Characterization of transcriptional initiation from promoters P1 and P2 of the pyrBI operon of Escherichia coli K12.

Expression of the pyrBI operon of Escherichia coli K12, which encodes the pyrimidine biosynthetic enzyme aspartate transcarbamylase, is negatively regulated by pyrimidine availability, primarily through an attenuation control mechanism, and also appears to be subject to stringent control. Previous in vitro transcription studies indicated that the pyrBI operon is transcribed from tandem promoters designated P1 and P2, which appeared to be of similar strength. In this study, we characterized these promoters in detail and examined their role in pyrBI expression. Our results show that although transcription is initiated at both promoters in vivo, greater than 99% of the pyrBI transcripts are initiated at promoter P2, indicating that this is the only physiologically significant promoter. The level of transcripts initiated at promoter P2 was found to be higher in cells grown under pyrimidine-limiting conditions compared to that in cells grown under conditions of pyrimidine excess, indicating pyrimidine-mediated regulation at the level of transcriptional initiation. In vitro characterization of transcription from the pyrBI promoter region showed that non-physiological reaction conditions used in the original identification of the two promoters greatly overestimated the strength of promoter P1. Further in vitro characterization of the two promoters showed that transcription from promoter P2, but not from promoter P1, is inhibited by guanosine tetraphosphate and exhibits salt and heparin sensitivity typical of a stringently controlled promoter. In addition, heparin-challenge experiments revealed a UTP-induced instability of transcriptional initiation complexes at promoter P2 which may be of regulatory significance.