Cobalamin (vitamin B12) repression of the Cob operon in Salmonella typhimurium requires sequences within the leader and the first translated open reading frame.

Expression of the Cob operon in Salmonella typhimurium is repressed by cobalamin (Cbl). Here it is shown that Cbl repression is mediated by a post-transcriptional regulatory mechanism that requires sequences within the leader and the first translated open reading frame, the cbiA gene. Transcriptional and translational Cob::lacZ fusions containing various lengths of Cob DNA were analysed. In a translational Cob::lacZ fusion 407 bp of leader sequence (+69 to +476) was sufficient to confer normal repression. However in a transcriptional Cob::lacZ fusion a 618 bp region (+69 to +687) was required for normal repression. This 618 bp region included sequences in the leader as well as sequences within the cbiA gene. Point mutations which resulted in loss of repression control were isolated and shown to be clustered in the leader sequence (+257 to +380). This region contains a putative hairpin-loop structure which we propose functions as an RNA operator site for a vitamin B12-responsive repressor protein.