Transcription increases multiple spontaneous point mutations in Salmonella enterica

Abstract

The spontaneous rate of G.C-->A.T mutations and a hotspot T.A-->G.C transversion are known to increase with the frequency of transcription-increases that have been ascribed primarily to processes that affect only these specific mutations. To investigate how transcription induces other spontaneous point mutations, we tested for its effects in repair-proficient Salmonella enterica using reversion assays of chromosomally inserted alleles. Our results indicate that transcription increases rates of all tested point mutations in the induced gene: induction significantly increased the individual rates of an A.T-->T.A transversion, an A.T-->G.C transition and the pooled rates of the three other point mutations assayed. Although the S.enterica genome is thought to have a mutational bias towards G.C base pairs, transitions creating A.T pairs were approximately 10 times more frequent than the reverse mutation, resulting in an overall mutation pressure to lower G+C contents. Transitions occurred at roughly twice the rate of transversions, similar to results from sequence comparisons; however, several individual transversions are more frequent than the least common transition.

Figure 1

Exponential-phase reversion rates in the presence (hatched bars) or absence (solid bars) of IPTG, calculated under the assumptions that post-plating mutations did or did not occur. Rates calculated under these two assumptions were identical for all strains in the presence of IPTG but differed for strains TT22914 and UB631 in its absence. In these two cases, the gray extended regions of the solid bars reflect rates that include post-plating mutations.