<p> <taxon tax_id="10710">Bacteriophage lambda</taxon> C1 repressor controls the expression of viral genes as part of the lysogeny/lytic growth switch. C1 is essential for maintaining lysogeny, where the phage replicates non-disruptively along with the host. If the host cell is threatened, then lytic growth is induced. The Lambda C1 repressor consists of two domains connected by a linker: an N-terminal DNA-binding domain that also mediates interactions with RNA polymerase, and a C-terminal dimerisation domain [<cite idref="PUB00014015"/>]. The DNA-binding domain consists of four helices in a closed folded leaf motif. Several different phage repressors from different helix-turn-helix families contain DNA-binding domains that adopt a similar topology. These include the Lambda Cro repressor, <taxon tax_id="10712">Bacteriophage 434</taxon> C1 and Cro repressors, P22 C2 repressor, and <taxon tax_id="10677">Bacteriophage Mu</taxon> Ner protein.</p><p>The DNA-binding domain of <taxon tax_id="1423">Bacillus subtilis</taxon> spore inhibition repressor SinR is identical to that of phage repressors [<cite idref="PUB00014013"/>]. SinR represses sporulation, which only occurs in response to adverse conditions. This provides a possible evolutionary link between the two adaptive responses of bacterial sporulation and prophage induction.</p><p>Other DNA-binding domains also display similar structural folds to that of Lambda C1. These include bacterial regulators such as the purine repressor (PurR), the lactose repressor (Lacr) and the fructose repressor (FruR), each of which has an N-terminal DNA-binding domain that exhibits a fold similar to that of lambda C1, except that they lack the first helix [<cite idref="PUB00014016"/>, <cite idref="PUB00014017"/>, <cite idref="PUB00014018"/>]. POU-specific domains found in transcription factors such as in Oct-1, Pit-1 and Hepatocyte nuclear factor 1a (LFB1/HNF1) display four-helical fold DNA-binding domains similar to that of Lambda C1 [<cite idref="PUB00014019"/>, <cite idref="PUB00007265"/>, <cite idref="PUB00014020"/>]. The N-terminal domain of cyanase has an alpha-helix bundle motif similar to Lambda C1, but it probably does not bind DNA. Cyanase is an enzyme found in bacteria and plants that catalyses the reaction of cyanate with bicarbonate to produce ammonia and carbon dioxide in response to extracellular cyanate [<cite idref="PUB00008198"/>].</p>