A SINGLE-BASE-PAIR MUTATION CHANGES THE SPECIFICITIES OF BOTH A TRANSCRIPTION ACTIVATION PROTEIN AND ITS BINDING-SITE

The C1 protein of bacteriophage P22 binds to a unique site in the -35 region of the P(RE) promoter and activates transcription of the phage c2 repressor gene. This -35 target has an approximate direct repeat that overlaps the 5' end of the c1 coding region. We have isolated a single-base-pair mutation in this region that changes the P(RE) -35 target as well as the amino-terminal region of the C1 protein. Although the mutant C1 protein activates the mutant P(RE) promoter, it fails to activate the wild-type P(RE) promoter. This suggests that a single-base-pair mutation changes the specificities of both a protein and its target site. These studies also indicate that C1 binding to DNA is influenced by contacts made through residues near the amino terminus.