In the preceding paper we report that satellite phage P4 can derepress P2 lysogens (Six and Lindqvist, 1978). We now show that derepression leads to the expression of the early as well as the late gene regions of the P2 prophage. In the absence of transactivation, P2 prophage late genes will be expressed only if the early P2 gene A function is available. This was demonstrated with P4 mutants defective in gene α. Such mutants are proficient in derepression but were found to be transactivation deficient, hence restricted to derepression as a means of turning on P2 prophage late genes. Thus two modes of P4-induced late gene expression can be distinguished: transactivation and derepression. Derepression precedes transactivation by about 30 min, but the extra time needed for transactivation appears not to be due to a requirement for early P2 gene products. © 1978.
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