Helix-turn-helix XRE-family like proteins
SMART accession number:SM00530
Description: -
Interpro abstract (IPR001387):

The cro/C1-type HTH domain is a DNA-binding, helix-turn-helix (HTH) domain of about 50-60 residues present in transcriptional regulators. The domain is named after the transcriptional repressors cro and C1 of temperate bacteriophages 434 and lambda, respectively. Besides in bacteriophages, cro/C1-type regulators are present in prokaryotes and in eukaryotes. The helix-turn-helix DNA-binding motif is generally located in the N-terminal part of these transcriptional regulators. The C-terminal part may contain an oligomerization domain, e.g. C1 repressors and CopR act as dimers, while SinR is a tetramer. The cro/C1-type HTH domain also occurs in combination with the TPR repeat and the C-terminal part of C-5 cytosine-specific DNA methylases contains regions related to the enzymatic function.

Several structures of cro/C1-type transcriptional repressors have been resolved and their DNA-binding domain encompasses five alpha helices, of which the extremities are less conserved [ (PUBMED:3187531) ]. The helix-turn-helix motif comprises the second and third helices, the third being called the recognition helix. The HTH is involved in DNA-binding into the major groove, where the recognition helix makes most DNA-contacts. The bacteriophage repressors regulate lysogeny/lytic growth by binding with differential affinity to the operators. These operators show 2-fold symmetry and the repressors bind as dimers. Binding of the repressor to the operator positions the DNA backbone into a slightly bent twist [ (PUBMED:3187531) (PUBMED:11972345) ].

Family alignment:
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There are 375913 HTH_XRE domains in 370972 proteins in SMART's nrdb database.

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