Department of Biochemistry & Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80045, USA; RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, CO 80045, USA
Gabriel Galindo
Department of Biochemistry & Molecular Biology, Colorado State University, Fort Collins, CO 80523, USA
Amanda L. Koch
Department of Biochemistry & Molecular Biology, Colorado State University, Fort Collins, CO 80523, USA
Emma R. Horton
Department of Biochemistry & Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80045, USA; RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, CO 80045, USA
Evan J. Morrison
Department of Biochemistry & Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80045, USA; RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, CO 80045, USA
Samantha Tisa
Department of Biochemistry & Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80045, USA; RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, CO 80045, USA
Timothy J. Stasevich
Department of Biochemistry & Molecular Biology, Colorado State University, Fort Collins, CO 80523, USA
Olivia S. Rissland
Department of Biochemistry & Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80045, USA; RNA Bioscience Initiative, University of Colorado School of Medicine, Aurora, CO 80045, USA; Corresponding author
Summary: Nonoptimal synonymous codons repress gene expression, but the underlying mechanisms are poorly understood. We and others have previously shown that nonoptimal codons slow translation elongation speeds and thereby trigger messenger RNA (mRNA) degradation. Nevertheless, transcript levels are often insufficient to explain protein levels, suggesting additional mechanisms by which codon usage regulates gene expression. Using reporters in human and Drosophila cells, we find that transcript levels account for less than half of the variation in protein abundance due to codon usage. This discrepancy is explained by translational differences whereby nonoptimal codons repress translation initiation. Nonoptimal transcripts are also less bound by the translation initiation factors eIF4E and eIF4G1, providing a mechanistic explanation for their reduced initiation rates. Importantly, translational repression can occur without mRNA decay and deadenylation, and it does not depend on the known nonoptimality sensor, CNOT3. Our results reveal a potent mechanism of regulation by codon usage where nonoptimal codons repress further rounds of translation.
