Polynucleotide sequence relatedness tests were carried out to determine the extent ofdeoxyribonucleic acid (DNA) divergence among species of Klebsiella and Enterobacter aerogenes strains. Labeled, denatured DNA fragments from K. pneumoniae type 2 and E. aerogenes 1627-66 were each incubated with an excess of unlabeled DNA fragments from Klebsiella species and strains of E. aerogenes. Reassociated DNA duplexes were separated from unreactedDNA on hydroxyapatite. The stability of reassociated DNA duplexes was determined in a series of thermal elutions from hydroxyapatite. Relative reassociation of DNA from 5 Klebsiella strains to K. pneumoniae type 2 was 80 to 91%. In no case did the related DNA duplexes exhibit evidence of greater than 1.2% unpaired bases. Similarly, DNA from 10 strains of E. aerogenes exhibited 83 to 100% relative reassociation with DNA from E. aerogenes 1627-66. In this case, 2.2% was the largest amount of unpaired bases within a reassociated DNA duplex. Conversely, only 56% relative relatedness was observed in 18 reactions between klebsiellae and E. aerogenes. In these reactions, related DNA duplexes exhibited an average of 9% unpaired bases. We conclude that klebsiellae and E. aerogenes each are highly related groups of strains and that these two groups have diverged significantly from one another. An E. cloacae strain exhibited some 40% relative relatedness with some 12% unpaired bases in reactions with K. pneumoniae and E. aerogenes.


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