Known functions of polyuridylation in the nucleus and in the mitochondria. (a) Functions in the nucleus. U6 snRNA is the only known substrate for polyuridylation in the nucleus by U6 TUTase. Polyuridylation is thought to regenerate the 3′-end of U6 snRNA following its shortening by exonucleases. If this RNA is adenylated, the polyuridylation event is inhibited and the U6 snRNA is degraded. If the RNA is polyuridylated, mature U6 snRNA is produced and incorporated in the splicing complex known as the spliceosome. (b) Function of polyuridylation in the mitochondria of trypanosomes. In order to be properly matured, gRNAs are polyuridylated by RET1 TUTase allowing the gRNA to “guide” the editing reaction. To be translationally competent, mitochondrial mRNAs require addition of a long A/U tail, which is performed by the RET1/KPAP1 complex and coordinated by the KPAF1/KPAF2 complex. The mRNA is then recognized by the ribosome and translation can be started.