THE INTERACTION OF THE VIBRIO-CHOLERAE TRANSCRIPTION FACTORS, FUR ANDIRGB, WITH THE OVERLAPPING PROMOTERS OF 2 VIRULENCE GENES, IRGA AND IRGB

irgA, a virulence gene in Vibrio cholerae, encodes a 77 kDa outer memb rane protein. irgA expression is activated by irgB, which encodes a Ly sR-type transcription factor and is divergently transcribed from a pro moter overlapping that of irgA. Expression of irgA and irgB is repress ed by iron and Fur. A 200 bp DNA fragment containing the irgA-irgB int ergenic 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. c holer ae. This DNA fragment was sufficient to produce regulation of ir gA-phoA and irgB-lacZ transcription by iron, Fur and IrgB. Purified V. cholerae Fur and IrgB overexpressed in E. coli bound simultaneously t o this DNA fragment in gel shift experiments, and footprints of both p roteins on the irgA-irgB intergenic region were observed using DNaseI footprinting. The Fur footprint overlapped a Fm box, previously identi fied by homology with the E. coli Fur box. The position of the IrgB fo otprint was consistent with activation of irgA transcription and repre ssion of irgB transcription by IrgB. We present a model for the intera ction of Fur and IrgB in transcriptional regulation of irgA. (C) 1998 Elsevier Science B.V.