HOMEOLOGOUS RECOMBINATION AND MISMATCH REPAIR DURING TRANSFORMATION IN STREPTOCOCCUS-PNEUMONIAE - SATURATION OF THE HEX MISMATCH REPAIR SYSTEM

The ability of the Hex generalized mismatch repair system to prevent r ecombination between partially divergent (also called homeologous) seq uences during transformation in Streptococcus pneumoniae was investiga ted, By using as donor in transformation cloned fragments 1.7-17.5% di vergent in DNA sequence from the recipient, it was observed that the H ex system prevents chromosomal integration of the least and the most d ivergent fragments but frequently fails to do so for other fragments, In the latter case, the Hex system becomes saturated (inhibited) due t o an excess of mismatches: it is unable to repair a single mismatch lo cated elsewhere on the chromosome, Further investigation with chromoso mal donor DNA, carrying only one genetically marked divergent region, revealed that a single divergent fragment can lead to saturation of th e Hex system, Increase in cellular concentration of either HexA, the M utS homologue that binds mismatches, or HexB, the MutL homologue for w hich the essential role in repair as yet remains obscure, was shown to restore repair ability in previously saturating conditions, Investiga tion of heterospecific transformation by chromosomal DNA from two rela ted streptococcal species, Streptococcus oralis and Streptococcus miti s, also revealed complete saturation of the Hex system, Therefore the Hex system is not a barrier to interspecies recombination in S, pneumo niae, These results are discussed in light of those described for the Mut system of Escherichia coli.