Codon usage trajectories and 7-cluster structure of 143 complete bacterial genomic sequences

Three results are presented. First, we prove the existence of a universal 7-cluster structure in all 143 completely sequenced bacterial genomes available in Genbank in August 2004, and explained its properties. The 7-cluster structure is responsible for the main part of sequence heterogeneity in bacterial genomes. In this sense, our 7 clusters is the basic model of bacterial genome sequence. We demonstrated that there are four basic “pure” types of this model, observed in nature: “parallel triangles”, “perpendicular triangles”, degenerated case and the flower-like type.