Phage repressor

Anderson, J.E., Ptashne, M., Harrison, S.C. A phage repressor-operator complex at 7 A resolution. Nature 316 S96-601, 1985. 

Examination of DNA-regulatory protein complexes have permitted reasonable guesses to be made about the precise nature of the contacts between amino acid side chains and DNA in many cases. Figure 30.27 illustrates three examples One for the 434 phage repressor (fig. 30.27a), one for the A cl repressor (fig. 30.27b), and one for the trp repressor (fig. 30.27c). In all cases only half-sites are depicted because symmetry considerations dictate that the two halfsites should have virtually identical structures. 

In the case of the two phage repressors, several contacts are observed between individual amino acid side chains and individual base pairs. In addition, one or two hydrogen bonds are observed to nearby phosphate groups. In both of these cases the side chain that is contacting a base participates in the hydrogen bond to a phosphate group, either directly or indirectly. 

A common but not universal feature of regulatory proteins is their sensitivity to small-molecule effectors. The lamb-doid phage repressors are examples of molecules that almost always function as repressors their action is controlled merely by their concentration. When they are needed, they are synthesized when they are no longer needed or their presence is undesirable, they are selectively removed by degradation. 

Specific interactions between three different repressors and their operator binding sites. Only half the operator binding site is shown because identical contacts are made with the other half. The numbers associated with the amino acid side chains refer to the distance of amino acids from the amino-terminal end of the protein. Nucleotides are numbered from the central dyad at the operator, (a) The 434 phage repressor (b) The A repressor, (c) The trp repressor. (Source Adapted from T. Steitz, Q. Rev. Biophys. 23 236, 1990.)... 

Ptashne, M. 1967a. Specific binding of the X phage repressor to X DNA. Nature (London), 214 232-234. 

Ackers, G.K., A.D. Johnson, and M.A. Shea. 1982. Quantitative model for gene regulation by I phage repressor. Proceedings of the National Academy of Sciences 79(4) 1129-1133. 

Bass and co-workers (1967) showed, however, that during the destruction of bacterial cells infected with phage T2, derepression of synthesis of bacterial DNA takes place. These workers consider that this process is evidence against the existence of specific phage repressors and they postulate the possibility of inhibition of synthesis of host DNA in the infected cells purely because of topographic separation of host DNA from DNA-polymerase. 

Steinberg, R. A., Ptashne, M. In vitro repression of RNA synthesis by purified A phage repressor. Nature (Lond.) New Biol. 230, 76-80 (1971). 

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