The key enzyme in methane metabolism is methane monooxygenase (MMO), which catalyses the oxidation of methane to methanol. Some methanotrophs, including Methylococcus capsulatus (Bath), possess two distinct MMOs. The level of copper in the environment regulates the biosynthesis of the MMO enzymes in these methanotrophs. Under low-copper conditions, soluble MMO (sMMO) is expressed and regulation takes place at the level of transcription. The structural genes of sMMO were previously identified as mmoXYBZ, mmoD and mmoC. Putative transcriptional start sites, containing a σ70- and a σN-dependent motif, were identified in the 5′ region of mmoX. The promoter region of mmoX was mapped using truncated 5′ end regions fused to a promoterless green fluorescent protein gene. A 9.5 kb region, adjacent to the sMMO structural gene cluster, was analysed. Downstream (3′) from the last gene of the operon, mmoC, four ORFs were found, mmoG, mmoQ, mmoS and mmoR. mmoG shows significant identity to the large subunit of the bacterial chaperonin gene, groEL. In the opposite orientation, two genes, mmoQ and mmoS, showed significant identity to two-component sensor-regulator system genes. Next to mmoS, a gene encoding a putative σN-dependent transcriptional activator, mmoR was identified. The mmoG and mmoR genes were mutated by marker-exchange mutagenesis and the effects of these mutations on the expression of sMMO was investigated. sMMO transcription was impaired in both mutants. These results indicate that mmoG and mmoR are essential for the expression of sMMO in Mc. capsulatus (Bath).
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