Key Concepts of Section 9.8

• The process of transcription initiation by Pol I and Pol III is similar to that by Pol II but requires different general transcription factors, is directed by different promoter elements, and does not require hydrolysis of ATP β-γ phosphodiester bonds to separate the DNA strands at the start site as Pol II transcription does.

• Pol I transcribes only a single RNA, the 45S precursor of 18S, 5.8S, and 28S rRNA, from multiple copies of the pre-rRNA gene.

• Pol III transcribes tRNAs from promoters within the genes that encode the tRNA regions common to all tRNAs. This internal promoter is bound by transcription factor TFIIIC, which in turn binds TFIIIB, a multisubunit factor that includes the TATA box–binding protein, TBP, which associates with the tRNA gene about 30 bp upstream of the transcription start site.

• Pol III transcribes 5s rRNA directed by a promoter within the 5S-rRNA coding region that is bound by transcription factor TFIIIA. TFIIIA then associates with TFIIIC and TFIIIB, which interact with Pol III in a manner similar to their interactions in tRNA transcription.

• Additional small stable RNAs, several with as yet unknown functions, are transcribed by Pol III as directed by TBP-containing transcription factors that bind immediately upstream of the genes (see Figure 9-52).

• Pol III transcription is regulated by a specific inhibitor, MAF1, whose transport from the cytoplasm into the nucleus is controlled in response to nutrient availability.