title: tRNASec is transcribed by RNA polymerase II in Trypanosoma brucei but not in humans.
abstract: Nuclear-encoded tRNAs are universally transcribed by RNA polymerase III (Pol-III) and contain intragenic promoters. Transcription of vertebrate tRNA(Sec) however requires extragenic promoters similar to Pol-III transcribed U6 snRNA. Here, we present a comparative analysis of tRNA(Sec) transcription in humans and the parasitic protozoa Trypanosoma brucei, two evolutionary highly diverged eukaryotes. RNAi-mediated ablation of Pol-II and Pol-III as well as oligo-dT induced transcription termination show that the human tRNA(Sec) is a Pol-III transcript. In T. brucei protein-coding genes are polycistronically transcribed by Pol-II and processed by trans-splicing and polyadenylation. tRNA genes are generally clustered in between polycistrons. However, the trypanosomal tRNA(Sec) genes are embedded within a polycistron. Their transcription is sensitive to α-amanitin and RNAi-mediated ablation of Pol-II, but not of Pol-III. Ectopic expression of the tRNA(Sec) outside but not inside a polycistron requires an added external promoter. These experiments demonstrate that trypanosomal tRNA(Sec), in contrast to its human counterpart, is transcribed by Pol-II. Synteny analysis shows that in trypanosomatids the tRNA(Sec) gene can be found in two different polycistrons, suggesting that it has evolved twice independently. Moreover, intron-encoded tRNAs are present in a number of eukaryotic genomes indicating that Pol-II transcription of tRNAs may not be restricted to trypanosomatids.
authors.affiliation: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors.forename: Eric
authors.initials: E
authors.lastname: Aeby
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Ullu
authors.forename: Hasmik
authors.initials: H
authors.lastname: Yepiskoposyan
authors.forename: Bernd
authors.initials: B
authors.lastname: Schimanski
authors.forename: Isabel
authors.initials: I
authors.lastname: Roditi
authors.forename: Oliver
authors.initials: O
authors.lastname: Mühlemann
authors.forename: André
authors.initials: A
authors.lastname: Schneider
mesh_terms: D053959:Alpha-Amanitin / Q000494:pharmacology; D001483:Base Sequence; D006367:HeLa Cells; D006801:Humans; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D034622:RNA Interference; D012319:RNA Polymerase II / Q000037:antagonists & inhibitors / Q000378:metabolism*; D012345:RNA, Transfer, Amino Acid-Specific / Q000096:biosynthesis / Q000235:genetics*; D014158:Transcription, Genetic / Q000187:drug effects; D014346:Trypanosoma brucei brucei / Q000235:genetics*
chemical_list: D053959:Alpha-Amanitin; D012345:RNA, Transfer, Amino Acid-Specific; C036928:tRNA, selenocysteine-; D012319:RNA Polymerase II
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