We have characterized the mismatch repair system (MMR) of the highly 
radiation-resistant type strain of Deinococcus radiodurans, ATCC 13939. We show 
that the MMR system is functional in this organism, where it participates in 
ensuring the fidelity of DNA replication and recombination. The system relies on 
the activity of two key proteins, MutS1 and MutL, which constitute a conserved 
core involved in mismatch recognition. Inactivation of MutS1 or MutL resulted in 
a seven-fold increase in the frequency of spontaneous RifR mutagenesis and a 
ten-fold increase in the efficiency of integration of a donor point-mutation 
marker during bacterial transformation. Inactivation of the mismatch 
repair-associated UvrD helicase increased the level of spontaneous mutagenesis, 
but had no effect on marker integration--suggesting that binding of MutS1 and 
MutL proteins to a mismatched heteroduplex suffices to inhibit recombination 
between non identical (homeologous) DNAs. In contrast, inactivation of MutS2, 
encoded by the second mutS -related gene present in D. radiodurans, had no 
effect on mutagenesis or recombination. Cells devoid of MutS1 or MutL proteins 
were as resistant to gamma-rays, mitomycin C and UV-irradiation as wild-type 
bacteria, suggesting that the mismatch repair system is not essential for the 
reconstitution of a functional genome after DNA damage.