GENETIC HETEROZYGOSITY IN UNREPLICATED BACTERIOPHAGE h RECOMBINANTS

Bacteriophage crosses using density-labeled parents have been carried out under conditions restricting DNA synthesis. The parental material and genetic contributions to progeny manifesting recombination within a genetic interval sufficiently short to exhibit high negative interference have been examined. The unreplicated products of recombination isolated as phage particles appear to contain long continuous heteroduplex regions which are heterozygous for the closely linked markers. Recombination between closely linked markers seems to be the consequence of the removal of base-pair mismatches that are present within the heteroduplex regions. This localized reduction of heterozygosity within the heteroduplex regions that join the parental components of recombinant DNA molecules can account for high negative interference. N a previous paper (WHITE and Fox 19749, we have suggested that recombiI nation between closely linked markers in phage h occurs by a two-step process. First a heterozygous heteroduplex overlap region is formed, presumably by a breakage and joining reaction, usually including most of the closely linked markers under consideration. Secondly, regions of base-pair mismatch present in these nonrecombinant heterozygotes are reduced to homozygosity or “repaired” by some unspecified process. Furthermore, it has been shown that the heterozygous heteroduplex overlaps have only one of the two possible chemical orientations of the DNA (Figure 1 b) . Here we wish to describe in more detail the genetic structure of the unreplicated products of the recombination events as revealed by examination of genotypes present among the progeny of individual phage products of a genetic cross. Crosses, involving closely linked markers, are carried out under conditions severely restricting DNA synthesis (MCMILIN and Russo 1972). The parental phage contain differential density labels and progeny phage mature without replication. The buoyant density of recombinant phage reflects their relative material contribution from each of the parents. In order to examine the parental genetic contributions, the “two-strand genotype” of the unreplicated recombinant phage is deduced from the genotypes found among the progeny of single infections. Although it is apparent from previous experiments that infection with heterozygous DNA frequently does not yield mixed or heterozygous bursts of 1 Present address: Department of Biochemistry, Stanford University School of Medicine, Stanford, California 94304. Genetics 81 : 33-50 September, 1975. 34 R. L. WHITE A N D M. S . FOX > 5 ' + b + x 3 a + c -A 5 ' a ) 3' < J formation of heteroduplex ove r l ap + b + a + cb) < r educ t ion t o homozygosity o f base\1 pair mismatches a t b and c, f o r exampie 3'