Ambivalent rII mutants of phage T4

Abstract

The rII mutants of phage T4 differ from the standard type in being inactive on strains of Escherichia coli lysogenic for phage X. While the mutants attach to the cells and inject their DNA, progeny fail to appear. In this paper it is shown that one or another subset of the rII mutants can become active as a result of modification of the host, either by mutation or by the action of 5-fluorouracil. The effects are characterized by extreme specificity, applying only to certain rII mutations at certain points within the two rII cistrons. The phenomenon resembles what has been called, in many other systems, "allele-specific suppression [1]," in which a mutation outside a genetic region controlling a particular function suppresses the effects of some of the mutations within the region. An advantage of the phage system is that a given genetic message, in the form of a phage mutant, can be readily inserted into different hosts. The stimulus for the present investigation derives from the possibility that modifications of the host affecting the expression of specific phage mutations might be a key to the detection of alterations in the components of the cellular system for translating genetic information.

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