Mismatch repair in Streptococcus pneumoniae: relationship between base mismatches and transformation efficiencies.

Abstract

Genetic transformation in S. pneumoniae involves the insertion of single-stranded pieces of donor DNA into a recipient genome. Efficiencies of transformation strongly depend on the mutations (markers) carried by donor DNA. Markers are classified according to their transforming efficiencies into very high, intermediate and low efficiency. The last is approximately 1/20th as efficient as the first. This marker effect is under the control of the Hex system, which is thought to correct mismatches at the donor-recipient heteroduplex stage in transformation. To investigate this effect, wild type, mutant and revertant DNA sequences at 5 genetic sites within the amiA locus were determined. Low-efficiency markers arise from transitional changes A .cntdot. T to G .cntdot. C. The transversion A .cntdot. T to T .cntdot. A corresponds to an intermediate-efficiency marker. Transversions G .cntdot. C to T .cntdot. A and G .cntdot. C to C .cntdot. G lead to high-efficiency markers. Among the 8 possible mismatches that could exist transiently at the heteroduplex stage in transformation, only 2, namely, A/G and C/C, are not corrected by the Hex system. The 4 possible base pairs (A .cntdot. T, T .cntdot. A, G .cntdot. C and C .cntdot. G) were encountered at the very same site (amiA6 site), which constitutes a good illustration of the marker effect. DNA sequence analysis also reveals that short deletions (33 or 34 bases long) are integrated with very high efficiencies. These results confirm that the Hex system corrects point mismatches harbored in donor-recipient heteroduplexes thousands of bases long. The correction pattern of the Hex system toward multiple-base mismatches was also investigated. Its behavior toward double-base mismatches is complex, suggesting that neighboring sequences may affect the detection of mispaired bases.