Expression of phage transcription in P2 lysogens infected with helper-dependent coliphage P4

The DNA of helper-dependent coliphage P4 and the DNA of its helper-P2-show no detectable sequence homology as measured by DNA.DNA hybridization. The lack of cross-hybridization permits direct analysis of P4 as well as of P2 transcription in P4-infected P2 lysogens by RNA.DNA hybridization. P4-transactivated P2 transcription can be detected around 20 min after P4 infection of the P2 lysogen and the rate (per infected cell) of that transcription becomes equal to that of the P4 transcription at the end of the latent period of P4. Furthermore, P4 transcription appears to be stimulated by the presence of the helper. Conceivably, P2 codes for a stimulator of P4 transcription. Rifamycin has been used to investigate the role of the host RNA polymerase during P4 transactivation of P2 transcription. The results exclude the participation of a P4-coded RNA polymerase and indicate that the original host RNA polymerase is responsible for the bulk of P4 and P2 transcription during transactivation.