Roles of Parental and Progeny DNA in Two Mechanisms of Phage S13 Recombination

THERE are at least two mechanisms for genetic recombination of phage S13–primary and secondary. They are distinguished by the amount of phage recombination observed in recombination-deficient (recA) bacterial hosts. An S13 cross performed in either of the recA hosts, Escherichia coli JC1553¹ or AB2463², yields a recombination frequency that is greatly reduced from that found in a rec⁺ host^{3, 4}; this suggests that the recA gene is required for the primary mechanism of S13 recombination. But even in a recA host the phage undergoes a small residual amount of recombination³ which has been attributed to a minor, secondary mechanism. Apparently the secondary mechanism functions in a recA cell and is only revealed when the primary mechanism is eliminated.