Positive transcriptional regulation of an iron-regulated virulence gene in Vibrio cholerae.

We have previously described a virulence gene in Vibrio cholerae (irgA) that is more than 850-fold regulated in response to iron. Negative regulation of irgA by iron occurred at the transcriptional level, and there was a dyad symmetric nucleotide sequence in the vicinity of the irgA promoter homologous to the Fur binding site in Escherichia coli. When irgA was cloned into E. coli, we showed that transcription of irgA required 900 base pairs of DNA upstream of the irgA promoter that contained an open reading frame in inverse orientation to irgA. In the present study, we show that this upstream region of DNA encodes a gene in inverse orientation to irgA (named irgB) that is also negatively regulated by iron. Insertional inactivation of irgB on the V. cholerae chromosome leads to loss of expression of a chromosomal irgA'-'phoA fusion (in which the primes indicate truncated genes), which is restored to normal by provision of irgB on a plasmid in trans. DNA sequencing of irgB shows that the protein product (IrgB) is homologous to the LysR family of positive transcriptional activators, and secondary structure analysis of IrgB predicts a helix-turn-helix DNA binding motif. The promoters of irgB and irgA are divergent but overlap each other and the previously defined Fur-binding site. We propose a model for iron regulation of irgA expression in V. cholerae. In the presence of sufficient iron, transcription of both irgA and irgB is negatively regulated by a Fur-like protein. In low iron conditions, negative regulation of transcription is removed, and production of IrgB leads to positive transcriptional activation of irgA. It seems likely that the high induction ratio of irgA expression under low- and high-iron conditions (850-fold) relates to the fact that its cognate positive transcriptional activator (irgB) is itself negatively regulated by iron.