A long-tailed and generalized transducing phage ES18 can recombine with a serologically unrelated phage . When ES18 phage stocks grown on lysogens were plated on lysogens resistant to ES18, plaques of a new hybrid phage P181 were found at a frequency of about 10. Serological analysis of phage P181 showed that it carries the protein coat of and has no serological cross-reactivity with ES18. Phage P181 expresses the clear plaque morphology similar to that of ES18. ES18 was also found to carry a homology with P22 in at least the regions. Thus, the markers (+, 1, 2 and 3) of P22 can be transferred to ES18. When these ES18 strains were used for isolation of P181 phage, the clear plaque morphology of P181 mimicked that of the P22 marker in the ES18 strain which gave rise to the P181 strain. strain Q1, lysogenic for P181, is immune to ES18 but not to . Furthermore, ES18 lysogens are immune to P181. Therefore it is concluded that the hybrid phage P181 conserves the protein coat of and carries at least the regions of ES18.

When ES18 was grown on lysogens of recombination-deficient () mutants, P181 hybrid particles were found in the ES18 stocks at a frequency of about 10. This frequency is about 1000-fold lower than that found in ES18 stocks grown on the wild type hosts lysogenic for . Therefore, it is concluded that the bacterial recombination mechanism plays an important role in the formation of P181 by recombination between the prophage and the superinfecting phage ES18.

Although phage ES18 belongs to B group phages, it possesses a generalized transducing capability. We have studied a possible origin of ES18.


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