Semiconservative DNA replication is initiated at a single site in recombination-deficient gene 32 mutants of bacteriophage T4.

We have investigated, by electron microscopy, replicative intermediate produced early after infection of Escherichia coli with two phage T4 gene 32 mutants (amA453 and tsG26) which replicate their parental DNA but are defective in secondary replications and in moderating the activities of recombination nucleases. Under conditions completely restrictive for progeny production, both of these mutant produced replicative intermediates, each containing a single internal loop. Both branches of these loops were double stranded; i.e., both leading and lagging strands were synthesized. The replicative intermediates of these mutants qualitatively and quantitatively resembled early replicating wild-type T4 chromosomes after solitary infection of E. coli. However, in contrast to intracellular wild-type T4 DNA isolated from multiple infection, the mutant DNAs showed neither multiple branches nor multiple tandem loops. These results demonstrate that a truncated gene 32 protein which consists of less than one-third of the wild-type T4 helix-destabilizing protein can facilitate the functions of T4 replication proteins, specifically those of T4 DNA polymerase and priming proteins. Our results also support the hypothesis that the generation of multiple tandem loops or branches in vegetative T4 DNA depends on recombination (Mosig et al., in B. Alberts, ed., Mechanistic Studies of DNA Replication and Genetic Recombination, p. 527-543, Academic Press, Inc., New York, 1980).