(157bb) Acetate Switch in Methanococcus Maripaludis S2

Acetate switch is a phenomenon which involves the transition from acetogenesis to acetotrophy during the growth phase of a microorganism. This behaviour has been reported in Escherichia coli and several other heterotrophs. In the early growth phase, these microorganisms dissimilate (produce and secrete) acetate when the carbon flux into the cells exceeds the capacity of their central pathways or when their tricarboxylic acid cycles do not operate fully due to limited oxygen. This dissimilation allows them to generate ATP and reduce acetyl-CoA accumulation. In the late growth phase, they switch to acetate assimilation (uptake and utilisation) to adapt to the scarcity of carbon sources. To date, all reported cases have involved heterotrophs and the tricarboxylic acid cycle.

In this work, we investigated the acetate metabolism in the methanogenic autotroph Methanococcus maripaludis S2. This microorganism is a mesophilic, hydrogenotrophic archaeon, which uses the Wolfe cycle for its anaerobic respiration. While acetate assimilation by M. maripaludis is known, its ability to dissimilate acetate has not been established thus far. We performed batch culture cultivation of M. maripaludis in minimal medium with carbon dioxide as the sole carbon source, either ammonium or dinitrogen as the sole nitrogen sources. We observed that M. maripaludis dissimilated acetate in the early growth phase and assimilated it back in the late growth phase. Furthermore, the acetate concentration was generally higher in the dinitrogen-grown cultures. These results provided evidence that, (1) M. maripaludis can dissimilate acetate, and (2) M. maripaludis exhibited an acetate concentration profile similar to the acetate switch profiles in E. coli and other heterotrophs. This is the first evidence that autotrophs can display an acetate switch behaviour.

We further examined the genome of M. maripaludis S2 and determined that the protein MMP0253 may catalyse acetate formation from acetyl-CoA. This protein was found to have similar primary and secondary structures to the ADP-forming acetyl-CoA synthetase in other archaea. We cloned MMP0253 in E. coli strain Rosetta (DE3) using pNIC28-Bsa4 as the vector and found that the recombinant protein indeed catalysed the acetate formation from acetyl-CoA.