Krulwich T A, Ito M, Guffanti A A
Department of Biochemistry and Molecular Biology, Mount Sinai School of Medicine, New York, NY 10029, USA.
Biochim Biophys Acta. 2001 May 1;1505(1):158-68. doi: 10.1016/s0005-2728(00)00285-1.
A Na(+) cycle plays a central role in the remarkable capacity of aerobic, extremely alkaliphilic Bacillus species for pH homeostasis. The capacity for pH homeostasis, in turn, appears to set the upper pH limit for growth. One limb of the alkaliphile Na(+) cycle consists of Na(+)/H(+) antiporters that achieve net H(+) accumulation that is coupled to Na(+) efflux. The major antiporter on which pH homeostasis depends is thought to be the Mrp(Sha)-encoded antiporter, first identified from a partial clone in Bacillus halodurans C-125. Mrp(Sha) may function as a complex. While this antiporter is capable of secondary antiport energized by an imposed or respiration-generated protonmotive force, the possibility of a primary mode has not been excluded. In Bacillus pseudofirmus OF4, at least two additional antiporters, including NhaC, have supporting roles in pH homeostasis. Some of these additional antiporters may be especially important for antiport at low [Na(+)] or at near-neutral pH. The second limb of the Na(+) cycle facilitates Na(+) re-entry via Na(+)/solute symporters and, perhaps, the ion channel associated with the Na(+)-dependent flagellar motor. The process of pH homeostasis is also enhanced, perhaps especially during transitions to high pH, by different arrays of secondary cell wall polymers in the two alkaliphilic Bacillus species studied most intensively. The mechanisms whereby alkaliphiles handle the challenge of Na(+) stress at very elevated [Na(+)] are just beginning to be identified, and a hypothesis has been advanced to explain the finding that B. pseudofirmus OF4 requires a higher [Na(+)] for growth at near-neutral pH than at very alkaline pH values.
钠循环在需氧、极端嗜碱芽孢杆菌物种卓越的pH稳态维持能力中起着核心作用。而pH稳态维持能力似乎又设定了生长的pH上限。嗜碱菌钠循环的一个分支由Na⁺/H⁺反向转运蛋白组成，这些蛋白实现了与Na⁺外流偶联的净H⁺积累。pH稳态维持所依赖的主要反向转运蛋白被认为是Mrp(Sha)编码的反向转运蛋白，它最初是从嗜碱芽孢杆菌C - 125的一个部分克隆中鉴定出来的。Mrp(Sha)可能作为一个复合体发挥作用。虽然这种反向转运蛋白能够通过施加的或呼吸产生的质子动力进行次级反向转运，但初级模式的可能性尚未排除。在类芽孢杆菌OF4中，至少还有另外两种反向转运蛋白，包括NhaC，在pH稳态维持中起辅助作用。其中一些额外的反向转运蛋白可能在低[Na⁺]或近中性pH条件下的反向转运中尤为重要。钠循环的第二个分支通过Na⁺/溶质同向转运蛋白，或许还有与Na⁺依赖性鞭毛马达相关的离子通道，促进Na⁺重新进入细胞。在所深入研究的两种嗜碱芽孢杆菌物种中，不同排列的次生细胞壁聚合物也增强了pH稳态维持过程，可能在向高pH转变期间尤为明显。嗜碱菌在非常高的[Na⁺]浓度下应对Na⁺胁迫挑战的机制刚刚开始被确定，并且已经提出了一个假说来解释类芽孢杆菌OF4在近中性pH下生长比在非常碱性的pH值下需要更高[Na⁺]浓度这一发现。
