GENE COPY NUMBER EFFECTS IN THE MER OPERON OF PLASMID-NR1

The level of resistance to Hg2+ determined by the inducible mer operon of plasmid NR1 was essentially the same for 3 gene copy number variants in Escherichia coli, less in Proteus mirabilis and intermediate in P. mirabilis transitioned to a high r-determinant gene copy number. Cell-free volatilization rates of radioactive Hg indicated increasing levels of intracellular mercuric reductase enzyme from low- to high-gene copy number forms in P. mirabilis and from low- to high-copy number forms in E. coli, but the additional enzyme in E. coli was effectively cryptic.