The cro/C1-type HTH domain is a DNA-binding, helix-turn-helix (HTH) domain ofabout 50-60 residues present in transcriptional regulators. The domain isnamed after the transcriptional repressors cro and C1 of temperatebacteriophages 434 and lambda, respectively. Besides in bacteriophages,cro/C1-type regulators are present in prokaryotes and in eukaryotes. Thehelix-turn-helix DNA-binding motif is generally located in the N-terminal partof these transcriptional regulators. The C-terminal part may contain anoligomerization domain, e.g. C1 repressors and CopR act as dimers, while SinRis a tetramer. The cro/C1-type HTH domain also occurs in combination with theTPR repeat and the C-terminal part of C-5 cytosine-specific DNA methylasescontains regions related to the enzymatic function.

Several structures of cro/C1-type transcriptional repressors have beenresolved and their DNA-binding domain encompasses five alpha helices, of whichthe extremities are less conserved [1]. The helix-turn-helixmotif comprises the second and third helices, the third being called therecognition helix. The HTH is involved in DNA-binding into the major groove,where the recognition helix makes most DNA-contacts. The bacteriophagerepressors regulate lysogeny/lytic growth by binding with differentialaffinity to the operators. These operators show 2-fold symmetry and therepressors bind as dimers. Binding of the repressor to the operator positionsthe DNA backbone into a slightly bent twist [2, 2].

1. Recognition of a DNA operator by the repressor of phage 434: a view at high resolution. Science 242, 899-907
2. CopR binds and bends its target DNA: a footprinting and fluorescence resonance energy transfer study. Nucleic Acids Res. 30, 2052-60

The cro/C1-type HTH domain is a DNA-binding, helix-turn-helix (HTH) domain ofabout 50-60 residues
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