Dual transcriptional initiation sites from the pyrC promoter control expression of the gene in Salmonella typhimurium.

Expression of the Salmonella typhimurium pyrC gene encoding dihydroorotase is negatively regulated by CTP and stimulated by GTP. This regulation does not occur at the level of transcription initiation but appears to involve translation attenuation of the transcripts. Alterations of specific bases in a region of hyphenated dyad symmetry located in the leader established that base pairing in the 5' terminal region of the pyrC leader transcript is required for normal regulation of dihydroorotase synthesis. Primer extension experiments on RNA from mutant strains that permit manipulation of the CTP and GTP pools showed that pyrC transcription may start at either a cytosine or a guanine residue, 2 bp apart. The ratio between G-starts and C-starts appeared to be determined by the intracellular [GTP]/[CTP] pool ratio. The larger transcript, starting with a C, is able to form a stable hairpin in the 5' end, sequestering part of the ribosome binding site in the stem. The leader of the shorter transcript, however, cannot form this secondary structure. Thus, translational initiation will occur unhindered only from the shorter transcript.