Gene expression correlates with substitution rate and gene essentiality. A) Nonsynonymous substitution rate dN (coding sequences averaged for all 36 strains) versus the expression level. Color indicates the density of data points ranging from blue (single data point) to deep red. B) Boxplots of gene expression levels grouped by gene essentiality as annotated in the Database of Essential Genes . Boxplots indicate 0.25 and 0.75 quantiles and the median (red line) with notches giving an estimate for the variance of the median. The green dashed line indicates the median dN for all genes.