FIG. 3.

Model for transcription start site switching and translational control of pyrC expression in E. coli and Salmonella. The nucleotide sequence of the pyrC promoter-regulatory region of E. coli is shown, with the −10 region, SD sequence, and pyrC initiation (Met) codon underlined and labeled. Asterisks indicate the four transcription start sites at the pyrC promoter, and the two major start sites, C7 and G9, are indicated. Inverted horizontal arrows indicate the region of dyad symmetry. The sequence and structure of transcripts initiated at start sites C7 (high CTP) and G9 (low CTP) are shown, with the SD sequence boxed. Only C7 transcripts form the hairpin that includes the SD sequence and prevents translation initiation.