Excision and repair of mismatched base pairs in transformation of Streptococcus pneumoniae.

The use of heteroduplex DNA molecules as donors in pneumococcal transformation makes it possible to follow the fate of each DNA strand. The integration efficiency of each strand depends strongly upon the single base changes it carries. The function (hex) which reduces drastically the transformation yield of markers referred to as low efficiency (LE) tends to remove either donor strand without respect ot which one is introduced. In the case of high efficiency (HE) markers the reduction in the transformation yield involves the elimination of only one donor strand. For a given locus it can be either one depending upon the mutation. The reduction in transformation yield can be less drastic for HE markers than for both strands of the LE markers. These data are discussed in terms of differences in the affinity for mismatched base pairs. We have studied the transfer of information from each donor DNA strand to the recipient genome, on the basis of differences in the rates of phenotypic expression of a given marker introduced on opposite strands. Results show that, as in the case of LE markers, the information from HE markers, when introduced on the strand recognized by the hex function, is transmitted to both strands of the recipient molecule. Correction of the recipient strand to homozygosis probably accounts for this information transfer. These results, together with earlier investigations, strongly suggest that the hex function is an excision-repair system acting on donor-recipient base pair mismatches.