ORIGINS OF THE OSMOPROTECTIVE PROPERTIES OF BETAINE AND PROLINE IN ESCHERICHIA-COLI K-12
ORIGINS OF THE OSMOPROTECTIVE PROPERTIES OF BETAINE AND PROLINE IN ESCHERICHIA-COLI K-12
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DOI：
10.1128/jb.174.5.1586-1595.1992
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发表时间：
1992-03-01
影响因子：
3.2
通讯作者：
RECORD, MT
中科院分区：
文献类型：
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作者：
CAYLEY, S;LEWIS, BA;RECORD, MT
测定了在含有0.5 M NaCl的3-(N-morpholino)丙烷磺酸（MOPS）缓冲葡萄糖-微量培养基中生长的大肠杆菌K-12在存在和不存在渗透保护剂甜菜碱和脯氨酸的情况下的细胞质水和所有具有渗透意义的细胞质溶质的数量。这项工作的目的是将渗透保护剂对细胞质组成的影响与它们提高渗透应激细胞生长速度的能力联系起来。生长培养基中浓度为1mm时，甜菜碱比脯氨酸更能提高生长速率；被大肠杆菌转化为甜菜碱的胆碱似乎对生长速度有中间作用。甜菜碱或脯氨酸的积累减少了细胞质中K+、谷氨酸、海藻糖和MOPS（在没有渗透保护剂的情况下积累的主要细胞质渗透物）的数量，因此在这种外部渗透压下，无论存在或不存在渗透保护剂，细胞质溶质的总量基本上是相同的。甜菜碱比脯氨酸积累得多，因此甜菜碱对细胞质溶质的替代程度大于脯氨酸。这些渗透保护剂的积累伴随着每单位细胞干重（V（细胞）BAR）的细胞质水体积（20%至50%）的大量增加。这种效应似乎是由于自由水V(f)BAR（相对于水合水或结合水）体积的增加，甜菜碱比脯氨酸更大。综上所述，这些结果表明甜菜碱和脯氨酸对水活性和细胞质渗透压的摩尔效应明显大于它们所取代的溶质。Cayley等人（S. Cayley， B. A. Lewis， H. J. Guttman, and M. T. Record, Jr.， J. Mol. Biol. 222:281-300, 1991）观察到，在没有渗透保护剂的细胞中，随着外部渗透压的增加，细胞的生长速度和V（细胞）BAR都降低了，而细胞质K+ (n(K+))的数量增加了。我们预测，观察到的n（K+）和V（细胞）BAR的变化将对基因表达的关键蛋白-核酸相互作用产生巨大的近似补偿作用，我们提出V(f)BAR是渗透胁迫细胞生长速度的基本决定因素。在甜菜碱和脯氨酸存在下培养的细胞的特性与我们以前的工作和建议完全一致。甜菜碱和脯氨酸的积累，在较小程度上，将一组相关的生理参数(n（K+)， V（细胞）BAR，生长速率）转移到没有渗透保护剂的低渗透压条件下的生长特征。讨论了渗透保护剂对V（细胞）BAR和V(f)BAR影响的热力学基础和生理后果的模型。
The amounts of cytoplasmic water and of all osmotically significant cytoplasmic solutes were determined for Escherichia coli K-12 grown in 3-(N-morpholino)propane sulfonate (MOPS)-buffered glucose-minimal medium containing 0.5 M NaCl in the presence and absence of the osmoprotectants betaine and proline. The goal of this work is to correlate the effects of osmoprotectants on the composition of the cytoplasm with their ability to increase the growth rate of osmotically stressed cells. At a concentration of 1 mM in the growth medium, betaine increases the growth rate more than does proline; choline, which is converted to betaine by E. coli, appears to have an intermediate effect on growth rate. The accumulation of either betaine or proline reduces the cytoplasmic amounts of K+, glutamate, trehalose, and MOPS (the major cytoplasmic osmolytes accumulated in the absence of osmoprotectants), so that at this external osmolarity the total amount of cytoplasmic solutes is essentially the same in the presence or absence of either osmoprotectant. More betaine than proline is accumulated, so the extent of replacement of cytoplasmic solutes is greater for betaine than for proline. Accumulation of these osmoprotectants is accompanied by a large (20 to 50%) increase in the volume of cytoplasmic water per unit of cell dry weight (V(cyto)BAR). This effect, which appears to result from an increase in the volume of free water, V(f)BAR (as opposed to water of hydration, or bound water), is greater for betaine than for proline. Taken together, these results indicate that the molar effects of betaine and proline on water activity and on the osmotic pressure of the cytoplasm must be significantly larger than those of solutes they replace. Cayley et al. (S. Cayley, B. A. Lewis, H. J. Guttman, and M. T. Record, Jr., J. Mol. Biol. 222:281-300, 1991) observed that, in cells grown in the absence of osmoprotectants, both growth rate and V(cyto)BAR decreased, whereas the amount of cytoplasmic K+ (n(K+)) increased, with increasing external osmolarity. We predicted that the observed changes in n(K+) and V(cyto)BAR would have large and approximately compensating effects on key protein-nucleic acid interactions of gene expression, and we proposed that V(f)BAR was the fundamental determinant of growth rate in osmotically stressed cells. The properties of cells cultured in the presence of betaine and proline appear completely consistent with our previous work and proposals. Accumulation of betaine and, to a lesser extent, proline shifts the set of linked physiological parameters (n(K+), V(cyto)BAR, growth rate) to those characteristic of growth at lower osmolarity in the absence of osmoprotectants. Models for the thermodynamic basis and physiological consequences of the effect of osmoprotectants on V(cyto)BAR and V(f)BAR are discussed.