Identification of Streptococcus pneumoniae mismatch repair genes by an additive transformation approach.

During transformation of Streptococcus pneumoniae, mismatch repair occurs on donor-recipient heteroduplexes harboring some mismatched base pairs. A few mutants defective in mismatch repair have been isolated and termed hex-. However, neither the number of genes involved nor their products have yet been identified. In an attempt to characterize such genes we have used an additive transformation approach--that is the inactivation of genes by insertion of chimeric plasmids. Pneumococcal DNA fragments were joined in vitro to a plasmid derivative of pBR325 that carries an erythromycin resistance determinant and does not replicate autonomously in S. pneumoniae. Ery-r transformants obtained with such a ligation mixture arise via homology-dependent integration of the chimeric plasmids into the chromosome. Hex- mutants have been selected among the ery-r population. Comparison of these mutants by Southern blot hybridization with a vector probe reveals that at least two genes are involved in mismatch repair.