Hungry Codons Promote Frameshifting in Human Mitochondrial Ribosomes

Human mitochondria are not strict adherents to the universal genetic code, with modifications that include the apparent recoding of two arginine triplets to termination signals. This use of both AGA and AGG occurs rarely in other mammals, and this putative change has long posed a challenging conundrum. A -1 mitoribosome frameshift upstream of the rare codons would necessitate recognition of only the conventional UAA and UAG termination codons. By using a sequence-specific endoribonuclease, we show that the rare arginine codons, presumably in association with other cis elements, promote frameshifting in human mitoribosomes.