Die Demethylierung von Dimethylselenid ist eine adaptive Antwort des Archaeons Methanococcus voltae

Selenium is a trace element which is important for many organisms. It is a constituent of proteins, where it is found as selenocysteine or selenomethionine, and of special nucleotides of tRNA bases. High concentrations of selenium are toxic for most organisms. Detoxification can be achieved by volatilization through methylation. Different methylation products have been found. The most abundant one is dimethylselenide which can probably be generated in different ways from inorganic selenium compounds. Dimethylselenide production is performed by microorganisms and plants and has been monitored in soil and marine environments. Although selenium is widely distributed in the environment, it is not always readily available. While inorganic selenium compounds, such as selenite and selenate are soluble, selenides can be very insoluble as is elementary selenium which can be formed from the oxidized species. Selenium can thus become limiting in anoxic environments. Access to selenium is essential for organisms depending on selenium-containing enzymes in their central metabolism. This is the case for at least two known methanogenic archaea Methanocaldococcus jannaschii and Methanococcus voltae. Both organisms convert hydrogen and carbon dioxide to methane, whereby the cells generate their energy. Two selenium containing hydrogenases, enzymes needed to oxidize hydrogen for the generation of electrons, are involved in the methanogenic pathway. Limiting selenium in growth media for M. voltae leads to a reduced growth rate and the knockout of a gene encoding a selenium-containing subunit of a hydrogenase has not been possible. While limited growth of M. voltae has been observed under selenium depletion, M. jannaschii cannot grow without selenium. It was previously shown that M. voltae carries genes encoding selenium-free isoenzymes of its selenium-containing hydrogenases which are only transcribed upon selenium limitation and most likely supplement the selenium enzymes. The cell can thus react to the deprivation of the trace element. We were interested in learning more about functions of proteins produced only under selenium limitation. The protein patterns in extracts obtained from of M. voltae cells grown with or without selenium were therefore analyzed. Subsequently, a protein that was induced by selenium deprivation was further characterized. This putative corrinoid protein together with a methyltransferase is involved in the liberation of selenium from the organic selenium compound dimethylselenide. The two respective genes are part of a common transcription unit. Their regulation occurs at the level of transcription or by regulation of transcript stability. This inducible demethylation of dimethylselenide constitutes a novel, alternative adaptation strategy of M. voltae to selenium limitation.