Role of a 300-Kilodalton Nuclear Complex in the Maturation of Trypanosoma brucei Initiator Methionyl-tRNA

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American Society for Microbiology

RESUMO

tRNAs are transcribed as precursors containing 5′ leader and 3′ extensions that are removed by a series of posttranscriptional processing reactions to yield functional mature tRNAs. Here, we examined the maturation pathway of tRNAMet in Trypanosoma brucei, an early divergent unicellular eukaryote. We identified an approximately 300-kDa complex located in the nucleus of T. brucei that is required for trimming the 5′ leader of initiator tRNAMet precursors. One of the subunits of the complex (T. brucei MT40 [TbMT40]) is a putative methyltransferase and a homolog of Saccharomyces cerevisiae Gcd14, which is essential for 1-methyladenosine modification in tRNAs. Down-regulation of TbMT40 by RNA interference resulted in the accumulation of precursor initiator tRNAMet containing 5′ extensions but processed 3′ ends. In addition, immunoprecipitations with anti-La antibodies revealed initiator tRNAMet molecules with 5′ and 3′ extensions in TbMT40-silenced cells, albeit at a much lower level. Interestingly, silencing of TbMT40, as well as of TbMT53, a second subunit of the complex, led to an increase in the levels of mature elongator tRNAMet. Taken together, our data provide a glance at the maturation of tRNAs in parasitic protozoa and suggest that at least for initiator tRNAMet, 3′ trimming precedes 5′ processing.

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