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. 1979 Oct;76(10):5061–5065. doi: 10.1073/pnas.76.10.5061

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

Alexander D Johnson 1, Barbara J Meyer 1, Mark Ptashne 1
PMCID: PMC413079  PMID: 159452

Abstract

The λ phage repressor binds cooperatively to the three 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 two 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. We infer 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). 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 OR 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 PRM can be changed from activation to repression by deletion of OR1. We explain this effect in terms of the alternative cooperative interactions described above.

Keywords: pairwise cooperativity, cro protein, DNA binding, amino- and carboxyl-terminal domains, operators and promoters

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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