Composition, Variation, and Dynamics of Major Osmotic Solutes in Methanohalophilus Strain FDF1

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Methanohalophilus strain FDF1, a member of the halophilic genus of methanogens, can grow over a range of external NaCl concentrations from 1.2 to 2.9 M and utilize methanol, trimethylamine, and dimethyl sulfide as substrates for methanogenesis. It produces the osmolytes glycine betaine, β-glutamine, and Nε-acetyl-β-lysine with increasing external NaCl, but the relative ratio of these zwitterions depends primarily on the methanogenic substrate and less on the external osmolarity. When the cells are grown on methanol in defined medium, accumulation of glycine betaine predominates over the other zwitterionic solutes. The cells also synthesized a carbohydrate which was not detected in cells grown on trimethylamine. This negatively charged compound, identified as α-glucosylglycerate from the 13C and 1H chemical shifts, does not act as an osmoregulatory solute in the salt range 1.4 to 2.7 M in this methanogen as evidenced by its invariant intracellular concentration. 13CH3OH-pulse/12CH3OH-chase experiments were used to determine half-lifes for these organic solute pools in the cells. l-α-Glutamate showed a rapid loss of heavy isotope, indicating that l-α-glutamate functions as a biosynthetic intermediate in these cells. Measurable turnover rates for both β-glutamine, which acts as an osmolyte, and α-glucosylglycerate suggest that they function as metabolic intermediates as well. Molecules which function solely as osmolytes (glycine betaine and Nε-acetyl-β-lysine) showed a slower turnover consistent with their roles as osmotic solutes in Methanohalophilus strain FDF1.

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