Sulphur islands in the Escherichia coli genome: markers of the cell's architecture?

Two highly contrasted images depict genomes: at first sight, genes appear to be distributed randomly along the chromosome. In contrast, their organisation into operons (or pathogenicity islands) suggests that, at least locally, related functions are in physical proximity. Analysis of the codon usage bias in orthologous genes in the genome of bacteria which diverged a long time ago suggested that some physical (architectural) selection pressure organised the distribution of genes along the chromosome. The metabolism of highly reactive species such as sulphur-containing molecules must be compartmentalised to escape the deleterious actions of diffusible reagents such as gases or radicals. We analysed the distribution of sulphur metabolism genes in the genome of Escherichia coli and found a number of them to be clustered into statistically significant islands. Another interesting feature of these genes is that the proteins they encode are significantly deprived of cysteine and methionine residues, as compared to the bulk proteins. We speculate that this clustering is associated to the organisation of sulphur metabolism proteins into islands where the sensitive sulphur-containing molecules are protected from reacting with elements in the environment such as dioxygen, nitric oxide or radicals.