rexAB mutants in Streptococcus pneumoniae.

Streptococcus pneumoniae is a human pathogen that is naturally transformable. In this study a major component of the homologous recombination pathway, the RexAB exonuclease/helicase, was characterized. rexA and rexB insertional mutants were constructed using mariner mutagenesis and found to have identical phenotypes. Both rexAB mutants displayed poor cell viability, reduced double-strand exonuclease activity, UV sensitivity and a reduced level of gene conversion compared to the wild-type strain. No effect was observed on plasmid and chromosomal transformation efficiencies. These results indicate that in S. pneumoniae, RexAB is required for DNA repair, but not for chromosomal transformation and plasmid establishment.