eIF1 discriminates against suboptimal initiation sites to prevent excessive uORF translation genome-wide

• Published in: RNA (Cambridge) Vol. 26; no. 4; pp. 419 - 438
• Main Authors: Zhou, Fujun, Zhang, Hongen, Kulkarni, Shardul D, Lorsch, Jon R, Hinnebusch, Alan G
• Format: Journal Article
• Language: English
• Published: United States Cold Spring Harbor Laboratory Press 01-04-2020
• Subjects: Alcohol Oxidoreductases - genetics; Alcohol Oxidoreductases - metabolism; Aminohydrolases - genetics; Aminohydrolases - metabolism; Codon, Initiator; Codons; Eukaryotic Initiation Factor-1 - metabolism; Gene Expression Regulation, Fungal; Genome, Fungal; Genomes; Glycine-tRNA Ligase - genetics; Glycine-tRNA Ligase - metabolism; Isoforms; Localization; Mitochondria; mRNA; Mutants; Open Reading Frames; Peptide Chain Initiation, Translational; Pyrophosphatases - genetics; Pyrophosphatases - metabolism; Ribosomes - metabolism; RNA, Messenger - chemistry; RNA, Messenger - genetics; RNA, Messenger - metabolism; Saccharomyces cerevisiae; Saccharomyces cerevisiae Proteins - genetics; Saccharomyces cerevisiae Proteins - metabolism; Translation; eIF1; translation; accuracy; initiation; yeast
• ISSN: 1355-8382; 1469-9001
• DOI: 10.1261/rna.073536.119

The translation preinitiation complex (PIC) scans the mRNA for an AUG codon in a favorable context. Previous findings suggest that the factor eIF1 discriminates against non-AUG start codons by impeding full accommodation of Met-tRNA
in the P site of the 40S ribosomal subunit, necessitating eIF1 dissociation for start codon selection. Consistent with this, yeast eIF1 substitutions that weaken its binding to the PIC increase initiation at UUG codons on a mutant
mRNA and particular synthetic mRNA reporters; and also at the AUG start codon of the mRNA for eIF1 itself owing to its poor Kozak context. It was not known however whether such eIF1 mutants increase initiation at suboptimal start codons genome-wide. By ribosome profiling, we show that the eIF1-L96P variant confers increased translation of numerous upstream open reading frames (uORFs) initiating with either near-cognate codons (NCCs) or AUGs in poor context. The increased uORF translation is frequently associated with the reduced translation of the downstream main coding sequences (CDS). Initiation is also elevated at certain NCCs initiating amino-terminal extensions, including those that direct mitochondrial localization of the
and
products, and at a small set of main CDS AUG codons with especially poor context, including that of eIF1 itself. Thus, eIF1 acts throughout the yeast translatome to discriminate against NCC start codons and AUGs in poor context; and impairing this function enhances the repressive effects of uORFs on CDS translation and alters the ratios of protein isoforms translated from near-cognate versus AUG start codons.