The rapid switch-off of nitrogenase activity in obligate methane-oxidizing bacteria

Abstract

Nitrogenase activity in the obligate methaneoxidizing bacterium Methylococcus capsulatus (Bath) was added ammonia. This observation was extended to include other ammonia. This observation was extended to include other representative N2-fixing species of methanotrophs. The ammonia “switch-off” of nitrogenase in M. capsulatus (Bath) was reversed on washing cells to remove excess ammonia, in the presence of chloramphenicol, suggesting that a form of covalent modification of nitrogenase may occur. Replacing the oxidizable substrate methanol with formaldehyde, formate, ethanol or hydrogen had no effect on nitrogenase “switch-off”. A number of potential nitrogen sources or intermediates of nitrogen metabolism such as glutamine, asparagine, glutamate and alanine when tested, did not effect “switch-off”. However, the rapid inhibition of nitrogenase activity of M. capsulatus (Bath) could be achieved by adding the uncoupler carbonylcyanide m-chlorophenylhydrazone or nitrite. The glutamine synthetase inhibitor methionine sulphoximine blocked the “switch-off” effect of ammonia, indicating that the metabolism of ammonia may be essential for “switch-off” to occur. Inhibitors of glutamate synthase did not alleviate the ammonia “switch-off” response. Methionine sulphoximine did not alleviate the rapid inhibition of nitrogenase by carbonylcyanide m-chlorophenylhydrazone indicating that the shortterm regulation of nitrogenase by uncouplers and ammonia proceed via different mechanisms.