Distribution of chromosome length variation in natural isolates of Escherichia coli

Large-scale variation in chromosome size was analyzed in 35 naturalisolates of Escherichia coli by physical mapping with a restriction enzymewhose sites are restricted to rDNA operons. Although the genetic maps andchromosome lengths of the laboratory strains E. coli K12 and Salmonellaenterica sv. Typhimurium LT2 are highly congruent, chromosome lengths amongnatural strains of E. coli can differ by as much as 1 Mb, ranging from 4.5to 5.5 Mb in length. This variation has been generated by multiple changesdispersed throughout the genome, and these alterations are correlated;i.e., additions to one portion of the chromosome are often accompanied byadditions to other chromosomal regions. This pattern of variation is mostprobably the result of selection acting to maintain equal distances betweenthe replication origin and terminus on each side of the circularchromosome. There is a large phylogenetic component to the observed sizevariation: natural isolates from certain subgroups of E. coli haveconsistently larger chromosome, suggesting that much of the additional DNAin larger chromosomes is shared through common ancestry. There is nosignificant correlation between genome sizes and growth rates, whichcounters the view that the streamlining of bacterial genomes is a responseto selection for faster growth rates in natural populations.