The Importance of Nonsense Errors: Estimating the Rate and Implications of Drop-Off Errors during Protein Synthesis

The process of mRNA translation is both energetically costly and relatively error-prone compared to transcription and replication. Nonsense errors during mRNA translation occur when a ribosome drops off a transcript before reaching a stop codon, resulting in energetic investment in an incomplete and likely non-functional protein. Nonsense errors impose a potentially significant energy burden on the cell, making it critical to quantify their frequency and energetic cost. Here, we present a model of ribosome movement for estimating protein production, elongation, and nonsense error rates from high-throughput ribosome profiling data. Applying this model to an exemplary ribosome profiling dataset in S. cerevisiae, we find that nonsense error rates vary between codons, in conflict with the general assumption of uniform rates across sense codons. Using our parameter estimates, we find multiple lines of evidence that selection against nonsense errors is a prominent force shaping coding-sequence evolution, including that nonsense errors place an energetic burden on cells comparable to ribosome pausing. Our results indicate greater consideration should be given to the impact of nonsense errors in shaping coding-sequence evolution.