The temperate Staphylococcus aureus phage

harbors cI and cro repressor genes similar to those of lambdoid phages. Using extremely pure

Cro (the product of the

cro gene) we demonstrated that this protein possesses a single domain structure, forms dimers in solution at micromolar concentrations and maintains a largely

-helical structure even at

.

Cro was sensitive to thermolysin at temperatures ranging from

and began to aggregate at

, suggesting that the protein is moderately thermostable. Of the three homologous 15-bp operators (O1, O2, and O3) in the

cI-cro intergenic region,

Cro only binds efficiently to O3, which is located upstream of the cI gene. Our comparative analyses indicate that the DNA binding capacity, secondary structure and dimerization efficiency of thermostable

Cro are distinct from those of P22 Cro and

Cro, the best characterized representatives of the two structurally different Cro families.