The HAP2,3,4 transcriptional activator is required for derepression of the yeast citrate synthase gene, CIT1.

The yeast nuclear gene CIT1 encodes mitochondrial citrate synthase, which catalyses the first and rate-limiting step of the tricarboxylic acid (TCA) cycle. Transcription of CIT1 is subject to glucose repression. Mutations in HAP2, HAP3 or HAP4 block derepression of a CIT1-lacZ gene fusion. The HAP2,3,4 transcriptional activator also activates nuclear genes encoding components of the mitochondrial electron transport chain, and thus it co-ordinates derepression of two major mitochondrial functions. Two DNA sequences resembling the consensus HAP2,3,4-binding site (ACCAATNA) are located at approximately -310 and -290, upstream of the CIT1 coding sequence. Deletion and mutation analysis indicates that the -290 element is critical for activation by HAP2,3,4. Glucose-repressed expression of CIT1 is largely independent of HAP2,3,4, is repressed by glutamate, and requires a DNA sequence between -367 and -348. Evidence is presented for a second HAP2,3,4-independent activation element located just upstream and overlapping the -290 HAP2,3,4 element.