Partial exclusion between T-even bacteriophages: an incipient genetic isolation mechanism.

Conditional lethal mutant systems developed in T-even bacteriophages T2, T4 and T6 have been used to study the partial exclusion which characterizes mixed infections of these phages. In bacteria mixedly infected with T2 and T4, the dominant phage (T4) acts against localized exclusion sensitivity determinants in the genome of the excluded phage (T2). These determinants are clustered near genes controlling early functions; the determinants themselves do not appear among the progeny, but markers located close to them appear infrequently, by recombination. The excluding action of T4 does not depend on the action of any gene so far identified by conditional lethal mutations, nor does it depend on differences in DNA glucosylation between infecting phages. Regardless of mechanism, the genetic consequence of this partial exclusion is to limit genetic exchange between T2 and T4 in the region of the genome controlling early functions, while retaining the capacity for extensive exchange in other regions; in short, partial exclusion constitutes a localized genetic isolating mechanism. Related forms of partial exclusion characterize mixed infections of other T-even phages, including those of some phages newly isolated from nature.