FIG 3 .

Tree based on codon usage distances of cas-E and cas-F genes. Sequenced E. coli and Shigella strains with the complete set of these genes were considered. The codon usage data of the entire K-12-MG1655 genome (labeled “Genome”) are also included for reference. The colors designate the different sets of cas genes: red, cas-F; green, cas-E1; blue, cas-E2. Shigella strains are abbreviated as described for Fig. 2.