Evidence for conversion of heteroduplex transforming DNAs to homoduplexes by recipient pneumococcal cells (DNA strand resolution-DNA repair-bacterial transformation-genetic recombination).

Heteroduplex molecules of pneumococcal DNA, prepared by cross-annealing resolved complementary strands, have been used as donors in transformation. A number of pairs of genetic markers are situated exclusively in the trans configuration in these heteroduplexes. Insertion of two markers from trans configuration into opposite DNA strands of a recipient genome should result in their segregation after one replication cycle. As a consequence, doubly transformed progeny would not appear, or would be markedly decreased. Contrary to these expectations, transformations with the heteroduplex DNAs give as many doubly transformed progeny for unlinked marker pairs as do homoduplex DNAs. For a pair of markers that normally are weakly linked, the frequencies of cotransfer are actually greater than those observed for a mixture of two singly marked homoduplex DNAs. These results lead to the conclusion that the heteroduplex DNAs are acted upon by repair enzymes of recipient cells so that markers introduced in the trans configuration are frequently converted to the cis configuration, either before or during integration. The efficiency of this conversion suggests that the repair and integration processes may be intimately connected. It is also concluded that complete breakdown of one strand of donor heteroduplex DNA does not occur during DNA uptake by recipient cells.