Characterization of the Trypanosoma brucei cap hypermethylase Tgs1.

Many U-snRNAs contain a hypermodified 2,2,7-trimethylguanosine (TMG) cap structure, which is formed by post-transcriptional methylation of an m(7)G cap. At present, little is known about the maturation of U-snRNAs in trypanosomes. The current evidence is consistent with the primary transcript containing an m(7)G moiety, but it is not clear whether the conversion of m(7)G to TMG takes place in the cytoplasm or in the nucleus. To address this issue, we characterized the Trypanosoma brucei homologue of the trimethylguanosine synthase (TbTgs1), a 28kDa protein, which is mainly composed of the conserved catalytic domain and lacks a large N-terminal domain present in higher eukaryotes. A GFP fusion with TbTgs1 revealed that this protein localizes throughout the nucleoplasm, as well as in one or two dots outside the nucleolus and RNAi-mediated downregulation of TbTgs1 suggests that this protein is responsible for hypermethylation of the m(7)G cap of both snRNAs and snoRNAs.