Visualization of a novel junction in bacteriophage lambda DNA.

At early times after infection of a recA derivative of Escherichia coli with lambdab221c126red270a42 phage, a low but significant proportion of intracellular lambda molecules show a novel junction. These junctions are also present, although in reduced numbers, in a lysate obtained at late times after infection of a recA+ host with lambdacIIcIII phage. Fine structure and denaturation mapping analyses showed that these junctions occur at homologous positions and that they are compatible with the occurrence of a cross-strand exchange between lambda DNA duplexes similar to the type proposed in most molecular models for genetic recombination. However, the results are also consistent with the structures expected if a replicating growing point undergoes branch migration.