VITAMIN-B-12 REPRESSION OF THE COB OPERON IN SALMONELLA-TYPHIMURIUM -TRANSLATIONAL CONTROL OF THE CBIA GENE

Expression of the cob operon is repressed by B-12 via a post-transcrip tional control mechanism which requires sequence elements within the l eader region of the mRNA and the first gene of the operon, the cbiA ge ne. Here we show that B-12 repression of cbiA gene expression occurs a t the level of translation initiation through sequestration of the rib osomal binding site (rbs) in an RNA hairpin. Analysis of mutations tha t destabilize or restabilize the secondary structure demonstrates that folding of the hairpin is essential for repression. The existence of the hairpin was confirmed by a secondary structure analysis of RNA fro m the wild type and three mutants. Deletions that remove the upstream part of the leader confer a drastic reduction in translation efficienc y. This low-level translation is caused by the hairpin, as indicated b y the finding that suppressor mutations that destabilize the hairpin r estore efficient translation. Thus, the native upstream RNA functions as a translation enhancer and acts to relieve the hairpin's inhibitory effect on translation initiation. The inhibitory effect of the hairpi n was confirmed by a ribosomal toeprinting analysis. We propose that t he translational control of the cbiA gene mediates repression of the e ntire cob operon.