B. Martin et al., THE MMSA LOCUS OF STREPTOCOCCUS-PNEUMONIAE ENCODES A RECG-LIKE PROTEIN INVOLVED IN DNA-REPAIR AND IN 3-STRAND RECOMBINATION, Molecular microbiology, 19(5), 1996, pp. 1035-1045
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.