Methanotrophs or methane-oxidizing bacteria exhibit a unique ‘copper-switch’ where expression of two forms of methane monooxygenase (MMO) is controlled by the availability of copper. In the absence of copper, a cytoplasmic or soluble methane monooxygenase (sMMO) is expressed. In the presence of copper, a membrane-bound or particulate methane monooxygenase (pMMO) is expressed. These two forms of MMO have very different properties, and elucidation of the basis of the copper-switch is of significant interest as methanotrophs are becoming increasingly popular for the valorization of methane. Recently, it was suggested via characterization of a mutant of Methylosinus trichosporium OB3b that expresses sMMO in the presence of copper (smmoC mutant) that the copper-switch may be based on copCD. These genes encode for a periplasmic copper-binding protein and an inner membrane protein, respectively, and are used by other bacteria for copper uptake. Specific knockouts of copCD in M. trichosporium OB3b wild type, however, show that these genes are not part of the copper-switch in methanotrophs, nor do they appear to be critical for copper uptake. Rather, it appears that the constitutive expression of sMMO in the smmoC mutant of M. trichosporium OB3b may be due to multiple lesions as smmoC was generated via random chemical mutagenesis.
- methane monooxygenase