PPR polyadenylation factor defines mitochondrial mRNA identity and stability in trypanosomes

In Trypanosoma brucei , most mitochondrial mRNA s undergo internal changes by RNA editing and 3′ end modifications. The temporally separated and functionally distinct modifications are manifested by adenylation prior to editing, and by post‐editing extension of a short A‐tail into a long A/U‐heteropolymer. The A‐tail stabilizes partially and fully edited mRNA s, while the A/U‐tail enables mRNA binding to the ribosome. Here, we identify an essential pentatricopeptide repeat‐containing RNA binding protein, kinetoplast polyadenylation factor 3 ( KPAF 3), and demonstrate its role in protecting pre‐ mRNA against degradation by the processome. We show that KPAF 3 recruits KPAP 1 poly(A) polymerase to the 3′ terminus, thus leading to pre‐ mRNA stabilization, or decay depending on the occurrence and extent of editing. In vitro , KPAF 3 stimulates KPAP 1 activity and inhibits mRNA uridylation by RET 1 TUT ase. Our findings indicate that KPAF 3 selectively directs pre‐ mRNA toward adenylation rather than uridylation, which is a default post‐trimming modification characteristic of ribosomal and guide RNA s. As a quality control mechanism, KPAF 3 binding ensures that mRNA s entering the editing pathway are adenylated and, therefore, competent for post‐editing A/U‐tailing and translational activation.