Metalloregulation in Bacillus subtilis: isolation and characterization of two genes differentially repressed by metal ions.

We have cloned two metal-regulated genes (mrgA and mrgC) from Bacillus subtilis by using transposon Tn917-lacZ. Both were isolated as iron-repressible gene fusions, but the metal specificity and sensitivity of gene repression are distinct. Transcription of mrgA-lacZ is induced at the end of logarithmic-phase growth in minimal medium, and this induction is prevented by excess manganese, iron, cobalt, or copper. Limitation for metal ions is sufficient for mrgA-lacZ induction, since resuspension in medium lacking both manganese and iron rapidly induces transcription. Transcription of mrgC-lacZ is also induced by iron deprivation but is not repressed by added manganese or other metal ions. Expression of mrgC-lacZ and a 2,3-dihydroxybenzoic acid-based siderophore is repressed in parallel by iron, and in both cases, only iron effects repression. We have cloned and sequenced the promoter and regulatory regions of both mrgA and mrgC. Both genes are preceded by a predicted sigma A-dependent promoter element with overlapping sequences similar to the iron box consensus element for recognition by the Escherichia coli ferric uptake regulator protein (Fur). Mutation of the putative iron box for gene mrgC leads to partial derepression in iron-replete medium.