The c4 gene of phage P1

The c4 gene of phage P1 has been localized to 335 bp of the P1EcoRI-9 fragment, within 50 bp of the EcoRI-9/14 junction. DNA sequence analysis of this fragment reveals a single open reading frame of 66 amino acids. The location of two c4 mutations, both of which produce changes in the predicted amino acid sequence in this reading frame, suggests that the reading frame codes for the c4 repressor. A region with high homology to the E. coli promoter consensus sequence is located approximately 50 bp upstream from the reading frame. Deletion of this potential promoter region abolishes expression of c4, as indicated by the loss of complementation of c4 mutants for lysogeny. Complementation is restored by the introduction of a heterologous promoter (the T7 phi 10 promoter), indicating that c4 expression is absolutely dependent on transcription of the 66-amino acid reading frame.