Specificity of partial exclusion of bacteriophage T2 in crosses with T4.

An attempt was made to isolate a bacteriophage T4 mutant generally disturbed in its ability to exclude the localized exclusion sensitivity determinants of T2 from the progeny of crosses. The method used was a modification of that used previously, which revealed T4 mutants unable to exclude more than one of the T2 sensitive sites specifically. One out of 1000 isolates from a mutagenized T4 am56:am32 stock was found to be mutated in its exclusion properties. However, the mutant was not generally exclusion deficient. Its properties were similar to those of a double mutant T4 ex 56:ex32, specifically unable to exclude the T2 sensitive sites near gene 56 and gene 32. The failure to isolate a generally excluding mutant at a frequency down to 10(-3) renders the occurrence of a general exclusion locus unlikely.