bims-micpro 2024-01-28 papers

Issue of 2024‒01‒28
two papers selected by
Thomas Farid Martínez, University of California, Irvine

Nat Chem Biol. 2024 Jan 24.

Boric acid intercepts 80S ribosome migration from AUG-stop by stabilizing eRF1.

Mayuki Tanaka, Takeshi Yokoyama, Hironori Saito, Madoka Nishimoto, Kengo Tsuda, Naoyuki Sotta, Hideki Shigematsu, Mikako Shirouzu, Shintaro Iwasaki, Takuhiro Ito, Toru Fujiwara.

In response to environmental changes, cells flexibly and rapidly alter gene expression through translational controls. In plants, the translation of NIP5;1, a boric acid diffusion facilitator, is downregulated in response to an excess amount of boric acid in the environment through upstream open reading frames (uORFs) that consist of only AUG and stop codons. However, the molecular details of how this minimum uORF controls translation of the downstream main ORF in a boric acid-dependent manner have remained unclear. Here, by combining ribosome profiling, translation complex profile sequencing, structural analysis with cryo-electron microscopy and biochemical assays, we show that the 80S ribosome assembled at AUG-stop migrates into the subsequent RNA segment, followed by downstream translation initiation, and that boric acid impedes this process by the stable confinement of eukaryotic release factor 1 on the 80S ribosome on AUG-stop. Our results provide molecular insight into translation regulation by a minimum and environment-responsive uORF.

DOI: https://doi.org/10.1038/s41589-023-01513-0

Elife. 2024 Jan 24. pii: RP92916. [Epub ahead of print]12

Yeast eIF2A has a minimal role in translation initiation and uORF-mediated translational control in vivo.

Swati Gaikwad, Fardin Ghobakhlou, Hongen Zhang, Alan G Hinnebusch.

Initiating translation of most eukaryotic mRNAs depends on recruitment of methionyl initiator tRNA (Met-tRNAi) in a ternary complex (TC) with GTP-bound eukaryotic initiation factor 2 (eIF2) to the small (40S) ribosomal subunit, forming a 43S preinitiation complex (PIC) that attaches to the mRNA and scans the 5'-untranslated region (5' UTR) for an AUG start codon. Previous studies have implicated mammalian eIF2A in GTP-independent binding of Met-tRNAi to the 40S subunit and its recruitment to specialized mRNAs that do not require scanning, and in initiation at non-AUG start codons, when eIF2 function is attenuated by phosphorylation of its α-subunit during stress. The role of eIF2A in translation in vivo is poorly understood however, and it was unknown whether the conserved ortholog in budding yeast can functionally substitute for eIF2. We performed ribosome profiling of a yeast deletion mutant lacking eIF2A and isogenic wild-type (WT) cells in the presence or absence of eIF2α phosphorylation induced by starvation for amino acids isoleucine and valine. Whereas starvation of WT confers changes in translational efficiencies (TEs) of hundreds of mRNAs, the eIF2AΔ mutation conferred no significant TE reductions for any mRNAs in non-starved cells, and it reduced the TEs of only a small number of transcripts in starved cells containing phosphorylated eIF2α. We found no evidence that eliminating eIF2A altered the translation of mRNAs containing putative internal ribosome entry site (IRES) elements, or harboring uORFs initiated by AUG or near-cognate start codons, in non-starved or starved cells. Thus, very few mRNAs (possibly only one) appear to employ eIF2A for Met-tRNAi recruitment in yeast cells, even when eIF2 function is attenuated by stress.

Keywords: IRES; S. cerevisiae; eIF2; eIF2A; genetics; genomics; translation; uORF; yeast

DOI: https://doi.org/10.7554/eLife.92916