A suppressor of a HIS4 transcriptional defect encodes a protein with homology to the catalytic subunit of protein phosphatases.

Reversion analysis has identified four suppressor genes that permit transcription of the Saccharomyces cerevisiae HIS4 gene in the absence of GCN4, BAS1, and BAS2, trans-acting proteins normally required for activation of HIS4 transcription. These suppressor genes encode factors that affect the transcription of many diverse genes. Two of these suppressors, SIT1 and SIT2, are encoded by RPB1 and RPB2, the genes for the two largest subunits of RNA polymerase II. All strains containing suppressor mutations in RPB1 and RPB2 have reduced transcription of the INO1 gene and an inositol requirement. Mutations in SIT3 or high copy number SIT3 increase HIS4 transcription in the absence of GCN4, BAS1, and BAS2. This increase in HIS4 transcription by high copy number SIT3 or by sit3 alleles is largely independent of the HIS4 TATA sequence. The SIT4 protein is over 50% identical to the catalytic subunit of bovine type 2A protein phosphatase. sit4 mutations in combination with suppressor mutations in RPB1 or RPB2 (sit1, sit4 or sit2, sit4) are lethal, suggesting an interaction between SIT4 and RNA polymerase II.