Kappes Rainer M, Bremer Erhard
Philipps University Marburg, Department of Biology, Laboratory for Microbiology, Karl-von-Frisch Straße, D-35032 Marburg, Germany.
Microbiology (Reading). 1998 Jan;144(1):83-90. doi: 10.1099/00221287-144-1-83.
It was found that low concentrations of the naturally occurring and structurally related betaines L-carnitine, crotonobetaine and γ-butyrobetaine conferred a high degree of osmotic tolerance to Kinetic analysis of L-[ C]carnitine uptake in cells grown in minimal medium revealed the presence of a high-affinity transport system with a value of 5 μM and a maximum rate of transport ( ) of 41 nmol min (mg protein). A rise in medium osmolarity moderately increased the maximum velocity [ 71 nmol min (mg protein)] of this transport system, but had little effect on its affinity. Growth and transport studies with a set of strains that carried defined mutations in the previously identified glycine betaine transport systems OpuA, OpuC and OpuD allowed the identification of the ATP-binding cassette (ABC) transport system OpuC as the only uptake route for L-carnitine in Competition experiments with crotonobetaine and γ-butyrobetaine revealed that the OpuC system also exhibited a high affinity for these trimethylammonium compounds with values of 6.4 μM. Tracer experiments with radiolabelled L-carnitine and C-NMR tracings of cell extracts demonstrated that these betaines are accumulated by in an unmodified form. In contrast, the β-substituted acylcarnitine esters acetylcarnitine and octanoylcarnitine both functioned as osmoprotectants for but were found to be accumulated as carnitine by the cells. None of these trimethylammonium compounds were used as sole carbon or nitrogen sources. The results thus characterize L-carnitine, crotonobetaine and γ-butyrobetaine as effective compatible solutes for and establish a crucial role of the ABC transport system OpuC for the supply of with a variety of osmoprotectants.
研究发现，天然存在且结构相关的甜菜碱L-肉碱、巴豆甜菜碱和γ-丁酸甜菜碱的低浓度赋予了 高度的渗透耐受性。对在基本培养基中生长的细胞摄取L-[ C]肉碱的动力学分析表明，存在一种高亲和力转运系统，其 值为5 μM，最大转运速率（ ）为41 nmol·min (mg蛋白质)。培养基渗透压的升高适度增加了该转运系统的最大速度[71 nmol·min (mg蛋白质)]，但对其亲和力影响不大。对一组在先前鉴定的甘氨酸甜菜碱转运系统OpuA、OpuC和OpuD中携带特定突变的菌株进行的生长和转运研究表明，ATP结合盒（ABC）转运系统OpuC是 中L-肉碱的唯一摄取途径。用巴豆甜菜碱和γ-丁酸甜菜碱进行的竞争实验表明，OpuC系统对这些三甲铵化合物也表现出高亲和力，其 值为6.4 μM。用放射性标记的L-肉碱进行的示踪实验和细胞提取物的C-NMR追踪表明，这些甜菜碱以未修饰的形式被 积累。相比之下，β-取代的酰基肉碱酯乙酰肉碱和辛酰肉碱对 均起渗透保护剂的作用，但发现它们被细胞作为肉碱积累。这些三甲铵化合物均未用作唯一的碳源或氮源。因此，这些结果将L-肉碱、巴豆甜菜碱和γ-丁酸甜菜碱表征为 有效的相容性溶质，并确立了ABC转运系统OpuC在为 提供多种渗透保护剂方面的关键作用。
