The PCF1-1 mutation increases the activity of the transcription factor (TF) IIIB fraction from Saccharomyces cerevisae

PCF1-1 is a dominant suppressor of a tRNA gene A block promoter mutation (A19) In Saccharomyces cerevisiae. Transcriptlonal activation by PCF1-1 was examined in vitro using whole-cell extracts and purified factors derived from mutant and wild-type strains. These experiments show that PCF1 Is a general activator of RNA polymerase III (pol III) gene transcription. The transcription of all pol III genes analyzed to date, including type I and numerous type II genes, is increased 3 - 7 fold In mutant cell extracts. Single round transcription assays indicate that the PCF1-1 mutation increases the number of functional preinltiation complexes and suggest that this is achieved by increasing the intrinsic activity of the encoded product rather than its amount. Point mutations throughout the A block of the sup3-e gene and numerous B block mutations fall to abolish transcrlptional activation suggesting that interactions between TFIIIC and the internal promoter are unaffected by PCF1-1. Moreover, TFIIIC purified from the mutant strain Is incapable of conferring PCF1-1 transcrlptional activity to a reaction in which the remaining components are wild-type. In contrast, the activity of the TFIIIB fraction is increased in PCF1-1extracts and can reconstitute mutant levels of transcription when added to wild-type TFIIIC and polymerase. We conclude that PCF1 is a component or regulator of TFIIIB.