Hyperrecombination in pneumococcus: A/G to C.G repair and requirement for DNA polymerase I.
During pneumococcal transformation, we had previously described that the ami36 mutation, which results from a C.G to A.T transversion, induces a large excess of wild-type recombinants in two point crosses. Upon donor-recipient DNA recombination, two heteroduplexes are generated by this mutation: A36/G+ and C+/T36. In two point crosses, hyperrecombination is observed only when transformation leads to the A/G mismatch. Here, we have studied the separate evolution of A36/G+ and C+/T36 heterozygotes created upon transformation of an ami36 mutant strain with artificial heteroduplex DNAs. We found that the A36/G+ mismatch leads to a preferential generation of wild-type progeny as compared with the complementary C+/T36 mismatch. This result suggests that A/G carrying transformants partly behave as wild-type homozygotes. The only way to account for such behavior is an excision repair correcting some A/G mispairs created upon transformation into C.G pairs. Moreover, we show that hyperrecombination triggered by ami36 is strongly reduced in a DNA polymerase I deficient strain. This strengthens the fact of DNA repair synthesis, which should be therefore prominently due to DNA polymerase I.
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Laboratoire de Microbiologie et Génétique cellulaire du C.N.R.S., Université Paul Sabatier, Toulouse, France.
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D003433:Crosses, Genetic; D004256:DNA Polymerase I / Q000378:metabolism*; D004260:DNA Repair / Q000502:physiology*; D004269:DNA, Bacterial / Q000235:genetics; D006579:Heterozygote; D007700:Kinetics; D009692:Nucleic Acid Heteroduplexes / Q000502:physiology*; D017354:Point Mutation / Q000235:genetics; D011995:Recombination, Genetic / Q000235:genetics*; D013296:Streptococcus pneumoniae / Q000235:genetics* / Q000254:growth & development; D014169:Transformation, Bacterial
D004269:DNA, Bacterial; D009692:Nucleic Acid Heteroduplexes; D004256:DNA Polymerase I