A reversion assay system previously implemented to demonstrate the existence of 
adaptive or stationary-phase-induced mutagenesis in Bacillus subtilis was 
utilized in this report to study the influence of the mismatch DNA repair (MMR) 
system on this type of mutagenesis. Results revealed that a strain deficient in 
MutSL showed a significant propensity to generate increased numbers of 
stationary-phase-induced revertants. These results suggest that absence or 
depression of MMR is an important factor in the mutagenesis of nongrowing B. 
subtilis cells because of the role of MMR in repairing DNA damage. In agreement 
with this suggestion, a significant decrease in the number of adaptive revertant 
colonies, for the three markers tested, occurred in B. subtilis cells which 
overexpressed a component of the MMR system. Interestingly, the single 
overexpression of mutS, but not of mutL, was sufficient to decrease the level of 
adaptive mutants in the reversion assay system of B. subtilis. The results 
presented in this work, as well as in our previous studies, appear to suggest 
that an MMR deficiency, putatively attributable to inactivation or saturation 
with DNA damage of MutS, may occur in a subset of B. subtilis cells that 
differentiate into the hypermutable state.