Transcriptional regulation by iron of a Vibrio cholerae virulence gene and homology of the gene to the Escherichia coli fur system.

We have previously described an iron-regulated virulence determinant in Vibrio cholerae. Strain MBG40, which contains a TnphoA insertion mutation in the iron-regulated gene irgA, has reduced virulence in a newborn mouse model and has lost the major 77-kDa iron-regulated outer membrane protein. We report here the cloning of the irgA'-'phoA gene fusion, the sequencing of the 5'-proximal portion of irgA, and the definition of its promoter region by primer extension. The deduced amino acid sequence of the amino-terminal portion of IrgA is homologous to the ferrienterochelin receptor of Escherichia coli (FepA), suggesting that IrgA may be the iron-vibriobactin outer membrane receptor. Iron regulation of irgA in an E. coli background and that of the E. coli gene slt-IA in a V. cholerae background are reciprocal, suggesting a common mechanism of iron regulation. Regulation of irgA by iron in V. cholerae occurs at the transcriptional level, and there is an interrupted dyad symmetric sequence in the vicinity of the promoter that is homologous to Fur binding sites of E. coli. Unlike iron-regulated genes in E. coli, however, transcription of irgA requires an additional 900 bp of upstream DNA that contains an open reading frame in inverse orientation to irgA.