Cobalt-dependent transcriptional switching by a dual-effector MerR-like protein regulates a cobalt-exporting variant CPx-type ATPase

CoaR associates with and confers cobalt-dependent activation of the coat operator-promoter. A CoaR mutant (Ser-Asn-Ser) in a carboxyl-terminal Cys- His-Cys motif bound the coat operator-promoter but did not activate expression in response to cobalt, implicating thiolate and/or imidazole ligands at these residues in an allosteric cobalt binding site. Deletion of 1 or 2 nucleotides from between near consensus, but with aberrant (20 base pairs) spacing, -10 and -35 elements enhanced expression from the coat operator-promoter but abolished activation by cobalt-CoaR. It is inferred that cobalt effects a transition in CoaR that underwinds the coat operator- promoter to realign promoter elements. In the absence of cobalt, CoaR represses expression (~50%). CoaR is a fusion of ancestral MerR (mercury- responsive transcriptional activator)- and precortin isomerase (enzyme of vitamin B12 biosynthesis)-related sequences. Expression from the coaT operator-promoter was enhanced in a partial mutant of cbiE (encoding an enzyme preceding precorrin isomerase in B12 biosynthesis), revealing that this pathway 'inhibits' coaT expression. Disruption of coat reduced cobalt tolerance and increased cytoplasmic 57Co accumulation, coaT-mediated restoration of cobalt tolerance has been used as a selectable marker.