The interaction of the Vibrio cholerae transcription factors, Fur and IrgB, with the overlapping promoters of two virulence genes, irgA and irgB

Abstract

irgA, a virulence gene in Vibrio cholerae, encodes a 77 kDa outer membrane protein. irgA expression is activated by irgB, which encodes a LysR-type transcription factor and is divergently transcribed from a promoter overlapping that of irgA. Expression of irgA and irgB is repressed by iron and Fur. A 200 bp DNA fragment containing the irgA–irgB intergenic region was inserted between the Escherichia coli phoA and lacZ genes, respectively, to generate operon fusions to the two promoters, and this construct was crossed into the chromosomal lacZ gene of V. cholerae. This DNA fragment was sufficient to produce regulation of irgA–phoA and irgB–lacZ transcription by iron, Fur and IrgB. Purified V. cholerae Fur and IrgB overexpressed in E. coli bound simultaneously to this DNA fragment in gel shift experiments, and footprints of both proteins on the irgA–irgB intergenic region were observed using DNaseI footprinting. The Fur footprint overlapped a Fur box, previously identified by homology with the E. coli Fur box. The position of the IrgB footprint was consistent with activation of irgA transcription and repression of irgB transcription by IrgB. We present a model for the interaction of Fur and IrgB in transcriptional regulation of irgA.