Mechanism of Genetic Recombination

Genetic studies of the circular single-stranded DNA phages were initiated in the expectation that the smallness of these phages reflected a simplicity conducive to understanding the recombination process on the molecular level. Early extensive efforts just to find genetic recombination failed; the most serious work (Zahler 1958) showed no recombinants at the level of 10 −3 in several crosses of plaque-type mutants of S13. This inability to find genetic recombination was rationalized for several years by inferences from radiosensitivity studies (Tessman et al. 1957) which suggested that ϕ X174 and S13 behaved radiologically like RNA viruses, which are notably single-stranded in their nucleic acid and do not undergo genetic recombination. In the end, however, it turned out that recombination of S13, ϕ X, and G4 was easy to observe even without special selective techniques (Tessman and Tessman 1959). The original success was due to the stimulatory action of UV light, observed first for phage λ (Jacob and Wollman 1955); with UV light the recombination frequencies observed were of the order of 10 −3 , just within the range needed at that time for easy observation. However, UV stimulation is not essential for observing recombinants, and the mechanism of stimulation remains a major puzzle in molecular genetics. The early recombination experiments revealed a useful feature of this phage system: the probability of a mating event (the interaction of two genotypes leading to recombination) is around 0.01. This means that the recombinants emerging from a single cell are, with rare exceptions, the products of a single mating...