Effect of mismatched base pairs on the fate of donor DNA in transformation of Streptococcus pneumoniae.

Investigation of the mechanism that discriminates against mismatched base pairs in transformation of Streptococcus pneumoniae of genotype hex+ was based on the use of a radioactively labeled cloned fragment of pneumococcal DNA as donor in transformation. The fate of the donor label was followed by lysis of the transformed cells and separation by agarose gel electrophoresis of DNA fragments generated by restriction endonucleases. As a result of Hex action, most of the donor DNA fragment, which was a few kilobases in length, was lost when a mismatched base pair occurred between donor and recipient DNA. This was not observed in hex- recipient cells. Kinetic studies of mismatch-induced donor DNA loss showed that the process is faster in strain 800, an R6 derivative, than in DP1601, a strain of different origin. In the latter strain, the amount of donor label that becomes double stranded rises substantially, indicating extensive formation of donor-recipient heteroduplex structures, before falling to the expected level. At 30 degrees C the process is essentially completed 15 min after entry.