ACT_TyrR: N-terminal ACT domain of the TyrR protein. The TyrR protein of Escherichia coli controls the expression of a group of transcription units (TyrR regulon) whose gene products are involved in the biosynthesis or transport of the aromatic amino acids. Binding to specific DNA sequences known as TyrR boxes, the TyrR protein can either activate or repress transcription at different sigma70 promoters. Its regulatory activity occurs in response to intracellular levels of tyrosine, phenylalanine and tryptophan. The TyrR protein consists of an N-terminal region important for transcription activation with an ATP-independent aromatic amino acid binding site (contained within the ACT domain) and is involved in dimerization; a central region with an ATP binding site, an ATP-dependent aromatic amino acid binding site and is involved in hexamerization; and a helix turn helix DNA binding C-terminal region. In solution, in the absence of cofactors or in the presence of phenylalanine alone, the TyrR protein exists as a dimer. However, in the presence of ATP and tyrosine the TyrR protein self-aggregates to form a hexamer. Members of this CD belong to the superfamily of ACT regulatory domains.