Operon distribution of different expression subclasses. (a) Comparison of nonsynonymous substitution rate between operon genes and non-operon genes in MED4 (Mann–Whitney U Test, two-tailed). A circle represents an outlier. (b) Operon rate of four expression subclasses (HEG, MEG, LEG, and VEG) or the core/flexible genomes (Fisher’s exact test, one-tailed). The operon rate was defined as the ratio of operon genes to total genes in a certain gene collection. The operon rate of each subclass was normalized by the whole genome operon rate (55.5%). P-value ≤ 0.05 was indicated in figure. Core, the core genome; Flexible, the flexible genome; HEG, highly expressed genes; MEG, moderately expressed genes; LEG, lowly expressed genes; VEG, variably expressed genes.