Distribution of the 6,629 orthologous groups into COG functional categories. The number of orthologous groups in each COG category is represented by the bar height. The proportion of each COG category that is composed of core (black) or accessory (grey) orthologous groups is shown. Categories are arranged within each class in order of size. In most categories, at least half the orthologs are found in the core proteome, but COG category T (Signal transduction mechanisms) is overrepresented in the accessory proteome.