Differential response of multiple epsilon-globin cap sites to cis- and trans-acting controls.
The human epsilon-globin gene has a number of alternative transcription-initiation sites located upstream of the canonical mRNA cap site. In three nonerythroid cell lines, "leaky" epsilon-globin transcription occurs exclusively from one of these upstream sites, the -200 cap site. Using a transient expression assay, we have shown that transcription initiation from the -200 cap site and the major cap site can be independently regulated in response to plasmid replication, SV40 enhancer sequences in cis, and the adenovirus E1A gene in trans. The -200 cap site is located within a region of S1 hypersensitivity in the supercoiled plasmid, and in the absence of viral enhancer sequences it is the main initiation site following transfection into a number of cell lines. We suggest that the -200 cap site acts as a polymerase entry site by virtue of its accessible chromatin structure. The efficiency of polymerase binding at this site may be altered by trans-acting regulatory molecules.