From genes to genomes: universal scale-invariant properties of microbial chromosome organisation

The availability of complete genome sequences for a large variety of organisms is a major advance in understanding genome structure and function. One attribute of genome structure is chromosome organisation in terms of gene localisation and orientation. For example, bacterial operons, i.e. clusters of co-oriented genes that form transcription units, enable functionally related genes to be expressed simultaneously. The description of genome organisation was pioneered with the study of the distribution of genes of the Escherichia coli partial genetic map before the full genome sequence was known. Deploying powerful techniques from circular statistics and signal processing, we revisit the issue of gene localisation and orientation using 89 complete microbial chromosomes from the eubacterial and archaeal domains. We demonstrate that there is no characteristic size pertinent to the description of chromosome structure, e.g. there does not exist any single length appropriate to describe gene clustering. Our results show that, for all 89 chromosomes, gene positions and gene orientations share a common form of scale-invariant correlations known as "long-range correlations" that we can reveal for distances from the gene length, up to the chromosome size. This observation indicates that genes tend to assemble and to co-orient over any scale of observation greater than a few kilobases. This unexpected property of chromosome structure can be portrayed as an operon-like organisation at all scales and implies that a complete scale range extending over more than three orders of magnitudes of chromosome segment lengths is necessary to properly describe prokaryotic genome organisation. We propose that this pattern results from the effects of the superhelical context on gene expression coupled with the structure and dynamics of the nucleoid, possibly accommodating the diverse gene expression profiles needed during the different stages of cellular life.