Antisense Transcripts Delimit Exonucleolytic Activity of the Mitochondrial 3′ Processome to Generate Guide RNAs
2016
Suematsu, Takuma | Zhang, Liye | Aphasizheva, Inna | Monti, Stefano | Huang, Lan | Wang, Qi | Costello, Catherine E. | Aphasizhev, Ruslan
Small, noncoding RNA biogenesis typically involves cleavage of structured precursor by RNase III-like endonucleases. However, guide RNAs (gRNAs) that direct U-insertion/deletion mRNA editing in mitochondria of trypanosomes maintain 5′ triphosphate characteristic of the transcription initiation and possess a U-tail indicative of 3′ processing and uridylation. Here, we identified a protein complex composed of RET1 TUTase, DSS1 3′-5′ exonuclease, and three additional subunits. This complex, termed mitochondrial 3′ processome (MPsome), is responsible for primary uridylation of ∼800 nt gRNA precursors, their processive degradation to a mature size of 40–60 nt, and secondary U-tail addition. Both strands of the gRNA gene are transcribed into sense and antisense precursors of similar lengths. Head-to-head hybridization of these transcripts blocks symmetrical 3′-5′ degradation at a fixed distance from the double-stranded region. Together, our findings suggest a model in which gRNA is derived from the 5′ extremity of a primary molecule by uridylation-induced, antisense transcription-controlled 3′-5′ exonucleolytic degradation.
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