The phenotypic consequence of a genetic mutation depends on many factors including the expression level of a gene. However, a comprehensive quantification of this expression effect is still lacking, as is a further general mechanistic understanding of the effect. Here, we measured the fitness effect of almost all (>97.5%) single-nucleotide mutations in GFP, an exogenous gene with no physiological function, and URA3, a conditionally essential gene. Both genes were driven by two promoters whose expression levels differed by around tenfold. The resulting fitness landscapes revealed that the fitness effects of at least 42% of all single-nucleotide mutations within the genes were expression dependent. Although only a small fraction of variation in fitness effects among different mutations can be explained by biophysical properties of the protein and messenger RNA of the gene, our analyses revealed that the avoidance of stochastic molecular errors generally underlies the expression dependency of mutational effects and suggested protein misfolding as the most important type of molecular error among those examined. Our results therefore directly explained the slower evolution of highly expressed genes and highlighted cytotoxicity due to stochastic molecular errors as a non-negligible component for understanding the phenotypic consequence of mutations.

基因突变的表型后果取决于许多因素，包括基因的表达水平。然而，仍然缺乏对这种表达效果的全面量化，以及对该效果的进一步一般机械理解。在这里，我们测量了GFP中几乎所有 (>97.5%) 单核苷酸突变的适应度效应，一种没有生理功能的外源基因，和URA3，一个条件必需基因。这两个基因都由两个启动子驱动，其表达水平相差约十倍。由此产生的适应度图显示，基因内至少 42% 的所有单核苷酸突变的适应度效应是依赖于表达的。尽管不同突变之间适应性效应的一小部分变化可以通过蛋白质的生物物理特性和基因的信使 RNA 来解释，但我们的分析表明，避免随机分子错误通常是突变效应和建议蛋白质的表达依赖性的基础错误折叠是被检查者中最重要的分子错误类型。