The transcriptional activators BAS1, BAS2, and ABF1 bind positive regulatory sites as the critical elements for adenine regulation of ADE5,7

Adenine repression of the purine nucleotide biosyn thetic genes in Saccharomyces cerevisiae involves downregulation of the activator protein BAS1 or BASS by an unknown mechanism, To determine the minimal cis-acting requirements for adenine regulation, hybrid promoter constructs were made between ADE5,7 promoter fragments and a CYC1-lacZ reporter, A 139-nucleotide fragment containing two BAS1 binding sites was sufficient to confer adenine regulation on the CYC1-lacZ reporter, Analysis of deletion and substitution mutations led to the conclusion that the proximal BAS1 binding site is both necessary and sufficient for regulation, whereas the distal site augments the function of the proximal site, By performing saturation mutagenesis, we found two essential regions that flank the proximal site, An ABF1 consensus sequence is within one of these regions, and mutations that impaired in vitro ABF1 binding impaired promoter activity in vivo. A second region is AT-rich and appears to bind BASS, No substitution mutations led to high level constitutive promoter activity as would be expected from removal of an upstream repression sequence. Our results indicate that ABF1, BAS1, and BASS are required for ADE5,7 pro meter function and that adenine repression most likely involves activator modification or a negative regulator that does not itself bind DNA.