More generally a protein that inhibits the initiation of transcription by interfering with RNA polymerase access to gene promoters, in phage biology the term is typically employed to describe these actions within the context of blocking prophage gene expression, thereby preventing induction.
A protein that is responsible for effecting lysogenization as well as maintaining temperate phage lysogenic cycles and which also is responsible for effecting immunity/superinfection immunity.
In phage lambda (λ), by far the most thoroughly studied of temperate phages, the repressor is the CI protein, also known as the CI repressor or lambda repressor.
In phage λ every gene required for lytic growth is controlled by just three promoters (dubbed PL,PR, and PR') and the lambda repressor blocks PL and PR. 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 PRE) during the lytic-lysogeny decision.
During lysogeny, production of the CI protein is dependent on yet another promoter, dubbed PRM. 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 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 homologously or analogously in all temperate phages.
For reviews providing discussion of the lambda CI repressor, see Little (2005), Ptashne (2004), etc.
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