Abstract
Formation of genetic recombinants in bacteriophage φX174 is stimulated up to 50-fold in host cells carrying the recA + allele by subjecting the virus particles to ultraviolet irradiation before infection, or by starving the host cell for thymine during infection; in recA host strains no such increases are observed. φX174 replicative form DNA molecules formed in vivo from ultraviolet-irradiated bacteriophage consist of an intact, circular full-length viral (+) strand and a partially complete complementary (−) strand extending from the point of origin of complementary strand DNA synthesis to an ultraviolet lesion. φX174 replicative form DNA molecules formed in thymine-deficient host strains during thymine starvation have nearly complete circular viral (+) and complementary (−) strands, which contain random single-strand nicks or gaps. Correlation of these structures with the observed increases in recombination suggests that single-strand “breaks” are aggressive intermediate structures in the formation of φX174 genetic recombinants mediated by the host recA + gene product.
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