Frequency distribution of DNA tetranucleotide usage profiles of selected prokaryotes. The observed/expected tetranucleotide frequency divergence (F(W)) was determined for the 256 tetranucleotide combinations for each genome, using both Markov chain and zero-order Markov analysis as described in Methods section. The F(W)values were sorted within 0.25 intervals and the ordinate represents the number of tetranucleotide combinations within each interval. (A) Zero-order Markov analysis. (B) Markov chain analysis.