RNA polymerase II subunit Rpb9 is important for transcriptional fidelity in vivo

The fidelity of yeast RNA polymerase 11 (Pol 11) was assessed in vivo with an assay in which errors in transcription of can1-100, a nonsense allele of CAN1, result in enhanced sensitivity to the toxic arginine analog canavanine. The Pol 11 accessory factor THIS has been proposed to play a role in transcript editing by stimulating the intrinsic nuclease activity of the RNA polymerase. However, deletion of DST1, the gene encoding the yeast homolog of THIS, had only a small effect on transcriptional fidelity, as determined by this assay. In contrast, strains containing a deletion of RPB9, which encodes a small core subunit of Pol 11, were found to engage in error-prone transcription. rpb9 Delta strains also had increased steady-state levels of can1-100 mRNA, consistent with transcriptional errors that decrease the normal sensitivity of the can1-100 transcript to nonsense-mediated decay, a pathway that degrades mRNAs with premature stop codons. Sequences of cDNAs from rpb9 Delta strains confirmed a significantly increased occurrence of transcriptional substitutions and insertions. These results suggest that Rpb9 plays an important role in maintaining transcriptional fidelity, whereas THIS may serve a different primary purpose.