Trypanosomes have a novel post-transcriptional process for editing mitochondrial rna. in this form of rna editing, several uracil bases are inserted at specific locations in an mrna (e. g., for purposes of shifting the reading frame into correct position). the insertions are directed by short guide rnas (grnas) that encode editing "instructions" in the form of complementary base-pairing sequences. which enzyme involved in this process catalyzes the addition of uracil (u) bases to the mrna?