THE MMSA LOCUS OF STREPTOCOCCUS-PNEUMONIAE ENCODES A RECG-LIKE PROTEIN INVOLVED IN DNA-REPAIR AND IN 3-STRAND RECOMBINATION

We describe the characterization of a mutant strain of Streptococcus p neomoniae previously isolated on the basis of its sensitivity to Methy l Methane Sulphonate (MMS). The mutant strain also exhibited increased sensitivity to UV light and to X-rays, together with a reduced capaci ty for recombination and Hex-mediated generalized mismatch repair. We show that the original mutant contains two unlinked mutations in the m msA and in the pms genes. The mmsA wild-type region was cloned and the nucleotide sequence of the mmsA gene was determined. mmsA encodes a p olypeptide of 671 amino acids related to a large family of DNA-RNA hel icases, with the highest similarity to Escherichia coli RecG, a protei n involved in the branch migration of Holliday junctions. A plasmid ca rrying the intact mmsA coding region was shown to restore UV resistanc e to E. coli recG mutant strains. An mmsA-null mutant constructed by i nsertion of a chloramphenicol-resistance gene exhibited a 25-fold redu ction in recombination during transformation, We suggest that MmsA rec ognizes and branch migrates three-strand transformation intermediates to extend donor-recipient heteroduplex regions. The mmsA-null mutant e xhibited the other phenotypes of the original mutant, apart from misma tch-repair deficiency and, in addition, an alteration in colony-formin g ability was noticed, In the pms mutant background, all phenotypes ca used by the mmsA mutation were attenuated. Therefore, the pms mutation , although it affected mismatch repair and, to some extent, DNA repair and recombination, acted as a suppressor of the mmsA mutation.