Protein material costs: single atoms can make an evolutionary difference.

The process of gene expression has material costs caused by the quantities of carbon, nitrogen, sulfur and phosphorus that are needed to make mRNAs and proteins. When any such chemical element is ecologically limiting, mutations increasing these costs can reduce growth. Here, we ask if such mutations are 'visible' to natural selection in the yeast Saccharomyces cerevisiae. We find that mutations causing small increases in expression and even single amino acid replacements can be subject to natural selection on the basis of their material costs.