  Coding information is the main source of heterogeneity (non-randomness) in
the sequences of bacterial genomes. This information can be naturally modeled
by analysing cluster structures in the "in-phase" triplet distributions of
relatively short genomic fragments (200-400bp). We found a universal 7-cluster
structure in bacterial genomic sequences and explained its properties. We show
that codon usage of bacterial genomes is a multi-linear function of their
genomic G+C-content with high accuracy. Based on the analysis of 143 completely
sequenced bacterial genomes available in Genbank in August 2004, we show that
there are four "pure" types of the 7-cluster structure observed. All 143
cluster animated 3D-scatters are collected in a database and is made available
on our web-site: http://www.ihes.fr/~zinovyev/7clusters The finding can be
readily introduced into any software for gene prediction, sequence alignment or
bacterial genomes classification.
