REPLICATIVE FRAGMENTATION IN T4 BACTERIOPHAGE DNA, II. BIPARENTAL MOLECULAR RECOMBINATION

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Abstract
When a light, thymine-dependent strain of E. coli B is infected simultaneously with light, P32 at a low multiplicity, and heavy 5 BU-labeled, cold phage at a high multiplicity in the absence of thymine and in the presence of FUDR, the resulting progeny phage and its DNA contain a biparental contribution. This is revealed in CsCl gradient as a displacement of the P32-containing moiety of DNA toward the heavy side. Shearing of this DNA separates heavy and light fragments; denaturation by alkali does not. These results indicate that parental contributions are integrated into the recombinant molecule as discrete subunits and that after exchanges the continuity is repaired and a large fraction of subunits derived from both parents is located within the same single strand of DNA. If infected bacteria are starved, or chloramphenicol [CM] is added between 0 to 5 min after infection and intracellular parental DNA analyzed, no recombinant molecules can be detected. However, the addition of CM at 9 min after infection permits a certain degree of recombination. If the labeling system is reversed, i.e., light bacteria are infected with heavy and radioactive phage in the minority, and cold, light phage in the majority, and the displaced moiety of DNA is separated in CsCl gradient and then subjected to shearing, no parental-like conservative subunits, composed of both heavy and radioactive strands, could be detected in the recombinant molecule. This indicates that the phenomenon of semiconservative replication must be closely associated with molecular recombination. Models of recombinant molecules are presented.