Comparison of predicted and shuffled operons. Complete operons contain all 5 essential che genes (A, B, R, W and Y). Incomplete operons are missing one or more of these genes. Complementary operons contain genes which make up a complete set of essential che genes. Shuffled operons are generated by keeping the number and size of operons within a species constant, but shuffling the che genes so that the allocation of che genes into the operons is randomised.