The many bacterial transcription regulation proteins which bind DNA through a'helix-turn-helix' motif can be classified into subfamilies on the basis ofsequence similarities. One such family is the AsnC/Lrp subfamily [1]. The Lrp family of transcriptional regulators appears to be widely distributed among bacteria and archaea, as an important regulatory system of the amino acid metabolism and related processes [2].

Members of the Lrp family are small DNA-binding proteins with molecular masses of around 15 kDa. Target promoters often contain a number of binding sites that typically lack obvious inverted repeat elements, and to which binding is usually co-operative. LrpA from Pyrococcus furiosus is the first Lrp-like protein to date of which a three-dimensional structure has been solved. In the crystal structure LrpA forms an octamer consisting of four dimers. The structure revealed that the N-terminal part of the protein consists of a helix-turn-helix (HTH) domain, a fold generally involved in DNA binding.The C terminus of Lrp-like proteins has a beta-fold, where the two alpha-helices are located at one side of the four-stranded antiparallel beta-sheet. LrpA forms a homodimer mainly through interactions between the beta-strands of this C-terminal domain, and an octamer through further interactions between the second alpha-helix and fourth beta-strand of the motif. Hence, the C-terminal domain of Lrp-like proteins appears to be involved in ligand-response and activation [3].

This entry represents a conserved site that spans the complete helix-turn-helix motif and extends one residue downstream and one upstream of the HTH extremities.

1. The eubacterial transcriptional activator Lrp is present in the archaeon Pyrococcus furiosus. Trends Biochem. Sci. 20, 140-1
2. The Lrp family of transcriptional regulators. Mol. Microbiol. 48, 287-94

The many bacterial transcription regulation proteins which bind DNA through a'helix-turn-helix' m
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