tRNA super(Sec) is transcribed by RNA polymerase II in Trypanosoma brucei but not in humans

• Published in: Nucleic acids research 2010-09, Vol.38 (17), p.5833-5843
• Main Authors: Aeby, Eric, Ullu, Elisabetta, Yepiskoposyan, Hasmik, Schimanski, Bernd, Roditi, Isabel, Muehlemann, Oliver, Schneider, Andre
• Format: Article
• Language: English
• Subjects: Trypanosoma brucei
• ISSN: 0305-1048; 1362-4962
• DOI: 10.1093/nar/gkq345

Nuclear-encoded tRNAs are universally transcribed by RNA polymerase III (Pol-III) and contain intragenic promoters. Transcription of vertebrate tRNA super(Sec) however requires extragenic promoters similar to Pol-III transcribed U6 snRNA. Here, we present a comparative analysis of tRNA super(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 super(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 super(Sec) genes are embedded within a polycistron. Their transcription is sensitive to a-amanitin and RNAi-mediated ablation of Pol-II, but not of Pol-III. Ectopic expression of the tRNA super(Sec) outside but not inside a polycistron requires an added external promoter. These experiments demonstrate that trypanosomal tRNA super(Sec), in contrast to its human counterpart, is transcribed by Pol-II. Synteny analysis shows that in trypanosomatids the tRNA super(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.