FIG. 6.

Amino acid sequences of the CI repressor protein. Eighty-six amino acids from TP901-1 CI (residues 49 to 133) were searched for sequence identity to other repressor proteins. Phages encoding proteins with sequence similarity to TP901-1 CI and the corresponding host bacteria are shown to the left of the sequences. Bacteriophages that encode MOR homologous are marked with an asterisk. CI encoded by phages ul36, φ31, and 4268 are 98% identical, whereas CI proteins encoded by phages 80α and φPVL180 are 94% identical. Amino acids shown in gray are identical to amino acids in CI of phage TP901-1. Numbers to the right of the sequences indicate the amino acid numbers in the CI proteins. ▿, positions where linker insertion in CI allows repression of P_L or P_R; ▾, positions where linker insertion abolishes repression of P_L or P_R; hatched triangle, positions where linker insertion in CI in the presence of MOR allows the choice between the two different states. Two conserved regions are marked with black lines. The gray box below the sequences at the left represents a region conserved only in phages encoding MOR-homologous proteins. Insertion of five amino acids at positions 78 and 89 in this region abolished repression of P_L in the presence of MOR, suggesting that this region is important for CI:MOR interactions. The second and the fourth gray boxes represent regions where insertion of five amino acids produced CI mutant proteins that can no longer be derepressed following activation of the host SOS response, suggesting that interaction with RecA may be abolished or that the multimeric form of the protein may be changed. The third gray box represents a conserved region in phage infection in Lactococcus. Insertion of five amino acids in this region abolishes repression of P_R, suggesting that the cooperative binding of CI at O_L-O_R is reduced and hence this region is involved in oligomerization of CI.