Gene clustering in a bacterium with a large genome, Pseudomonas fluorescens, as a function of their frequency in bacterial genomes (redrawn and modified after supplementary figure 1, 118 (Danchin et al. 2007)). (a) On the abscissa, genes are grouped by clusters of 50 genes, as a function of their frequency in available bacterial genomes longer than 1,500 genes: the leftmost groups are present in most if not all genomes, and this number progressively decreases along the abscissa, with the group of 50 genes present on the rightmost bars present only in P. fluorescens. On the ordinate is represented the tendency for the genes in each group of 50 genes to remain clustered together in the genomes where they are present. The grouping of the genes on the left, making the paleome, is reminiscent of a scenario of the origin of life, while the genes on the right, making the genome, permit cells to occupy a particular niche (Danchin 2007). (b) The genes in the paleome make two categories, persistent essential genes and persistent non-essential genes (Fang et al. 2005). The latter category codes for proteins that use energy to maintain and repair the cell functions as well as genes involved in managing energy involving polyphosphates