HTH_ASNC

helix_turn_helix ASNC type		 HTH_ASNC
SMART accession number:SM00344
Description: AsnC: an autogenously regulated activator of asparagine synthetase A transcription in Escherichia coli)
Interpro abstract (IPR019888):

The many bacterial transcription regulation proteins which bind DNA through a 'helix-turn-helix' motif can be classified into subfamilies on the basis of sequence similarities. One such family is the AsnC/Lrp subfamily [ (PUBMED:7770911) ]. 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 [ (PUBMED:12675791) ].

Members of the Lrp family are small DNA-binding proteins with molecular masses of around 15kDa. 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 [ (PUBMED:12675791) ].

This entry also includes some siroheme decarboxylases, which contain the AsnC-like ligand binding domain.

Family alignment:
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There are 72894 HTH_ASNC domains in 70987 proteins in SMART's nrdb database.

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