PCF4 encodes an RNA polymerase III transcription factor with homology to TFIIB

A dominant mutation in the PCF4 gene of S. cerevisiae was isolated as a suppressor of a tRNA gene A block promoter mutation. In vitro studies indicate that PCF4 is a stoichiometrically-required RNA polymerase III (pol III) transcription initiation factor. We show that the PCF4-1 mutation increases the number of transcriptionally competent preinitiation complexes by affecting a limiting activity in yeast cell extracts that is squelched by excess TFIIIC. The PCF4 gene encodes a TFIIB homolog whose size, biochemical, and genetic properties are consistent with those of the 70 kd subunit of TFIIIB. The TFIIB homology of PCF4 suggests a means for determining the polymerase specificity of a gene.