Specific A/G-to-C.G mismatch repair in Salmonella typhimurium LT2 requires the mutB gene product

Abstract

An assay has been developed that permits analysis of repair of A/G mismatches to CG base pairs in cell extracts of Salmonella typhimurium LT2. This A/G mismatch repair is independent of ATP, dam methylation, and mutS gene function. The gene product of mutB has been shown to be involved in the dam-independent pathway through the in vitro assay. Moreover, specific DNA-protein complexes and an endonuclease can be detected in S. typhimurium extracts by using DNA fragments containing an A/G mismatch. These activities are not observed with substrates which have a T/G mismatch or no mismatch. The S. typhimurium endonuclease, like the AMG endonuclease found in Escherichia coli (A-L. Lu and D.-Y. Chang, Cell 54:805-812, 1988), makes incisions at the first phosphodiester bond 3' to and the the second phosphodiester bond 5' to the dA of the A/G mismatch. No incision site was detected on the other DNA strand. Extracts prepared from mutB mutants cannot form A/G mismatch-specific DNA-protein complexes and do not contain the A/G endonuclease activity. Thus the A/G mismatch specific binding and nicking activities are probably involved in the A/G mismatch repair pathway. Preliminary analysis of the mutational spectrum of the mutB strain has indicated that this mutator allele causes an increase in CG-to-A-T transversions without affecting the frequencies of other transversion or transition events. In addition, the mutB gene has been mapped to the 64-min region of the S. typhimurium chromosome. Together, this biochemical and genetic evidence suggests that the mutB gene product of S. typhimurium is the homolog of the E. coli micA (and/or mutY) gene product.

Competence Repression under Oxygen Limitation through the Two-Component MicAB Signal-Transducing System in Streptococcus pneumoniae and Involvement of the PAS Domain of MicB

“…ORFs flanking micB were designated micA and orfC. The micAB genes are located close to the antimutator, mutY (2,16,24) (Fig. 1A).…”

“…This antimutator gene in E. coli and S. pneumoniae encodes an adenine glycosylase specific for A/G miss-match pairs. The major in vivo substrate for this enzyme is probably the adenine from A/7,8-dihydro-8-oxoguanine, a product of oxidative damage to DNA (2,16,18). The MutY of E. coli has the highly conserved stretch of four cysteines, Cys-X6-Cys-X2-Cys-X5-Cys, that coordinates the (4Fe-4S) 2ϩ cluster loop (FCL).…”

Homologous Recombination between thetufGenes ofSalmonella typhimurium

“…The significance of these numbers is discussed later in terms of models for recombination. Finally, mutB (equivalent to mutY in E. coli ), which is specifically involved in repairing G-A and A-C mispairs to G-C (Radicella et al, 1988;Lu et al, 1990;Modrich, 1991;Desiraju et al, 1993), has no influence on the wild-type rate of sequence transfer as measured in these assays. The event we are selecting for is Ala375 to Thr, GCA to ACA, and potentially requires either an A-C or a G-T mispair as an intermediate.…”

Nucleotide sequence of the Salmonella typhimurium mutB gene, the homolog of Escherichia coli mutY

“…It has been suggested that the mutB gene of S. typhimurium is the homolog of E. coli mutY (6). The mutB mutant exhibits a higher rate of C. G-to-A-T transversions than wild-type cells and has no effects on other transversion or transition events (6,15). The mutB mutant extract is defective in A/G-specific binding and nicking and repair activities (6).…”