Abstract
RNA polymerase III (Pol III) transcribes a limited set of short genes in eukaryotes producing abundant small RNAs, mostly tRNA. The originally defined yeast Pol III transcriptome appears to be expanding owing to the application of new methods. Also, several factors required for assembly and nuclear import of Pol III complex have been identified recently. Models of Pol III based on cryo-electron microscopy reconstructions of distinct Pol III conformations reveal unique features distinguishing Pol III from other polymerases. Novel concepts concerning Pol III functioning involve recruitment of general Pol III-specific transcription factors and distinctive mechanisms of transcription initiation, elongation and termination. Despite the short length of Pol III transcription units, mapping of transcriptionally active Pol III with nucleotide resolution has revealed strikingly uneven polymerase distribution along all genes. This may be related, at least in part, to the transcription factors bound at the internal promoter regions. Pol III uses also a specific negative regulator, Maf1, which binds to polymerase under stress conditions; however, a subset of Pol III genes is not controlled by Maf1. Among other RNA polymerases, Pol III machinery represents unique features related to a short transcript length and high transcription efficiency.
Highlights
Transcription of nuclear DNA in eukaryotes is carried out by at least three different RNA polymerases (Pols), designated Pol I, II and III
Pol I and polymerase III (Pol III) are specialized in highlevel synthesis of protein-non-coding RNA species, rRNA and tRNA, which are fundamental components of the translation machinery. tRNA and rRNA genes are highly transcribed, leading to the production in yeast of 3 million tRNAs per generation and 300 000 ribosomes, compared with about 60 000 molecules of mRNA
Starting from the biogenesis of Pol III complex, we describe promoter recognition by Pol III general factors and the cascade of DNA –protein interactions leading to recruitment of Pol III to their genes
Summary
Transcription of nuclear DNA in eukaryotes is carried out by at least three different RNA polymerases (Pols), designated Pol I, II and III. One topic which is not covered is the chromatin connections of Pol III-transcribed genes, but there is an excellent review available on the subject [1]. Another aspect of Pol III control not discussed here is nonuniform regulation of tRNA genes that can shift the translation profiles of key codon-biased mRNAs. Another aspect of Pol III control not discussed here is nonuniform regulation of tRNA genes that can shift the translation profiles of key codon-biased mRNAs For this topic, the readers may be referred to other recent reviews [2,3]
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