Mutator gene studies in Escherichia coli: the mutS gene.

We report here on a study of a mutator gene (mutS) that causes transition mutations in Escherichia coli. We have used the trpA system to show that A:T-->G:C and G:C-->A:T transitions occur. Not all A:T pairs are equally susceptible to mutS action however, since the A:T pair at the trpA223 site reverts at a frequency similar to, if not identical with, the frequency in a mut(+) background. Presumably this is a consequence of neighboring bases, because other A:T pairs are reverted by mutS in the same gene; and an A:T pair in the lac operon is reverted at two widely separated points on the chromosome, and in two orientations relative to the trp sense strand. In addition, we have shown that the mutS1 allele is recessive to wild type, and trans active.