Tn5 insertion mutations in the mercuric ion resistance genes derived from plasmid R100.

The mercuric resistance (mer) genes of plasmid R100 were cloned into plasmid pBR322. A series of transposon Tn5 insertion mutations in the mer genes were isolated and mapped. The mutants were characterized phenotypically by their sensitivity to Hg2+ and by binding and volatilization of 203Hg2+. Dominance and complementation tests were also performed. Mutations affecting the previously described mer genes merR (regulation), merT (transport), and merA (reductase) were characterized. Evidence was obtained for two new mer genes, which have been called merC and merD. A restriction enzyme map of the mer region was drawn with the gene order merRTCAD. Transcriptional merR-lac and merA-lac fusions were generated by insertion of phage Mu d amp lac into plasmid R100-1. These were used to study regulation of mer gene expression. The merR gene product appears to regulate negatively its own expression as well as acting as both a negative and a positive regulator of the merTCA genes.