USE OF THE G+C CONTENT OF DNA FOR DETERMINATION OF MOLECULAR PHYLOGENY OF NITRIFYING BACTERIA

It is shown that the content of guanine + cytosine pairs in DNA (the G +C content of DNA) can be used in studies of bacterial molecular phylo geny. The method involves taking into account the absolute value of th e difference between the DNA G+C contents in species branching from a single phylogenetic line. The magnitudes of this value, which we calle d the ''G+C evolutionary distance,'' were determined for nitrifying ba cteria using the well-known phylogenetic tree of 16S rRNA primary stru ctures. The correlation coefficient between the evolutionary distance of 16S rRNA sequences and the ''G+C evolutionary distance'' was found to be as high as 0.97. Nitrite-oxidizing Nitrospira marina and Nitrosp ina gracilis were shown to make up a group lacking close phylogenetic affinity with other nitrifying bacteria. Thus, in addition to being an important taxonomic feature, the G+C content of DNA can be used as su pplementary data in studies of bacterial molecular phylogeny.