Recombination in Bacteriophage Lambda II. The Mechanism of General Recombination

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It has been shown that recombination in λ is accompanied by material exchange of DNA (Meselson and Weigle, 1961; Kellenberger, Zichichi, and Weigle, 1961; Meselson, 1964). In the preceding paper (Kellenberger and Weisberg, this volume) we have demonstrated that, in fact, materialexchange takes place in each of the three separate recombination systems that can be used by λ: the site-specific int system and the general recombination systems red (specified by λ ) and rec (the bacterial recombination system). The mechanism of this exchange and the possible differences existing among the three recombination systems still remain to be elucidated. Meselson (1964) has demonstrated that material exchange in recombination can occur by break and reunion of pre-existing DNA strands. He crossed in light medium two phages labeled with heavy isotopes. Among the recombinants formed in the interval h–c , two markers roughly equidistant from the ends of the linear chromosome (Fig. 1), he found some fully heavy phages. It is now known, however, that about half of the recombinants in the interval h–c are due to the int system (Echols, Gingery, and Moore, 1968; Weil and Signer, 1968). The heavy recombinants in Meselson’s experiment could therefore be due to the int system alone or to either one of the two general recombination systems red and rec, or to any combination of the three. In their first study of material exchange, Meselson and Weigle (1961) studied recombination between the markers c and mi, an interval in which only general recombination is effective. They...