The transcription factor CYS3 of Neurospora crassa is a positive regulator of the sulfur regulatory circuit which contains many structural genes involved in sulfur metabolism. Expression and degradation of the CYS3 protein are precisely regulated in a sulfur-dependent manner. cys-3 expression was found to be fully repressed by high concentrations of methionine or inorganic sulfate present in the culture medium and to be derepressed when these favored sulfur sources were limited. cys-3 transcripts could be readily detected within 2 h after derepression, whereas the CYS3 protein was not found until after 4 h. CYS3 is stable, with a half-life greater than 4 h under low-sulfur conditions when it is required for cell growth. However, it is degraded relatively quickly when methionine or inorganic sulfate becomes available. Upon sulfur repression, cys-3 transcripts disappeared within 30 min with an estimated half-life of 5 min whereas CYS3 protein almost entirely disappeared in 1 h with a half-life of approximately 10 min. These results suggest that a selective elimination of CYS3 is a highly regulated process. Site-directed mutagenesis showed that Lys-105 of CYS3 is important for its instability. The change of this single residue from lysine to glutamine resulted in a prolonged half life of CYS3 and impaired responsiveness of CYS3 degradation to sulfur level changes.