Particulate methane monooxygenase genes in methanotrophs

A 45-kDa membrane polypeptide that is associated with activity of the particulate methane monooxygenase (pMMO) has been purified from three methanotrophic bacteria, and the N-terminal amino acid sequence was found to be identical in 17 of 20 positions for all three polypeptides and identical in 14 of 20 positions for the N terminus of AmoB, the 43-kDa subunit of ammonia monooxygenase. DNA from a variety of methanotrophs was screened with two probes, an oligonucleotide designed from the N-terminal sequence of the 45-kDa polypeptide from Methylococcus capsulatus Bath and an internal fragment of amoA, which encodes the 27-kDa subunit of ammonia monooxygenase. In most cases, two hybridizing fragments were identified with each probe. Three overlapping DNA fragments containing one of the copies of the gene encoding the 45-kDa pMMO polypeptide (pmoB) were cloned from Methylococcus capsulatus Bath. A 2.1-kb region was sequenced and found to contain both pmoB and a second gene, pmoA. The predicted amino acid sequences of these genes revealed high identity with those of the gene products of amoB and amoA, respectively. Further hybridization experiments with DNA from Methylococcus capsulatus Bath and Methylobacter albus BG8 confirmed the presence of two copies of pmoB in both strains. These results suggest that the 45- and 27-kDa pMMO-associated polypeptides of methanotrophs are subunits of the pMMO and are present in duplicate gene copies in methanotrophs.