RECOGNITION OF NONCONSERVED BASES IN THE P22 OPERATOR BY P22 REPRESSOR REQUIRES SPECIFIC INTERACTIONS BETWEEN REPRESSOR AND CONSERVED BASES

The ability of P22 repressor protein to distinguish between the six na turally occurring operator binding sites is critically important in de termining whether the bacteriophage chooses to grow lytically or lysog enically. We have shown that changes in the highly conserved bases at P22 operator positions 3, 5, 6, and 7 prevent specific binding of P22 repressor, Moreover, studies of mutant proteins identified the three r epressor amino acids that directly contact these conserved bases. The pattern of operator sequence conservation permits these direct amino a cid-base pair interactions to occur in all except one of the 12 operat or half-sites in the phage chromosome, Therefore, repressor differenti al affinity for these sites cannot be due to these highly conserved ba se pair-amino acid interactions, Our binding studies show that the non conserved bases at positions 2 and 4 also play an important role in de termining the relative affinity of the naturally occurring P22 operato rs for P22 repressor, Our data indicate that the direct contacts betwe en the three solvent-exposed amino acids and the conserved bases in th e binding site lock these amino acids in place, forming a scaffold all owing the rest of the amino acids side chains to form weaker interacti ons with the nonconserved bases in the binding site.