The U6 gene of Saccharomyces cerevisiae is transcribed by RNA polymerase C (III) in vivo and in vitro.

Unlike the majority of genes encoding small nuclear RNAs, which are transcribed by RNA polymerase B, the U6 gene contains features found in both class B and class C genes, indicating the involvement of a combination of transcription factors normally specific to each class of genes. We present direct genetic and biochemical evidence that the U6 gene of Saccharomyces cerevisiae is transcribed by RNA polymerase C in vivo as well as in vitro. A mutant strain with a temperature-sensitive defect in the large subunit of RNA polymerase C that results in defective transcription of tRNA and 5S RNA genes shows a corresponding defect in U6 RNA levels. Also, purified RNA polymerase C transcribes the U6 gene when supplemented with partially purified TFIIIB. The other class C transcription factors, TFIIIA and Tau (TFIIIC), are not required in this system.