Interactions between DNA-bound repressors govern regulation by the λ phage repressor

Abstract

The λ phage repressor binds cooperatively to the three sites in the right operator ( O R ) according to the following pattern. If the DNA is wild type, O R 1 and O R 2 are filled coordinately because of interactions between repressor dimers bound to these two sites. Site O R 3 is filled only at higher repressor concentrations. In contrast, if O R 1 is mutant, O R 2 and O R 3 are filled coordinately because of interactions between repressors bound to these sites. In this case, the affinity of O R 3 is increased and that of O R 2 is decreased relative to the wild type. We infer that a repressor dimer bound to the middle site O R 2 can interact either with another repressor dimer bound to O R 1 (wild-type case) or, alternatively, with one bound to O R 3 (mutant O R 1 case). We argue that these repressor interactions are mediated by protein-protein contacts between adjacent repressor dimers, because the isolated amino-terminal domains of repressor bind to the operator sites noncooperatively. The cro protein of phage λ, a second regulatory protein, which recognizes the same three sites in O R as does repressor, binds non-cooperatively. Experiments performed in vivo show that regulation of gene expression by repressor can be influenced critically by cooperative interactions. We demonstrate that the effect of repressor in a lysogen on the activity of the promoter P RM can be changed from activation to repression by deletion of O R 1. We explain this effect in terms of the alternative cooperative interactions described above.