ANALYSIS OF 16S RIBOSOMAL-RNA AND METHANE MONOOXYGENASE GENE-SEQUENCES REVEALS A NOVEL GROUP OF THERMOTOLERANT AND THERMOPHILIC METHANOTROPHS, METHYLOCALDUM GEN. NOV
L. Bodrossy et al., ANALYSIS OF 16S RIBOSOMAL-RNA AND METHANE MONOOXYGENASE GENE-SEQUENCES REVEALS A NOVEL GROUP OF THERMOTOLERANT AND THERMOPHILIC METHANOTROPHS, METHYLOCALDUM GEN. NOV, Archives of microbiology, 168(6), 1997, pp. 493-503
Two methanotrophic bacteria with optimum growth temperatures above 40
degrees C were isolated. Thermotolerant strain LK6 was isolated from a
gricultural soil, and the moderately thermophilic strain OR2 was isola
ted from the effluent of an underground hot spring. When compared to t
he described thermophilic methanotrophs Methylococcus capsulatus and M
ethylococcus thermophilus, these strains are phenotypically similar to
Methylococcus thermophilus. However, their 16S rRNA gene sequences ar
e markedly different from the sequence of Methylococcus thermophilus (
similar to 8% divergence) and, together with Methylomonas gracilis, th
ey form a distinct, new genus within the gamma-subgroup of the Proteob
acteria related to extant Type I methanotrophs. Further phenotypic cha
racterisation showed that the isolates possess particulate methane mon
ooxygenase (pMMO) but do not contain soluble methane monooxygenase. Th
e nucleotide sequence of a gene encoding pMMO (pmoA) was determined fo
r both isolates and for Methylomonas gracilis. PmoA sequence compariso
ns confirmed the monophyletic nature of this newly recognised group of
thermophilic methanotrophs and their relationship to previously descr
ibed Type I methanotrophs. We propose that strains OR2 and LK6, togeth
er with the misclassified thermophilic strains Methylomonas gracilis V
KM-14L(T) and Methylococcus thermophilus IMV-B3122, comprise a new gen
us of thermophilic methanotrophs, Methylocaldum gen. nov., containing
three new species: Methylocaldum szegediense, Methylocaldum tepidum an
d Methylocaldum gracile.