Gene conversion in Streptococcus pneumoniae.

In pneumococcus, correction of mutations by the repair of mismatched bases results in gene conversion, i.e. transfer of genetic information from one strand of DNA to the other. Three different correction systems act upon a variety of mutations: 1) A long-patch process (a few Kb) is efficient in the elimination of transitions, mostly +/- 1 base-pair mutations and less efficient in eliminating some transversions without affecting fairly long deletions. Neighboring sequences may interfere with this process. It is directed by at least two genes. 2) A localized conversion system acts on a six base-pair heteroduplex structure such as 5'ATTAAT/3'TAAGTA by specifically converting the mutated A base to the wild type C base. Modifications of this configuration by site-directed mutagenesis lead to reduced conversion. 3) Fairly long deletions are eliminated during recombination by events that extend several scores of bases around the heterologus region. Although only the first conversion system has been shown to participate efficiently in protecting Streptococcus pneumoniae against spontaneous mutation, the two other processes may also eliminate mutations of different natures.