Structure of the Aspergillus nidulans qut repressor-encoding gene: implications for the regulation of transcription initiation.

The nucleotide (nt) sequence of the qutR gene has been determined and shown to encode an inferred protein (QUTR) of 929 amino acids (aa). The inferred aa sequence shows a high level of similarity throughout its length with the aa sequence of the three C-terminal domains (shikimate kinase; 3-dehydroquinase; shikimate dehydrogenase) of the pentafunctional AROM protein of Aspergillus nidulans that catalyses steps 2-6 in the shikimate pathway. The inferred QUTR aa sequence has a completely conserved aa sequence motif, Gly, Xaa4, Gly, Lys, Ser, that is found in proteins that bind purine nt, suggesting that the inferred protein may have an in vivo kinase activity. The inferred QUTR protein also has a peptide sequence, DMVRLTQPAT, related to the active-site peptide in type-I 3-dehydroquinases. In active 3-dehydroquinases, the Arg (of QUTR) is replaced by Lys, which is involved in Schiff base formation as part of the reaction mechanism. The change from Lys----Arg in the inferred QUTR protein may allow the protein to bind but not metabolise the substrate for 3-dehydroquinase enzymes, namely 3-dehydroquinate. These observations are entirely consistent with the genetical model for how the QUTR protein functions, as it predicts that the protein can recognise and bind, but not metabolise, quinate, 3-dehydroquinate, and dehydroshikimate.