Postsynthesis mismatch repair is an important contributor to mutation avoidance 
and genomic stability in bacteria, yeast, and humans. Regulation of its activity 
would allow organisms to regulate their ability to evolve. That mismatch repair 
might be down-regulated in stationary-phase Escherichia coli was suggested by 
the sequence spectrum of some stationary-phase ("adaptive") mutations and by the 
observations that MutS and MutH levels decline during stationary phase. We 
report that overproduction of MutL inhibits mutation in stationary phase but not 
during growth. MutS overproduction has no such effect, and MutL overproduction 
does not prevent stationary-phase decline of either MutS or MutH. These results 
imply that MutS and MutH decline to levels appropriate for the decreased DNA 
synthesis in stationary phase, whereas functional MutL is limiting for mismatch 
repair specifically during stationary phase. Modulation of mutation rate and 
genetic stability in response to environmental or developmental cues, such as 
stationary phase and stress, could be important in evolution, development, 
microbial pathogenicity, and the origins of cancer.