Genes encoding transcription factor IIIA and the RNA polymerase common subunit RPB6 are divergently transcribed in...

The gene encoding Saccharomyces cerevisiae transcription factor TFIIIA has been found adjacent to RPB6, a gene that specifies a subunit shared by nuclear RNA polymerases. Analysis of DNA upstream of the RPB6 gene revealed an open reading frame that predicts a protein, designated PZF1, with nine C2H2 zinc fingers. The presence of nine C2H2 zinc fingers in PZF1 protein, a hallmark of amphibian TFIIIA proteins, suggested that PZF1 might be a TFIIIA homologue. We found that purified recombinant PZF1 specifically binds the internal control region (ICR) of the 5S rRNA gene in S. cerevisiae. The presence of nine C2H2 zinc fingers, the specific binding to ICR DNA, and the similarity of the predicted molecular mass of PZF1 with that determined for purified yeast TFIIIA, together indicate that PZF1 is TFIIIA. The yeast and amphibian TFIIIA proteins share only a limited number of residues outside of those normally conserved in C2H2 zinc fingers; these conserved residues may provide clues to the sequence specificity of these proteins. The PZF1 gene was found to be single copy, transcribed into a 1.5-kilobase mRNA, and essential for yeast cell viability. Interestingly, the yeast RPB6 and TFIIIA coding sequences are divergently transcribed and are separated by only 233 base pairs, providing the potential for coregulated expression of components of RNA polymerases and the 5S rRNA component of ribosomes.