The Minimum Open Reading Frame, AUG-Stop, Induces Boron-Dependent Ribosome Stalling and mRNA Degradation

AUG-stops in the 5′ untranslated regions of Arabidopsis genes, including NIP5;1, which encodes a boron influx transporter, regulate B-dependent mRNA accumulation through ribosome stalling. Upstream open reading frames (uORFs) are often translated ahead of the main ORF of a gene and regulate gene expression, sometimes in a condition-dependent manner, but such a role for the minimum uORF (hereafter referred to as AUG-stop) in living organisms is currently unclear. Here, we show that AUG-stop plays an important role in the boron (B)-dependent regulation of NIP5;1, encoding a boric acid channel required for normal growth under low B conditions in Arabidopsis thaliana. High B enhanced ribosome stalling at AUG-stop, which was accompanied by the suppression of translation and mRNA degradation. This mRNA degradation was promoted by an upstream conserved sequence present near the 5′-edge of the stalled ribosome. Once ribosomes translate a uORF, reinitiation of translation must take place in order for the downstream ORF to be translated. Our results suggest that reinitiation of translation at the downstream NIP5;1 ORF is enhanced under low B conditions. A genome-wide analysis identified two additional B-responsive genes, SKU5 and the transcription factor gene ABS/NGAL1, which were regulated by B-dependent ribosome stalling through AUG-stop. This regulation was reproduced in both plant and animal transient expression and cell-free translation systems. These findings suggest that B-dependent AUG-stop-mediated regulation is common in eukaryotes.

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