High-mobility-group proteins NHP6A and NHP6B participate in activation of the RNA polymerase IIISNR6 gene

Transcription of yeast class III genes involves the formation of a transcription initiation complex that comprises RNA polymerase III (Pol III) and the general transcription factors TFIIIB and TFIIIC. Using a genetic screen for positive regulators able to compensate for a deficiency in a promoter element of the SNR6 gene, we isolated the NHP6A and NHP6B genes. Here we show that the high-mobility-group proteins NHP6A and NHP6B are required for the efficient transcription of the SNR6 gene both in vivo and in vitro. The transcripts of wild-type and promoter-defective SNR6 genes decreased or became undetectable in an nhp6A Delta nhp6B Delta double-mutant strain, and the protection over the TATA box of the wild-type SNR6 gene was lost in nhp6A Delta nhp6B Delta cells at 37 degreesC. In vitro. NHP6B specifically stimulated the transcription of SNR6 templates up to fivefold in transcription assays using either cell nuclear extracts from nhp6A Delta ,nhp6B Delta cells or reconstituted transcription systems. Finally, NHP6B activated SNR6 transcription in a TFIIIC-independent assay. These results indicate that besides the general transcription factors TFIIIB and TFIIIC, additional auxilliary factors are required for the optimal transcription of at least some specific Pol III genes.