Deoxyribonucleic acid homologies among 96 strains of ammonia-oxidizing bacteria

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Abstract
DNA of 96 strains of the genera Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus, and Nitrosovibrio was isolated and analysed spectrophotometrically. Percentages of guanine plus cytosine (G+C) content, genome sizes, and DNA-DNA homologies were determined. The results indicated the presence of eight Nitrosomonas species, three or four Nitrosococcus species, five Nitrosospira species, and two species of both Nitrosolobus and Nitrosovibrio. DNA homologies between strains of a separate species ranged from 56–100%. Average homologies between strains of different species were 33% in Nitrosococcus, 36% in Nitrosomonas, 37% in Nitrosolobus, 40% in Nitrosospira, and 42% in Nitrosovibrio. Average homologies between species of different genera were 33% and thus not significantly above the background value of 30% detected between DNA of ammonia-oxidizing bacteria and Escherichia coli. Genome sizes ranged from 1.90–2.74×109 dalton in Nitrosomonas, 2.09–2.37×109 dalton in Nitrosococcus, 1.87–2.15×109 dalton in Nitrosospira, 1.92–2.10×109 dalton in Nitrosolobus, and 1.91–2.15×109 dalton in Nitrosovibrio. Differences in genome sizes were in accordance with DNA homologies.

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