 riM, respectively}. Templates are indicated above the lanes. Shown is an autoradiograph of the gel on which transcripts were isolated. ( Fig. 4B, lanes 3, 5, and 7}. In contrast, transcriptionally competent complexes formed on the heteroduplex promoters in the absence of NTRC initiated primarily at + 3 (see especially Fig. 4B, lane 4), providing a first line of evidence that these complexes are not on the normal reaction pathway.
To confirm that the two sorts of complexes formed between crS4-holoenzyme and the heteroduplex promoters initiated differently, we labeled the 5' ends of such transcripts formed at the promoter carrying heterodu-plex between -1 0 and -2 with [~/-32P]ATP or [-¢-32p]GTP to identify transcripts initiating at + 1 or + 3, respectively; we also labeled with [a-a2P]CTP, as usual, to detect all transcripts. As at the wild-type glnA promoter, complexes between crS4-holoenzyrne and the heteroduplex promoter that were formed in the presence of NTRC initiated at both + 1 and + 3 {Fig. 5A, lanes 1-3}. However, complexes formed in the absence of NTRC initiated almost exclusively at + 3 {Fig. 5A, lanes 4-6}. Because the same high concentration of ATP was present in reaction mixtures with or without NTRC, the failure of complexes formed in the absence of NTRC to initiate at the + 1 A residue did not result from a lack of substrate. It is particularly interesting that the complexes formed without NTRC preferred to initiate at the + 3 rather than + 1 position because base rnispairing in the template extended only through -2 with respect to the start site of transcription. Nevertheless, crS4-holoenzyme alone had access to the template a few bases downstream of the end of the heteroduplex region. The same effect of NTRC on the start point of initiation was obtained with the templates carrying heteroduplex between -1 and + 4 and between -1 0 and + 4 (not shown}. To compare the stabilities of transcription complexes formed in the presence or absence of NTRC, we formed them on the promoter carrying heteroduplex from -10 to -2 at a subsaturating concentration of crS4-holoenzyme and in the presence of ATP. We then diluted them 50-fold into buffer containing heparin to minimize the formation of new complexes. After a further 30-rain incubation at 37°C, complexes formed in the presence of NTRC had decayed by only 1.6-fold relative to the undiluted control {i.e., 36% of the complexes had decayed; To provide further evidence that transcription complexes formed without NTRC on heteroduplex promoters are not on the natural reaction pathway, we constructed a "mutant" version of the -1 0 to -2 heteroduplex promoter lacking the conserved GG dinucleotide in the -2 4 region {deleted from -2 7 to -18; see Materials and methods}. In the absence of NTRC, transcription from this mutant promoter was the same as that from the parental heteroduplex {Fig. 5C, lanes 2,4}. However, whereas NTRC greatly stimulated transcription from the parental promoter {Fig. 4B, cL lane 3 with lane 4; Fig. 5C, cf. lane 1 with lane 21, it had no effect on transcription from the mutant promoter {Fig. 5C, cf. lane 3 with lane 4). This indicates that initiation of transcription from the parental heteroduplex promoter in the absence of NTRC and ATP results from a complex that is not dependent on the conserved promoter recognition sequence in the -24 region and therefore lies outside the normal reaction pathway.
Because the behavior of transcription complexes formed on heteroduplex promoters in the absence of GENES   reactions, each containing ATP at 400 ~M. Each reaction mixture was then split; one portion was immediately provided with elongation mix containing both ADP and heparin; the other was diluted 50-fold into a buffer lacking polyethylene glycol and containing ADP at 100 p~M and heparin at 100 ~g/ml and incubated for a further 30 min at 37°C before elongation mix was added. The initial formation of complexes between crS4-holoenzyme and the promoter was carried out with limiting core RNA polymerase (0.5 nM) SO that the forward association reaction of holoenzyme with the promoter would be minimized after dilution (to 0.01 nM), allowing any difference in the stabilities of complexes formed with or without NTRC to be detected. Elongation was carried out with the same nucleotide concentrations and the same specific activity of the labeled nucleotide in each case, and transcripts were precipitated from the entire reaction mixture and run on a denaturing gel; hence, all lanes are directly comparable. The presence or absence of NTRC and the use of dilution are indicated above the lanes. Shown are autoradiographs of the gels on which transcripts were isolated. There are four transcript bands rather than the two seen in A because the template was not digested with BstXI and hence there are two stops within the bank of T residues used for termination, as well as two starts. I C) Requirement for conserved promoter sequences.
Complexes were formed between 60 nM ~r54-holoenzyme (60 nM core polymerase and 180 nM cr 54) and template (1-kb PvuII fragments; 0.8 riM)with or without NTRC sl6°F and NTRB (each at 20 riM). Formation of complexes and their assessment by transcription were as described in B and Materials and methods, except that ADP was not added with heparin before elongation. Because heparin does not disrupt complexes between ~r54-holoenzyme and heteroduplex promoters, the prominent short transcript band in lane 1 is probably attributable to a second cycle of initiation. Templates were the -1 0 to -2 heteroduplex promoter (lanes 1,2J or the -1 0 to -2 heteroduplex promoter lacking conserved promoter sequences in the -24 region (lanes 3,4; see Materials and methods). The templates were not digested with BstXI (see legend to B). NTRC and NTRB were present for the samples in lanes 1 and 3, as indicated.  Aiyar et al. 1994). NTRC is a molecular machine that have been tested; these include promoters that are activator dependent in vivo and at which open complex formation is activator dependent under stringent conditions in vitro (e.g., the lac promoter; R. Ebright, per. comm.), promoters at which open complexes do not accumulate sufficiently to be detected physically (e.g., the trp promoter; Kierkegaard et al. 1983), and promoters at which physical detection of open complexes depends on the presence of initiating nucleotides (e.g., the rrnB Pl promoter; Newlands et al. 1991;Ohlsen and Gralla 1992).

There is a thermodynamic barrier to open complex formation by ~4-holoenzyme
In addition to the kinetic barrier to open complex formation by (rS4-holoenzyme, there is a thermodynamic barrier. That is, in the absence of ATP hydrolysis by NTRC, the position of equilibrium favors closed complexes. As indicated above, open complexes do not form in the absence of NTRC or ATP hydrolysis. Moreover, we have now demonstrated that open complexes formed in the presence of both of these factors and then separated from them decay back to closed complexes, as opposed to inactive polymerase (Fig. 3). Because NTRC does not remain associated with the polymerase in open complexes, a thermodynamic requirement indicates that there must be a mechanism for coupling ATP hydrolysis by NTRC to open complex formation by ~S4-holoenzyme. In this sense NTRC acts as a simple molecular machine (A1berts and Miake-Lye 1992). In contrast, nucleotide hydrolysis by G proteins plays a regulatory role (Bourne et al. 1991;Webb 1992). Binding of nucleoside triphosphate confers an "active conformation" on G proteins, whereas binding of nucleoside diphosphate yields an inactive conformation. In the case of NTRC, neither ATP~/S nor ADP yields an active conformation; rather, nucleotide hydrolysis is required for activity .

NTRC is not a site-specific helicase
We have considered two possible stages for the involvement of NTRC in open complex formation by orSa-holoenzyme. One is that NTRC acts as a site-specific helicase to perform the DNA strand-denaturation reaction per se, a mode of action that has been assumed by others (Laurent et al. 1993). A second possibility is that NTRC couples the energy of nucleotide hydrolysis to a change in conformation of (rS4-holoenzyme and that this conformational change enables the polymerase itself to carry out the strand-denaturation reaction. We have obtained evidence against the first possibility by showing that preopened promoter templates do not circumvent the requirement for either NTRC or ATP, although they are used very effectively if both are provided (Figs. 4 and 5). Hence, DNA strand separation does not appear to be the rate-limiting step in open complex formation by ~54-holoenzyme and NTRC is not a site-specific helicase. Therefore, we take as a working hypothesis that NTRC mediates a conformational change in polymerase. Such a conformational change is known to accompany (i.e., to occur before or concomitant with) open complex formation, as evidenced by a lengthening of the polymerase "footprint" to a position downstream of the transcriptional start site ( Fig. 1; Popham et al. 1989).
The inferred mode of action of NTRC is of interest with respect to the mechanism of open complex formation by crT°-holoenzyme and (r32-holoenzyme. Kinetic studies indicate that a conformational change in these polymerases is rate limiting for open complex formation, rather than denaturation of the DNA strands per se (Kadesch et al. 1982;Buc and McClure 1985;Roe et al. 1985;Duval-Valentin and Ehrlich 1987;for review, see Buc 1989;Leirmo and Record 1989). This conformational change characterizes the transition from the "first closed complex," an initial weak-binding recognition complex, to the "second closed complex," a tighter binding intermediate complex that is formed before DNA melting. The second closed complex has been characterized physically at temperatures too low to allow DNA strand separation [10-20°C for linear templates; Spassky et al. (1985) for lacUV5; Hofer et al. (1985) for fd PVIII; Duval-Valentin and Ehrlich (1986)for pSC101 tetR; Li and Mc-Clure (1988) for ~ PRM (up-l, A265); Schickor et al. (1990) for T7/A1; Cowing et al. (1989) and Mecsas et al. (1991) for groE]. In it, polyrnerase usually has a longer footprint than in the initial closed complex, and this footprint extends downstream of the transcriptional start site; lengthening of the footprint is inferred to precede DNA strand denaturation at higher temperatures.

The A TP requirement differs from that for initiation of transcription by eukaryotic RNA polymerase H
In contrast to the case for ~S4-holoenzyme, the requirement for ATP hydrolysis for initiation of transcription by eukaryotic RNA polymerase II is circumvented by use of a supercoiled plasmid template or a preopened linear template (Parvin and Sharp 1993;Tantin and Carey 1994;Timmers 1994). Use of a supercoiled plasmid template also obviates the need for the general transcription factor TFIIH, which has an ATP-dependent helicase activity (Schaeffer et al. 1993}. [Requirements for general transcription factors have not yet been determined with preopened linear templates (Tantin and Carey 1994).] At present it is not clear whether the helicase activity of TFIIH is required for open complex formation per se, promoter clearance, or both (Wang et al. 1992;Parvin and Sharp 1993;Goodrich and Tjian 1994;Timmers 1994).

Templates
Plasmid pIES750 is derived from pIES463, which carries the glnA promoter-regulatory region containing a number of synthetic restriction sites and yields a transcript containing 64 C residues (Porter et al. 1993;Wedel 1993). In pIES750, the sequence between the BclI site at position -96 of PIES463 and the NcoI site at -38 (containing the low-affinity NTRC binding sites III, IV, and V)has been replaced with the 403-bp BclI-NcoI fragment from pACYC184, positioning NTRC-binding sites I and II, which constitute the glnA enhancer, at -484 and -452 with respect to the start site of transcription (Porter et al. 1993). In addition, pJES750 contains four tandem oligonucleotides lacking T residues in the nontranscribed strand (ACCACA-GACCACACCACAGAC) that were inserted into the BstXI site of pJES463 at +28 (the site was regenerated). When UTP is omitted during the elongation phase of a single-cycle transcription assay, transcription from pJES750 terminates at about position + 113, the beginning of a bank of T residues in the nontranscribed strand, yielding a transcript of 112 bases containing 50 C residues. If pJES750 is cleaved with BstXI, the runoff transcript contains 108 bases and 49 C residues.
Plasmids pJES752, pJES754, and pJES755 are the same as pJES750 with the exception of the bases in the melting region downstream of the promoter. In pJES752, the sequence ATTTT from positions -6 to -2 is replaced with GCTCC; in pJES754, the sequence TTATATTTT from positions -10 to -2 is replaced with CCGCGCCCC; and in pJES755, the sequence TACGG from -1 to +4 is replaced with CGAAC. The base changes were made by PCR using the NcoI site at -38 and the HindIII site in the polylinker downstream of the transcript terminus, pJES760 is the same as pJES752, except that it carries six rather than four tandem oligonucleotides and hence yields a transcript of ---154 bases containing 70 C residues.
Plasmids pJES761 and pJES762 are the same as pJES750 and pJES754, respectively, with the exception of bases in the -24 region of the promoter. In both cases, PCR was used to delete bases -27 through -18.

Heteroduplex templates
Plasmid pJES750 was constructed in the vector pTZ19U (Mead et al. 1986), which produces and packages the template strand when replicated from the phage fl origin. The EcoRI-HindIII fragments bearing the promoter regions from pJES750, pJES754, pJES755, and pJES763 were also cloned into pTZ19R, which produces the opposite strand when replicated from the fl origin. Single-stranded plasmid carrying each of the three promoter regions was made from the constructs in both pTZ19U and pTZ19R (Ausubel et al. 1989) and purified using Qiagen tip-500 columns (Qiagen, cat. no. 12162). Two complementary single strands (10 }*g each)were heated to 95°C for 10 min in 800 }*1 of full-strength PvuII buffer (New England Biolabs buffer 2)and slowly cooled to room temperature. Then 100 units of PvuII was added, and digestion was allowed to proceed for 2 hr at 37°C. DNA was precipitated with ethanol, and the 1-kb PvuII fragment bearing the heteroduplex promoter region was purified on an agarose gel. After isolation from the gel, templates used for the experiments of Figures 3, 4, and 5A were digested with BstXI, which cleaves at the 3' end of the transcript-coding sequence just before the bank of T residues. When A, C, and G nucleotides are provided, the resulting 700-bp fragment yields a runoff transcript with a single 3' end. It was important that the concentration of single-stranded DNA in the annealing step be low to avoid formation of a large network of partially annealed DNA circles.
Core RNA polymerase from Escherichia coh was kindly provided by D. Hager and R. Burgess (University of Wisconsin, Madison).

Open complex formation
Formation of open complexes was detected in a single-cycle transcription assay, as detailed below. Studies from several laboratories have established the validity of this assay by showing that the requirements for the formation of open complexes are the same whether this assay is used to detect them or any one of four additional assays is used (e.g., Sasse-Dwight and Gralla 1988;Popham 1989;Popham et al. 1989;Weiss et al. 1991;Wedel 1993).
Core RNA polymerase and cr 54 were mixed together on ice, to which buffer [final concentration, 50 mM HEPES-acetate (pH 8.0), 100 mM potassium acetate, 8 mM magnesium acetate, 27 mM ammonium acetate, 1 mM dithiothreitol, and 3.5% (wt/vol) polyethylene glycol (6000-8000; Sigma), final pH 8.0], template and, if present, NTRC and NTRB, were added. The final concentrations of template and protein components for individual experiments are detailed in the figure legends.
Mixtures were then warmed to 37°C for 10 min, and reactions were initiated by adding ATP to a final concentration of 0.4 mM in a total volume of 24 }*1. After 10 min, formation of open complexes was terminated by addition of heparin to a final concentration of 0.1 mg/ml and ADP to a final concentration of 4 mM. After an additional 10 min, nucleotides were added to allow synthesis of transcripts [GTP to 0.4 mM and CTP to 0.1 raM; 5 mCi of [c~-'UP]CTP was also added (10 mCi/mmole, New England Nuclear)]. After 10 min, transcripts were precipitated and isolated by gel electrophoresis. Gels were dried and transcript bands were quantified using a Molecular Dynamics Phosphor-Imager. Any deviations from this protocol are detailed in the relevant figure legend.
The specific activity of CTP was determined by spotting 1 }*1 of a 1:1000 dilution of the heparin-nucleotide mix on the dried gel before visualization/quantitation on the PhosphorImager. The specific activity of transcripts was then determined by multiplying the specific activity of CTP by the number of C residues per transcript--70 for the transcript from pJES760 and 50 for transcripts from pJES750, pJES752, pJES754, pJES755, and each of the templates carrying a region of heteroduplex downstream of the promoter ("heteroduplex promoter templates"). Finally, radioactivity in transcript bands (pixel volume corrected for background} was converted to fmoles of transcript.

Decay and reformation of open complexes
Open complexes were formed on a PvuII fragment of ---1 kb derived from pJES760, as detailed below; this template yields a transcript of -~ 154 bases containing 70 C residues. Core RNA polymerase (50 nM), cr 54 (200 riM), NTRC (100 riM), NTRB (20 nM1, and template (10 nM) were mixed in a final volume of 140}.1 in the transcription buffer described above and incubated at 37°C for 10 min. Formation of open complexes was then initiated by addition of ATP to 1 mM. After a further incubation of 8 min, heparin was added to 100 }*g/ml and the mixture was chilled on ice. To assess the number of open complexes at this point, a 20-},1 sample was taken and incubated with elongation mix to allow synthesis of transcripts.
To separate open complexes from free proteins, 100 t*1 of the mixture described above was loaded on a 24-ml Superose-12 column (Pharmacia HR 10/30 prepacked) that had been equilibrated in transcription buffer without polyethylene glycol, and open complexes were eluted at 0.3 ml/min in the same buffer. The column fraction containing the bulk of the template and, hence, of open complexes, was identified by absorbance at 260 nm and contained DNA at a concentration of 0.3 nM. To monitor decay and reformation of open complexes, the column fraction identified above was split into three portions of 80 ~1 each on ice. Each portion was made 1 mM in ATP and 3.5% in polyethylene glycol, the second and third portions were also made 100 nM in NTRC and 20 nM in NTRB, and the third portion was made 5 nM {representing an excess of 17-fold over the first tem-plate} in the -10 to + 4 heteroduplex promoter template, which yields a transcript of ---113 bases containing 50 C residues (see Fig. 3). The heteroduplex template was used because ~54-holoenzyme binds to it with a very low apparent KD (Wedel 1993}. The final volume of all three portions was 100 ~1. The three mixtures were transferred to prewarmed glass tubes at 37°C to initiate the decay and reformation reactions. Beginning 2 min after transfer to 37°C, 20-~1 samples were removed at 5-rain intervals and added to an elongation mix containing heparin, GTP, and CTP plus oL32p]CTP to allow any open complexes to form transcripts. This procedure takes a "snapshot" of the number of open complexes at any given moment, and in portion 3, it shows the partitioning of holoenzyme between the two templates.
Before sieving on the Superose-12 column, 30% of the template molecules carried polymerase in an open complex, whereas after sieving that fraction was ---3%, meaning that ---10% of the open complexes survived the 45 min between addition of heparin before loading the column and collection of the template fraction off the column. The half-life of open complexes on this template is --30 min at 0°C in the absence of heparin and is shorter in the presence of heparin. Addition of heparin was found to be necessary to prevent free holoenzyme from remaining associated with the template during passage through the column, and the open complexes were therefore in contact with heparin until some time after they entered the column.
Control experiments indicated that incubation of polymerase with template at low concentrations of each (0.01 nM core polymerase, 0.01 nM cr 54 and 0.3 nM template in the presence of polyethylene glycol~ conditions of Fig. 3A, lanes 5-8) resulted in no loss of polymerase activity after 1 hr at 37°C. Hence, loss of activity in lanes 5-8 of Figure 3A cannot be accounted for on this basis.