In crosses between bacteriophages T2 and T4 most early genes of T2 are partially excluded from the progeny. Six genes of T4 affect the exclusion of six corresponding exclusion-sensitive sites in T2, each gene being specific for the exclusion of one site. Mutants of T4 in these genes have been isolated ( mutants). The induction of the gene product (deoxycytidine triphosphate nucleotidohydrolase, dCTPase) of the strongly excluded T2 gene 56 was determined. The dCTPase was induced in the presence of the replication inhibitor oxolinic acid to prevent possible artefacts from unequal replication rates of T2 and T4.

The rate of T2 dCTPase induction was 30% of the control in T2 × T4 infections in which all six exclusion-sensitive sites were excluded and was 57% in infections in which the sites except 56 were excluded. The dCTPase induction was 67% of the control with exclusion of the 56 site only and equalled the control when no T2 site was excluded. Inhibition of dCTPase induction under conditions of exclusion is partly due to exclusion of neighbouring sites and partly due to exclusion of the 56 site. The simplest explanation for the last type of inhibition is exclusion-related transcription inhibition of gene 56.


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