ABSTRACT
In Escherichia coli, expression of the pyrC gene is regulated primarily by a translational control mechanism based on nucleotide-sensitive selection of transcriptional start sites at the pyrC promoter. When intracellular levels of CTP are high, pyrC transcripts are initiated predominantly with CTP at a site 7 bases downstream of the Pribnow box. These transcripts form a stable hairpin at their 5' ends that blocks ribosome binding. When the CTP level is low and the GTP level is high, conditions found in pyrimidine-limited cells, transcripts are initiated primarily with GTP at a site 9 bases downstream of the Pribnow box. These shorter transcripts are unable to form a hairpin at their 5' ends and are readily translated. In this study, we examined the effects of nucleotide sequence and position on the selection of transcriptional start sites at the pyrC promoter. We characterized promoter mutations that systematically alter the sequence at position 7 or 9 downstream of the Pribnow box or vary the spacing between the Pribnow box and wild-type transcriptional initiation region. The results reveal preferences for particular initiating nucleotides (ATP > or = GTP > UTP >> CTP) and for starting positions downstream of the Pribnow box (7 >> 6 and 8 > 9 > 10). The results indicate that optimal nucleotide-sensitive start site switching at the wild-type pyrC promoter is the result of competition between the preferred start site (position 7) that uses the poorest initiating nucleotide (CTP) and a weak start site (position 9) that uses a good initiating nucleotide (GTP). The sequence of the pyrC promoter also minimizes the synthesis of untranslatable transcripts and provides for maximum stability of the regulatory transcript hairpin. In addition, the results show that the effects of the mutations on pyrC expression and regulation are consistent with the current model for translational control. Possible effects of preferences for initiating nucleotides and start sites on the expression and regulation of other genes are discussed.
