Bacteriophage T4 late transcription is unusual, among prokaryotes, in its complexity. Late transcription requires the host RNA polymerase, the products of T4 genes 33, 45 and 55, and other small polypeptides, the genes of which have not been identified1. In addition the DNA template must be 'competent' for late transcription. First the DNA must contain the substituted base 5-hydroxymethyl cytosine in place of cytosine (this requirement is eliminated by a mutation in the T4 alc gene)1,2. Second, the DNA must be replicating, although late transcription can be uncoupled from DNA replication by mutations in the T4 genes coding for DNA ligase (gene 30) and DNA exonuclease (gene 46)1,3. We report here the location of the initiation sites of the messenger RNAs (mRNAs) synthesized in vivo from four late genes (genes 21, 22, 23 and 36) by S1 nuclease mapping and we have determined the DNA sequences at these sites. We have found strong homology to the sequence TATAAATAC-TATT immediately upstream from the 5′ ends of the late messages and we suggest that this sequence is specifically recognized by the complex responsible for late transcription. Also, we have examined gene 23 mRNA synthesized in the absence of DNA replication using the 30- 46- mutant described above and find that it is identical to the true late transcript synthesized in normal infections.
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