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].

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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