unYc462 is a gain-of-function mutation in the purine catabolism positive regulatory gene of Aspergillus nidulans. This allele leads to a constitutive, hyperinducible and derepressed expression of a least three genes controlled by uaY, and this occurs at different levels depending on the target gene. The uaYc462 allele was mapped physically in relation to known loss-of-function alleles and sequenced. uaYc462 is a one-base change in codon 222, resulting in a serine to leucine change. We propose that this mutation maps in a functional domain involved, directly or indirectly, in the interaction of UaY with other components of the transcriptional apparatus. A sequence similar to the motif surrounding serine 222 may play similar roles in the PPR1 and ADR1 proteins of Saccharomyces cerevisiae.