Mitchell W J
Department of Biological Sciences, Heriot-Watt University, Edinburgh, UK.
Adv Microb Physiol. 1998;39:31-130. doi: 10.1016/s0065-2911(08)60015-6.
The solvent-forming clostridia have attracted interest because of their ability to convert a range of carbohydrates to end-products such as acetone, butanol and ethanol. Polymeric substrates such as cellulose, hemicellulose and starch are degraded by extracellular enzymes. The majority of cellulolytic clostridia, typified by Clostridium thermocellum, produce a multi-enzyme cellulase complex in which the organization of components is critical for activity against the crystalline substrate. A variety of enzymes involved in degradation of hemicellulose and starch have been identified in different strains. The products of degradation, and other soluble substrates, are accumulated via membrane-bound transport systems which are generally poorly characterized. It is clear, however, that the phosphoenolpyruvate-dependent phosphotransferase system (PTS) plays a major role in solute uptake in several species. Accumulated substrates are converted by intracellular enzymes to end-products characteristic of the organism, with production of ATP to support growth. The metabolic pathways have been described, but understanding of mechanisms of regulation of metabolism is incomplete. Synthesis of extracellular enzymes and membrane-bound transport systems is commonly subject to catabolite repression in the presence of a readily metabolized source of carbon and energy. While many genes encoding cellulases, xylanases and amylases have been cloned and sequenced, little is known of control of their expression. Although the mechanism of catabolite repression in clostridia is not understood, some recent findings implicate a role for the PTS as in other low G-C Gram-positive bacteria. Emphasis has been placed on describing the mechanisms underlying the switch of C. acetobutylicum fermentations from acidogenic to solventogenic metabolism at the end of the growth phase. Factors involved include a lowered pH and accumulation of undissociated butyric acid, intracellular concentration of ATP and reduced pyridine nucleotides, nutrient limitation, and the interplay between pathways of carbon and electron flow. Genes encoding enzymes of solvent pathways have been cloned and sequenced, and their expression correlated with the pattern of end-product formation in fermentations. There is evidence that the initiation of solvent formation may be subject to control mechanisms similar to other stationary-phase phenomena, including sporulation. The application of recently developed techniques for genetic manipulation of the bacterium is improving understanding of the regulatory circuits, but a complete molecular description of the control of solvent formation remains elusive. Experimental manipulation of the pathways of electron flow in other species has been shown to influence the range and yield of fermentation end-products. Acid-forming clostridia can, under appropriate conditions, be induced to form atypical solvents as products. While the mechanisms of regulation of gene expression are not at all understood, the capacity to adapt in this way further illustrates the metabolic flexibility of clostridial strains.
能形成溶剂的梭菌因其能将多种碳水化合物转化为丙酮、丁醇和乙醇等终产物而备受关注。纤维素、半纤维素和淀粉等聚合底物会被胞外酶降解。以嗜热栖热菌为代表的大多数纤维素分解梭菌会产生一种多酶纤维素酶复合物，其中各组分的组织方式对于其作用于结晶底物的活性至关重要。在不同菌株中已鉴定出多种参与半纤维素和淀粉降解的酶。降解产物及其他可溶性底物通过膜结合转运系统进行积累，而这些系统通常特征不明。然而，很明显磷酸烯醇式丙酮酸依赖性磷酸转移酶系统（PTS）在多个菌种的溶质摄取中起主要作用。积累的底物被胞内酶转化为该生物体特有的终产物，并产生ATP以支持生长。代谢途径已被描述，但对代谢调控机制的理解并不完整。在存在易于代谢的碳源和能源时，胞外酶和膜结合转运系统的合成通常会受到分解代谢物阻遏。虽然许多编码纤维素酶、木聚糖酶和淀粉酶的基因已被克隆和测序，但对其表达调控知之甚少。尽管梭菌中分解代谢物阻遏的机制尚不清楚，但最近的一些发现表明PTS在其中发挥作用，如同在其他低G-C含量革兰氏阳性细菌中一样。重点已放在描述丙酮丁醇梭菌在生长阶段末期发酵从产酸代谢转变为产溶剂代谢的潜在机制上。涉及的因素包括pH降低、未解离丁酸的积累、ATP的胞内浓度以及还原型吡啶核苷酸、营养限制，以及碳流和电子流途径之间的相互作用。编码溶剂途径酶的基因已被克隆和测序，其表达与发酵中终产物形成模式相关。有证据表明溶剂形成的起始可能受类似于其他稳定期现象（包括孢子形成）的控制机制调控。最近开发的用于该细菌基因操作的技术正在增进对调控回路的理解，但对溶剂形成控制的完整分子描述仍然难以捉摸。在其他菌种中对电子流途径进行实验操作已表明会影响发酵终产物的范围和产量。在适当条件下，产酸梭菌可被诱导形成非典型溶剂作为产物。虽然对基因表达调控机制完全不了解，但以这种方式适应的能力进一步说明了梭菌菌株的代谢灵活性。
