A second positive regulatory function in the mer (mercury resistance) operon

Transpositional mutagenesis of the mer operon of the IncFII plasmid, R100, has revealed a second, trans-acting positive regulatory function. Mutants in this function do not synthesize any of the three small mer operon peptides and have no inducible Hg(II) uptake activity. This second regulatory function is part of complementation group B and so depends upon the activity of the previously described trans-acting positive regulatory function merR. All mutants in this new function map in the amino-terminal 20 kDal of the Hg(II) reductase, suggesting either that this enzyme is also a regulatory protein or that there is a distinct protein whose reading frame is superimposed on that of the Hg(II) reductase. While we have only seen the five previously described mer operon peptides of 69, 66, and 12 (13) kDal encoded in minicells by single-copy plasmids, we have observed two new HgCl 2-inducible polypeptides of approx. 20 kDal in minicells carrying a multicopy derivative of the mer operon of R100. Sequence data for the Hg(II) reductase region of the related mer operon of the transposon, Tn 501 [Brown, N.L., Ford, S.J., Pridmore, R.D. and Fritzinger, D.C., Biochemistry 22 (1983) 4089–4095], shows a second reading frame very rich in cysteine and arginine which overlaps the amino-terminal 20 kDal of the Hg(II) reductase structural gene. We believe that this reading frame is the structural gene for this new regulatory function and propose the name merC (for control).