Clear plaque mutants of phage P7

Abstract

Clear plaque mutants of the P1-related phage P7 were isolated and classified by complementation tests with one another and with the P1 clear plaque mutants. In addition to several types of virulent mutants, a total of eight complementation groups of clear plaque mutants has been identified in these phages. A class of nonvirulent clear mutants unable to complement either P7c1 or P7c4 was also found. Mutants in this class have no genetic defect in the c4 region and are probably single mutants. Like c1 ts lysogens of P1, P7c 1 ts lysogens are thermally inducible, which supports the idea that gene c1 of both phages controls a repressor. P7c4 mutants cannot be complemented by any P1 clear mutants and P1c4 mutants cannot be complemented by any P7 clear mutants. This complementation behavior is expected if gene c4 codes for the immunity specific repressor in each of these heteroimmune phages. Unlike P1c4ts lysogens, P7c4ts lysogens are thermally induced to produce phage. This difference is discussed, but has not yet been explained.