Branched DNA molecules: Intermediates in T4 recombination

Abstract

Multiple infection of Escherichia coli with T4 phages defective in both DNA polymerase and ligase leads to the accumulation of DNA structures that are intermediates in recombination. When examined with the electron microscope, up to 25% of these phage DNA molecules contained one or more double-stranded branches of varying lengths and arrangements. Infection at a multiplicity of one failed to give rise to branched molecules. Differences in the frequency of the branched molecules were observed when the infecting phage carried genetic defects in addition to those in polymerase and ligase. DNA of hybrid density extracted from bacteria mixedly-infected with 13C 15N 32P-labeled and 3H-labeled T4 polymerase-ligase mutants was enriched in branched molecules, indicating that the branches were generated in the course of DNA exchange. These findings have suggested a molecular mechanism for recombination based on a process termed “branch migration”, in which there is a concerted dissociation of two sets of homologous parental DNA double helices and reassociation of two sets of hybrid recombinant complements, with a resulting displacement of the branch point sequentially along the interacting DNA molecules. Strand exchange is thus effected without the necessity of exposing long stretches of single-stranded DNA.