Abstract
There exists an inverse relationship between the rate of molecular evolution and the level of gene expression. Among the many explanations, the "toxic-error" hypothesis is a most general one, which posits that processing errors may often be toxic to the cells. However, toxic errors that constrain the evolution of highly expressed genes are often difficult to measure. In this study, we test the toxic-error hypothesis by using microRNA (miRNA) genes because their processing errors can be directly measured by deep sequencing. A miRNA gene consists of a small mature product (≈22 nt long) and a "backbone." Our analysis shows that (i) like the mature miRNA, the backbone is highly conserved; (ii) the rate of sequence evolution in the backbone is negatively correlated with expression; and (iii) although conserved between distantly related species, the error rate in miRNA processing is also negatively correlated with the expression level. The observations suggest that, as a miRNA gene becomes more highly (or more ubiquitously) expressed, its sequence evolves toward a structure that minimizes processing errors.