Two modes of in vivo transcription for genes 43 and 45 of phage T4.

Sensitivities of the expression of early genes of phage T4 to UV light were determined at various stages of intracellular development of T4 wild type, a DNA-negative mutant (T4 DO), and T4 tsG1, (a mutant that exhibits delayed expression of some T4 early genes). Whereas the sensitivities of some genes in the T4 wild type and T4 DO remain constant, genes 43 and 45 exhibit greatly reduced sensitivities several minutes after the onset of phage development. Since UV sensitivities are a measure of the distance of a gene from its promotor, these observations indicate a switch from distal, "immediate early" promotors to proximal, "delayed early" promotors for genes 43 and 45. In the tsG1 mutant this decrease in UV sensitivities of genes 43 and 45 does not occur at 42 C, suggesting that at high temperature this mutant does not utilize the delayed early promotors.