A single amino acid, outside the AlcR zinc binuclear cluster, is involved in DNA binding and in transcriptional regulation of the alc genes in Aspergillus nidulans

In Aspergillus nidulans, the transcriptional activator AlcR mediates specif ic induction of a number of ale genes. The AlcR DNA-binding domain is a zin c binuclear duster that differs from the other members of the Zn(2)Cys(6) f amily in several respects, Of these, the most remarkable is its ability to bind in vitro as a monomer to single sites, whereas only repeated sites (di rect or inverted) are necessary and functional in vivo. Deletion of the fir st five amino acids (following the N-terminal methionine) upstream of the A lcR zinc cluster or mutation of a single residue, Arg-6, impairs the AlcR i n vitro binding mainly to symmetrical sites. in vivo, the same mutations re sult in the inability of A. nidulans to grow on ethanol, The alc(-) phenoty pe results from a drastic decrease in activation of its own transcription a nd, in addition, that of the two structural genes, alcA and aldA, required for ethanol oxidation, This defect seems to be correlated to the inability of the Arg-6 AlcR mutant protein to bind to AlcR palindrome targets, which are essential in the three ale promoters. AlcR shows a unique pattern of bi nding and of transactivation among the Zn(2)Cys(6) family.