Sowers K R, Robertson D E, Noll D, Gunsalus R P, Roberts M F
Department of Microbiology, University of California-Los Angeles 90024.
Proc Natl Acad Sci U S A. 1990 Dec;87(23):9083-7. doi: 10.1073/pnas.87.23.9083.
Methanosarcina thermophila, a nonmarine methanogenic archaebacterium, can grow in a range of saline concentrations. At less than 0.4 M NaCl, Ms. thermophila accumulated glutamate in response to increasing osmotic stress. At greater than 0.4 M NaCl, this organism synthesized a modified beta-amino acid that was identified as N epsilon-acetyl-beta-lysine by NMR spectroscopy and ion-exchange HPLC. This beta-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, N epsilon-acetyl-beta-lysine synthesis was repressed and glycine betaine was accumulated. N epsilon-acetyl-beta-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 epsilon-acetyl-beta-lysine in response to extracellular solute concentration indicates that the methanogenic archaebacteria have a unique beta-amino acid biosynthetic pathway that is osmotically regulated.
嗜热甲烷八叠球菌是一种非海洋产甲烷古细菌，能在一定范围的盐浓度中生长。在氯化钠浓度低于0.4M时，嗜热甲烷八叠球菌会随着渗透胁迫的增加而积累谷氨酸。在氯化钠浓度高于0.4M时，这种生物体合成了一种修饰的β-氨基酸，通过核磁共振光谱和离子交换高效液相色谱法鉴定为Nε-乙酰基-β-赖氨酸。这种β-氨基酸衍生物在嗜热甲烷八叠球菌以及另一种被检测的产甲烷菌——海洋物种卡里亚产甲烷菌中积累到高细胞内浓度（高达0.6M）。该化合物具有相容性溶质的特征：在生理pH值下呈中性电荷且高度可溶。当细胞在生理相容性溶质外源性甘氨酸甜菜碱存在的情况下生长时，Nε-乙酰基-β-赖氨酸的合成受到抑制，甘氨酸甜菜碱得以积累。当在高溶质浓度下生长时，来自三个系统发育家族的物种都会合成Nε-乙酰基-β-赖氨酸，这表明它可能在产甲烷菌中普遍存在。响应细胞外溶质浓度控制Nε-乙酰基-β-赖氨酸生物合成的能力表明，产甲烷古细菌具有独特的、受渗透压调节的β-氨基酸生物合成途径。
