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

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Abstract
The .lambda. phage repressor binds cooperatively to the 3 sites in the right operator (OR) according to the following pattern. If the DNA is wild type, OR1 and OR2 are filled coordinately because of interactions between repressor dimers bound to these 2 sites. Site OR3 is filled only at higher repressor concentrations. In contrast, if OR1 is mutant, OR2 and OR3 are filled coordinately because of interactions between repressors bound to these sites. In this case, the affinity of OR3 is increased and that of OR2 is decreased relative to the wild type. It is inferred that a repressor dimer bound to the middle site OR2 can interact either with another repressor dimer bound to OR1 (wild-type case) or, alternatively, with one bound to OR3 (mutant OR1 case). These repressor interactions may be 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 .lambda., a 2nd regulatory protein, which recognizes the same 3 sites in OR as does repressor, binds noncooperatively. Experiments performed in vivo show that regulation of gene expression by repressor can be influenced critically by cooperative interactions. The effect of repressor in a lysogen on the activity of the promoter PRM can be changed from activation to repression by deletion of OR1. This effect is explained in terms of the alternative cooperative interactions described above.