Bacteriophage P22 Cro protein: sequence, purification, and properties.

The DNA sequence of part of the bacteriophage P22 early regulatory region, including genes cro and c1, was determined. The protein product of the cro gene consists of 61 amino acid residues, and that of c1, 92 amino acid residues. Both genes were placed separately in plasmids from which they are expressed from a controllable promoter in vivo. Induced cells bearing the cro-expressing plasmid were used as a source for purifying and characterizing the Cro protein. The amino-terminal sequence of this protein was found to be as predicted by the DNA sequence; close agreement was also observed between its predicted and experimentally determined amino acid composition and molar extinction coefficient at 280 nm. In gel filtration experiments, Cro protein at concentrations around 10(-5) M appears to have a molecular weight of 8600, which is more consistent with monomers (6800) than with dimers (13 600). Cro protein binds specifically to the three repressor binding sites in the P22 right operator; in order of decreasing affinity, these are OR3, OR1, and OR2.