Role of the merT and merP gene products of transposon Tn501 in the induction and expression of resistance to mercuric ions

The bacterial transposon Tn501 carries, in addition to the genetic information for its own transposition, the genes of the mer operon (in the order merRTPAD), which code for resistance to Hg2+ ions. The basis for the resistance to Hg2+ is the enzymatic reduction of Hg2+ to Hg0 by mercuric reductase, the product of the Tn501 merA gene. We show here that deletion of the merT and merP genes from Tn501 leads to almost complete loss of the resistance phenotype, even if mercuric reductase is still present in the cytoplasm. Expression of the merT and merP genes in the absence of mercuric reductase gives a mercury-supersensitive phenotype. Mercury-dependent induction of transcription of the mer operon can occur in the absence of the merT and merP gene products. However, this induction is reduced by the presence of mercuric reductase in the cell. We conclude that one or both of the merT and merP genes of Tn501 are required for the expression of the mercury-resistance phenotype. They may in addition have a role in maximising the induction of the mer operon in the presence of Hg2+ ions. This is consistent with their proposed role in transport of Hg2+ into the cytoplasm.