Regulation of transcription of the late genes of bacteriophage T7.

The transcription program of bacteriophage T7 in vivo was analyzed by hybridizing T7 mRNAs, labeled at intervals after infection, to Hpa I restriction fragments of T7 DNA. Transcripcion of the late genes is temporally regulated: class II genes are transcribed between 4 and 16 min after infection; most class III genes are transcribed from 8 min after infection until lysis. Genes 8--10 are transcribed as both class II and class III genes. The rate of T7 RNA synthesis decreases sharply at 10 min after infection. The rapid decrease in the rate of T7 RNA synthesis and the shutoff of class II RNA synthesis were not observed in cells infected with phage defective in gene 3.5 (lysozyme). Although the decrease in the rate of T7 RNA synthesis is independent of DNA replication, the failure to shut off class II RNA synthesis normally in 3.5-- -infected cells may reflect the role of T7 lysozyme in DNA replication. In vitro, the regions of T7 DNA transcribed by the phage RNA polymerase were found to be dependent upon ionic conditions.