Polynucleotide Sequence Divergence Among Strains ofEscherichia coliand Closely Related Organisms

Abstract

Polynucleotide sequence similarity tests were carried out to determine the extent of divergence present in a number ofEscherichia colistrains, obtained from diverse human, animal, and laboratory sources, and closely related strains ofShigella, Salmonella, and the Alkalescens-Dispar group. At 60 C, relative reassociation of deoxyribonucleic acid (DNA) from the various strains withE. coliK-12 DNA ranged from 100 to 36%, with the highest level of reassociation found for three strains derived from K-12, and the lowest levels for two “atypical”E. colistrains andS. typhimurium. The change in thermal elution midpoint, which indicates the stability of DNA duplexes, ranged from 0.1 to 14.5 C, with thermal stability closely following the reassociation data. Reassociation experiments performed at 75 C, at which temperature only the more closely related DNA species form stable duplexes, gave similar indications of relatedness. At both temperatures, Alkalescens-Dispar strains showed close relatedness toE. coli, supporting the idea that they should be included in the genusEscherichia. Reciprocal binding experiments withE. coliBB, 02A, and K-12 yielded different reassociation values, suggesting that the genomes of these strains are of different size. The BB genome was calculated to be 9% larger than that of K-12, and that of 02A 9% larger than that of BB. Calculation of genome size for a series ofE. colistrains yielded values ranging from 2.29 × 10⁹to 2.97 × 10⁹daltons.E. colistrains and closely related organisms were compared by Adansonian analysis for their relatedness to a hypothetical median strain.E. coli0128a was the most closely related to this median organism. In general, these data compared well with the data from reassociation experiments amongE. colistrains. However, anomalous results were obtained in the cases ofShigella flexneri, S. typhimurium, and “atypical”E. colistrains.