CONTRIBUTION OF GENOME CHARACTERISTICS TO ASSESSMENT OF TAXONOMY OF OBLIGATE METHANOTROPHS

The DNAs of a variety of obligate methanotrophic bacteria were analyzed for base composition and nucleotide distribution. Genome molecular weights were determined for representative strains. Similarity maps attained by plotting DNA base composition versus nucleotide distribution and genome molecular weight showed that related species formed distinct clusters. Group I methanotrophs were found to form three clusters in both nucleotide distribution and genome size analyses. The first cluster consisted of five Methylomonas species, Methylomonas methanica, Methylomonas fodinarum, Methylomonas aurantiaca, "Methylomonas rubra," and "Methylomonas agile." The other clusters included both Methylomonas species and Methylococcus species, indicating the heterogeneity within these genera. One cluster contained low-G+C-content Methylococcus strains and included Methylococcus whittenburyi, Methylococcus bovis, Methylococcus vinelandii, Methylococcus luteus, and "Methylococcus ucrainicus." The type strains of Methylomonas pelagica and "Methylomonas alba" and a marine methanotrophic strain also clustered with the low-G+C-content Methylococcus group rather than with the genus Methylomonas. "Methylomonas gracilis" also appeared to be genetically distinct from the true Methylomonas species and clustered with the high-G+C-content Methylococcus strains. This cluster included Methylococcus capsulatus, Methylococcus thermophilus, and other moderately thermophilic group I methanotrophic strains. The group II methanotrophs belonging to the genera "Methylosinus" and "Methylocystis" formed separate generic clusters according to genome molecular weight data but not according to nucleotide distribution data.