Methane monooxygenase gene expression mediated by methanobactin in the presence of mineral copper sources

doi:10.1073/pnas.0702879104

Methane monooxygenase gene expression mediated by methanobactin in the presence of mineral copper sources

*Department of Civil, Environmental, and Architectural Engineering and
^‡Department of Geology, University of Kansas, Lawrence, KS 66045; and
^†School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom

Edited by William H. Schlesinger, Duke University, Durham, NC, and approved June 5, 2007 (received for review March 28, 2007)

Abstract

Methane is a major greenhouse gas linked to global warming; however, patterns of in situ methane oxidation by methane-oxidizing bacteria (methanotrophs), nature's main biological mechanism for methane suppression, are often inconsistent with laboratory predictions. For example, one would expect a strong relationship between methanotroph ecology and Cu level because methanotrophs require Cu to sustain particulate methane monooxygenase (pMMO), the most efficient enzyme for methane oxidation. However, no correlation has been observed in nature, which is surprising because methane monooxygenase (MMO) gene expression has been unequivocally linked to Cu availability. Here we provide a fundamental explanation for this lack of correlation. We propose that MMO expression in nature is largely controlled by solid-phase Cu geochemistry and the relative ability of Cu acquisition systems in methanotrophs, such as methanobactins (mb), to obtain Cu from mineral sources. To test this hypothesis, RT-PCR expression assays were developed for Methylosinus trichosporium OB3b (which produces mb) to quantify pMMO, soluble MMO (the alternate MMO expressed when Cu is “unavailable”), and 16S-rRNA gene expression under progressively more stringent Cu supply conditions. When Cu was provided as CuCl₂, pMMO transcript levels increased significantly consistent with laboratory work. However, when Cu was provided as Cu-doped iron oxide, pMMO transcript levels increased only when mb was also present. Finally, when Cu was provided as Cu-doped borosilicate glass, pMMO transcription patterns varied depending on the ambient mb:Cu supply ratio. Cu geochemistry clearly influences MMO expression in terrestrial systems, and, as such, local Cu mineralogy might provide an explanation for methane oxidation patterns in the natural environment.

Footnotes

^§To whom correspondence should be addressed. E-mail: d.graham{at}ncl.ac.uk
Author contributions: J.A.R. and D.W.G. designed research; C.W.K. and E.K. performed research; C.W.K. contributed new reagents/analytic tools; C.W.K., D.A.F., J.A.R., and D.W.G. analyzed data; and C.W.K., D.A.F., J.A.R., and D.W.G. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The sequence reported in this paper (for the partial 16S-rRNA gene sequence from M. sporium strain NR3K described in SI Text and SI Figs. 3 and 4) has been deposited in the GenBank database (accession no. EF619620).
This article contains supporting information online at www.pnas.org/cgi/content/full/0702879104/DC1.
Abbreviation:

MMO,

particulate methane monooxygenase;

pMMO,

particulate MMO;

sMMO,

soluble MMO;

mb,

methanobactin.
Freely available online through the PNAS open access option.