🔬 Repressor

Discussion

<p>In phage λ every gene required for lytic growth is controlled by just three promoters (dubbed P_L,P_R, and P_R') and the lambda repressor blocks P_L and P_R. Binding of the lambda repressor consequently blocks induction of the lambda lytic cycle. The expression of a second protein, CII, is responsible for stimulating (or activating) production of the CI protein (from a promoter dubbed P_RE) during the lytic-lysogeny decision.

During lysogeny, production of the CI protein is dependent on yet another promoter, dubbed P_RM. The CI protein up-regulates rather than represses transcription from this promoter, resulting in positive-feedback increases in CI production. The CI protein, however, also Negative control of gene expression">negatively regulates CI production, resulting in a maintenance of lambda repressor densities within lambda lysogens at fairly constant levels (thereby maintaining the lysogenic state).

It is important to keep in mind that though many temperate phages resemble phage lambda (particularly the so-called lambdoid phages), the lambda regulatory mechanisms (etc.) nonetheless are not seen either Homology">homologously or Analogy">analogously in all temperate phages. For reviews providing discussion of the lambda CI repressor, see Little (2005), Ptashne (2004), etc.

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