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Background   Acinetobacter baylyi  ADP1 is an ideal bacterial strain for high-throughput genetic analysis as the bacterium is naturally transformable. Thus, ADP1 can be used to investigate DNA mismatch repair, a mechanism for repairing mismatched bases. We used the  mutS  deletion mutant (XH439) and  mutL  deletion mutant (XH440), and constructed a  mutS mutL  double deletion mutant (XH441) to investigate the role of the mismatch repair system in  A. baylyi .    Results  We determined the survival rates after UV irradiation and measured the mutation frequencies, rates and spectra of wild-type ADP1 and  mutSL  mutant via rifampin resistance assay (Rif R  assay) and experimental evolution. In addition, transformation efficiencies of genomic DNA in ADP1 and its three mutants were determined. Lastly, the relative growth rates of the wild type strain, three constructed deletion mutants, as well as the rifampin resistant mutants obtained from Rif R  assays, were measured. All three mutants had higher survival rates after UV irradiation than wild type, especially the double deletion mutant. Three mutants showed higher mutation frequencies than ADP1 and favored transition mutations in Rif R  assay. All three mutants showed increased mutation rates in the experimental evolution. However, only XH439 and XH441 had higher mutation rates than the wild type strain in Rif R  assay. XH441 showed higher transformation efficiency than XH438 when donor DNA harbored transition mutations. All three mutants showed higher growth rates than wild-type, and these four strains displayed higher growth rates than almost all their  rpoB  mutants. The growth rate results showed different amino acid mutations in  rpoB  resulted in different extents of reduction in the fitness of rifampin resistant mutants. However, the fitness cost brought by the same mutation did not vary with strain background.    Conclusions  We demonstrated that inactivation of both  mutS  and  mutL  increased the mutation rates and frequencies in  A. baylyi , which would contribute to the evolution and acquirement of rifampicin resistance. The  mutS  deletion is also implicated in increased mutation rates and frequencies, suggesting that MutL may be activated even in the absence of  mutS . The correlation between fitness cost and rifampin resistance mutations in  A. baylyi  is firstly established.

The mismatch repair system (mutS and mutL) in Acinetobacter baylyi ADP1