Physicochemical properties and distinct DNA binding capacity of the repressor of temperate Staphylococcus aureus phage φ11

The repressor protein and cognate operator DNA of any temperate Staphylococcus aureus phage have not been investigated in depth, despite having the potential to enrich the molecular biology of the staphylococcal system. In the present study, using the extremely pure repressor of temperate Staphylococcus aureus phage phi 11 (CI), we demonstrate that CI is composed of alpha-helix and beta-sheet to a substantial extent at room temperature, possesses two domains, unfolds at temperatures above 39 degrees C and binds to two sites in the phi 11 cI-cro intergenic region with variable affinity. The above CI binding sites harbor two homologous 15 bp inverted repeats (O1 and O2), which are spaced 18 bp apart. Several guanine bases located in and around O1 and O2 demonstrate interaction with CI, indicating that these 15 bp sites are used as operators for repressor binding. CI interacted with O1 and O2 in a cooperative manner and was found to bind to operator DNA as a homodimer. Interestingly, CI did not show appreciable binding to another homologous 15 bp site (O3) that was located in the same primary immunity region as O1 and O2. Taken together, these results suggest that phi 11 CI and the phi 11 CI-operator complex resemble significantly those of the lambdoid phages at the structural level. The mode of action of phi 11 CI, however, may be distinct from that of the repressor proteins of lambda and related phages.