Moderately thermostable phage Phi 11 Cro repressor has novel DNA-binding capacity and physicochemical properties

Abstract

The temperate Staphylococcus aureus phage Phi 11 harbors cl and cro repressor genes similar to those of lambdoid phages. Using extremely pure Phi 11 Cro (the product of the Phi 11 cro gene) we demonstrated that this protein possesses a single domain structure, forms dimers in solution at micromolar concentrations and maintains a largely alpha-helical structure even at 450 degrees C. Phi 11 Cro was sensitive to thermolysin at temperatures ranging from 55-75 degrees C and began to aggregate at similar to 630 degrees C, suggesting that the protein is moderately thermostable. Of the three homologous 15-bp operators (O1, O2, and O3) in the Phi 11 cl-cro intergenic region, Phi 11 Cro only binds efficiently to O3, which is located upstream of the cl gene. Our comparative analyses indicate that the DNA binding capacity, secondary structure and dimerization efficiency of thermostable Phi 11 Cro are distinct from those of P22 Cro and lambda Cro, the best characterized representatives of the two structurally different Cro families.