The Escherichia coli genome varies in size from 4.5 to 5.5 Mb. It is unclear whether this variation may be distributed finely throughout the genome or is concentrated at just a few chromosomal loci or on plasmids. Further, the functional correlates of size variation in different genome copies are largely unexplored. We carried out comparative macrorestriction mapping using rare-restriction-site alleles (made with the Tn10dRCP2 family of elements, containing the NotI, BlnI, I-CeuI, and ultra-rare-cutting I-SceI sites) among the chromosomes of laboratory E. coli K-12, newborn-sepsis-associated E. coli RS218, and uropathogenic E. coli J96. These comparisons showed just a few large accessory chromosomal segments accounting for nearly all strain-to-strain size differences. Of 10 sepsis-associated and urovirulence genes, previously isolated from the two pathogens by scoring for function, all were colocalized exclusively with one or more of the accessory chromosomal segments. The accessory chromosomal segments detected in the pathogenic strains from physical, macrorestriction comparisons may be a source of new virulence genes, not yet isolated by function
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