Base specificity of mismatch repair in Streptococcus pneumoniae.

DNA sequence analysis was undertaken to investigate the structural basis of mutations showing different integration efficiencies in Streptococcus pneumoniae. Wild type, mutant and revertant sequences at two sites in the amiA locus were determined. It appears that markers which transform efficiently or inefficiently can result from single base pair changes. A low efficiency (LE) marker corresponds to a C:G to T:A change and a high efficiency (HE) marker to a G:C to T:A change. In the latter case, two mismatches, G/A and T/C, can exist at the heteroduplex stage in transformation; only T/C appears to be recognized by the hex system which controls transforming efficiencies in pneumococcus. Each of the recognized mismatches, T/G and C/A, which result from transitional change, and T/C appears to involve at least one pyrimidine. It is proposed that the mismatch repair system of S. pneumoniae is directed against mismatched pyrimidines. DNA sequence analysis also reveals that short deletions (33 or 34 bases long) behave as very high efficiency markers, confirming that deletions are not recognized by the hex system.