Enzymes and Sites of Genetic Recombination: Studies with Gene-3 Endonuclease of Phage T7 and with Site-affinity Mutants of Phage  

Abstract

Branched DNA is an intermediate in genetic recombination (Broker and Lehman 1971; Kemper and Brown 1976; Tsujimoto and Ogawa 1977; Bell and Byers 1979; Ikeda and Kobayashi 1979). One form of branched DNA, the Holliday structure, arises by a reciprocal exchange of strands between duplexes (Holliday 1964). It has recently been shown that gene 49 of bacteriophage T4 specifies or controls an endonuclease (Endo VII) that cleaves Holliday structures (Mizuuchi et al. 1982; Fig. 1). Endo VII is essential for T4 growth; when it is defective, large branched DNA structures accumulate, and these cannot be packaged into mature viral particles (Kemper and Brown 1976; Kemper and Janz 1976). Because bacteriophages T4 and T7 have similar modes of replication, involving complex concatemeric intermediates, high recombination frequencies, and maturation of the DNA during packaging, it seems likely that a similar endonuclease...