Methanosarcina thermophila, a nonmarine methanogenic archaebacterium, can grow in a range of saline concentrations. At less than 0.4 M NaCi, Ms. thermophila accumulated glutamate in response to increasing osmotic stress. At greater than 0.4 M NaCI, this organism synthesized a modified (3-amino acid that was identified as N6-acetyl-.8-lysine by NMR spectroscopy and ion-exchange HPLC.This (-amino acid derivative accumulated to high intracellular concentrations (up to 0.6 M) in Ms. thermophila and in another methanogen examined-Methanogenium cariaci, a marine species. The compound has features that are characteristic of a compatible solute: it is neutrally charged at physiological pH and it is highly soluble. When the cells were grown in the presence of exogenous glycine betaine, a physiological compatible solute, N6-acetyl--lysine synthesis was repressed and glycine betaine was accumulated. N8-Acetyl-(3-lysine was synthesized by species from three phylogenetic families when grown in high solute concentrations, suggesting that it may be ubiquitous among the methanogens. The ability to control the biosynthesis of N'-acetyl-(3-lysine in response to extracellular solute concentration indicates that the methanogenic archaebacteria have a unique (3amino acid biosynthetic pathway that is osmotically regulated.Microorganisms can proliferate in a diverse range of saline concentrations from low saline environments such as freshwater lakes to saturated brines found in solar salterns (1, 2). Cell size and the intracellular water activity must remain relatively constant to maintain physiological processes. Eukaryotic and eubacterial microorganisms have evolved mechanisms that enable them to minimize water loss when the extracellular solute concentration exceeds that of the cell cytoplasm (1-3). The mechanism of this adaptation involves the uptake or synthesis of low molecular weight organic compounds known as physiological compatible solutes (4, 5). These compounds reduce the osmotic potential between the extracellular milieu and the cytoplasm and protect enzymes from the low water activity that results from solute accumulation.Methanogenic archaebacteria have been isolated from environments with NaCl concentrations ranging from <0.05 M for many nonmarine species to >4 M for halophilic species (6). Individual species can also adapt to a range of saline concentrations. Methanosarcina thermophila, which was isolated from a thermophilic sludge digestor, grows in medium containing 0.05-1.2 M NaCl (7-9), and the marine species Methanogenium cariaci (10) grows in medium containing 0.17-1.4 M NaCl (D.N. and M.F.R., unpublished results). Compatible solutes have been detected in methanogenic bacteria (11-13). Several species of marine methanogens synthesize ,-glutamate in response to external NaCl (11, 13). Methanogens have also been reported to accumulate glycine betaine if it is provided in the medium (12). However, the mechanism for adaptation to high osmotic stress has not been investigated. Here we report the structure of ...