The compatible solute N -acetyl-␤-lysine is unique to methanogenic archaea and is produced under salt stress only. However, the molecular basis for the salt-dependent regulation of N -acetyl-␤-lysine formation is unknown. Genes potentially encoding lysine-2,3-aminomutase (ablA) and ␤-lysine acetyltransferase (ablB), which are assumed to catalyze N -acetyl-␤-lysine formation from ␣-lysine, were identified on the chromosomes of the methanogenic archaea Methanosarcina mazei Gö1, Methanosarcina acetivorans, Methanosarcina barkeri, Methanococcus jannaschii, and Methanococcus maripaludis. The order of the two genes was identical in the five organisms, and the deduced proteins were very similar, indicating a high degree of conservation of structure and function. Northern blot analysis revealed that the two genes are organized in an operon (termed the abl operon) in M. mazei Gö1. Expression of the abl operon was strictly salt dependent. The abl operon was deleted in the genetically tractable M. maripaludis. ⌬abl mutants of M. maripaludis no longer produced N -acetyl-␤-lysine and were incapable of growth at high salt concentrations, indicating that the abl operon is essential for N -acetyl-␤-lysine synthesis. These experiments revealed the first genes involved in the biosynthesis of compatible solutes in methanogens.Living cells cope with salt stress by accumulation of osmolytes within their cytoplasm; this prevents loss of water and adjusts the turgor of the cell. The extremely halophilic halobacteria (Archaea) and the anaerobic halophilic Haloanaerobiales (Bacteria) accumulate KCl in their cytoplasm to counterbalance external salt (12, 52). However, this strategy, termed the salt-in-cytoplasm strategy, requires far-reaching adaptations of intracellular machineries to high salt concentrations and limits growth to certain salt concentrations (42). The second strategy, which is used by the majority of living cells, is to accumulate small, soluble, organic molecules that are termed compatible solutes. The ability to respond to an osmotic upshock by accumulating compatible solutes is found in all three lines of descent of life (2,17,30,35). However, the spectrum of compatible solutes used comprises only a limited number of compounds, and these can be divided into two major groups: (i) sugars and polyols and (ii) ␣-and ␤-amino acids and their derivatives, including methylamines. This limitation to a rather small number of compounds reflects the fundamental constraints on solutes which are compatible with macromolecular and cellular functions (21).Most archaeal compatible solutes resemble their bacterial counterparts, with the difference that the majority of them carry a negative charge. This anionic character is conferred to the solutes by the addition of a carboxylate, phosphate, or sulfate group (23,35). The compatible solutes accumulated by archaea include, depending on the species and the salt concentration, di-myo-1,1Ј-inositol phosphate, 2-O-D-mannosyl-Dgycerate, diglycerol phosphate, cyclic-2,3-bisphosphoglycerate, ␣-...

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