Selection conflicts, gene expression, and codon usage trends in yeast.

Synonymous codon usage in yeast appears to be influenced by natural selection on gene expression, as well as regional variation in compositional bias. Because of the large number of potential targets of selection (i.e., most of the codons in the genome) and presumed small selection coefficients, codon usage is an excellent model for studying factors that limit the effectiveness of selection. We use factor analysis to identify major trends in codon usage for 5836 genes in Saccharomyces cerevisiae. The primary factor is strongly correlated with gene expression, consistent with the model that a subset of codons allows for more efficient translation. The secondary factor is very strongly correlated with third codon position GC content and probably reflects regional variation in compositional bias. We find that preferred codon usage decreases in the face of three potential limitations on the effectiveness of selection: reduced recombination rate, increased gene length, and reduced intergenic spacing. All three patterns are consistent with the Hill-Robertson effect (reduced effectiveness of selection among linked targets). A reduction in gene expression in closely spaced genes may also reflect selection conflicts due to antagonistic pleiotropy.