repressor

I. Introduction

The repressor of temperate bacteriophages (e.g. lambda, 434, P22), works in opposition to the phage's cro protein to control the genetic switch that determines whether a lytic or lysogenic cycle will follow infection. When phage DNA enters a new bacterial host cell, the beginning of both a lytic and lysogenic infection is initiated by the expression of the immediate early and delayed early genes of the phage. A competition between the cro and repressor proteins ensues, the outcome of which determines whether the phage embarks on a lytic or lysogenic lifecycle.

Repressor and cro compete for control of an operator region containing three operators that determine the state of the lytic/lysogenic genetic switch. If this competition is won by repressor, transcription of the cro gene is blocked and repressor transcription is maintained. Lysogeny will result. A competition won by cro, however, means that the late genes of phage l will be expressed; this will result in lysis. In this case, cro blocks transcription that occurs from P_RM, the promoter that is responsible for the maintenance of repressor transcription. If you are unfamiliar with the mechanisms of the lambda genetic switch, you may wish to study Figure 1 (Genetic Switch Overview) and Figure 2a,b (Operator DNA Sequences) before moving on to structural stuidies of repressor. Reload molecule for the next sections.

The structure at left is a dimer of the amino terminal domains of phage 434 repressor complexed with operator DNA.

The nucleotide sequences of operator DNA to which repressor protein binds consists of fourteen base pairs .

The outer four base pairs at the ends of the operator, ACAA, serve as key recognition sequences for repressor binding.

Repressor binds to operator DNA as dimers. Each monomer binds to an operator half site. The amino-terminal domain of repressor (shown at left) is responsible for DNA binding and the carboxy-terminal domain (not shown) is primarily responsible for dimerization of the repressor monomers.

Helices two and three of each monomer form a helix-turn-helix motif found in a variety of DNA binding proteins, including some eukaryotic transcription factors. This motif comprises ~20 residues. Helix three of the repressor helix-turn-helix motif inserts into the major groove of operator DNA.

Helix three of the repressor is primarily responsible for binding to the ACAA recognition sequence in the major groove. Glutamine 28 forms hydrogen bonds with adenine one of this sequence.

Hydrogen bonds also link glutamine 29 of the repressor with guanine 2, as well as glutamine 33 with thymine 4.

Packing of glutamine 29 and threonine 27 forms a hydrophobic pocket that accepts the methyl group from thymine three.

The interactions just described are largely responsible for allowing the repressor protein to recognize operator DNA, with each monomer recognizing the ACAA sequence of an operator half site.

Aggarwal, A. K., Rodgers, D. W., Drottar, M., Ptashne, M., Harrison, S. C.: Recognition of a DNA operator by the repressor of phage 434: a view at high resolution. Science 242: 899-907 (1988).

Shimon, L. J., Harrison, S. C.: The phage 434 OR2/R1-69 complex at 2.5 A resolution. J Mol Biol 232: 826-838 (1993).