We examined a region of high variability in the mosaic mercury resistance (
mer) operon of natural bacterial isolates from the primate intestinal micro
biota. The region between the merP and merA genes of nine mer loci was sequ
enced and either the merC, the merF, or no gene was present. Two novel merC
genes were identified. Overall nucleotide diversity, pi (per 100 sites), o
f the merC gene was greater (49.63) than adjacent merP (35.82) and merA (32
.58) genes. However, the consequences of this variability for the predicted
structure of the MerC protein are limited and putative functional elements
(metal-binding ligands and transmembrane domains) are strongly conserved,
Comparison of codon usage of the merTP, merC, and merA genes suggests that
several merC genes are not coeval with their flanking sequences. Although e
vidence of homologous recombination within the very variable merC genes is
not apparent, the flanking regions have higher homologies than merC, and re
combination appears to be driving their overall sequence identities higher.
The synonymous codon usage bias (ENC) values suggest greater variability i
n expression of the merC gene than in flanking genes in six different bacte
rial hosts. We propose a model for the evolution of MerC as a host-dependen
t, adventitious module of the mer operon.