Correlation between RNA synthesis and basal level guanosine 5'-diphosphate 3'-diphosphate in relaxed mutants of Escherichia coli.
Through the use of a new nucleotide extraction procedure, we had previously shown that relaxed mutants of Escherichia coli exhibit a unique response to amino acid starvation (Lagosky, P. A., and Chang, F. N. (1980) J. Bacteriol. 144, 499-508). The basal level amounts of guanosine 5'-diphosphate 3'-diphosphate (ppGpp) in both relA and phenotypically relaxed relA+ rplK (relC) strains were shown to decrease at the onset of amino acid limitation and to remain severely depressed throughout the course of the starvation. Upon resupplementation of amino acid-starved relaxed mutants, the production of ppGpp resumes and results in the temporary overaccumulation of this nucleotide beyond its original basal level amount. We now show that the basal level ppGpp content of relaxed bacteria, as well as its subsequent fluctuations in response to amino acid starvation, is inversely correlated with the initial rates of RNA synthesis in these strains. The ability of ppGpp to control the rate of protein synthesis in relA mutants was also examined. It was observed that ppGpp had no apparent direct effect on the initial rates of protein synthesis in relA mutants. The constant inverse correlation which exists between ppGpp content in relA mutants, and their rates of RNa synthesis provide evidence which indicates that basal level ppGpp synthesis has definite physiological significance. It also suggests that the synthesis of basal level ppGpp might be an absolute requirement needed for normal bacterial growth.