Dürre Peter, Hollergschwandner Concha
Mikrobiologie und Biotechnologie, Universität Ulm, 89069 Ulm, Germany.
Anaerobe. 2004 Apr;10(2):69-74. doi: 10.1016/j.anaerobe.2003.11.001.
Endospore formation in bacilli and clostridia shows remarkable similarities in morphology as well as in physiological and molecular biological cellular events. Major differences are the formation of clostridial stage cells and granulose accumulation in clostridia. In both genera, a cascade of sigma factors is activated after septation (by help of sigma(H) and Spo0A approximately P) in the sequence sigma(F), sigma(E), sigma(G), and sigma(K). Of these, sigma(F) and sigma(G) are active inside the forespore and are regulated by anti-sigma factors and anti-anti-sigma factors, whereas sigma(E) and sigma(K) (mother cell-specific sigma factors) are synthesized as precursor proteins and activated by proteolysis. Each of these sigma factors allows transcription of a specific set of genes and operons, thus leading to the orchestral expression of stage-specific proteins required for successful sporulation. Both, the genetic organization of the respective operons and the expression pattern of the sigma factors are very similar in Bacillus subtilis and Clostridium acetobutylicum, the model organisms of the two genera. However, a major regulatory difference is found in initiation of endospore formation. Genome sequencing revealed that clostridia do not contain components of the so-called phosphorelay, with the exception of the essential transcription factor Spo0A. This might reflect recognition of different environmental signals, as for clostridia nutrient limitation is no prerequisite for sporulation. In contrast to Bacillus, the clostridial sigH gene is constitutively expressed at a low level, with no increase at the onset of spore formation. The spo0A gene in C. acetobutylicum is also constitutively expressed, but Spo0A synthesis only occurs during the early and mid-exponential growth phase, indicating a posttranscriptional or cotranslational regulation. Mutational studies revealed an important regulatory function of a dual palindrome region upstream of the spo0A gene of C. acetobutylicum, part of which overlaps with a Spo0A-binding site. In addition to controlling sporulation genes, phosphorylated clostridial Spo0A is involved in regulation of acetone and butanol synthesis.
芽孢杆菌属和梭菌属中的芽孢形成在形态以及生理和分子生物学细胞事件方面表现出显著的相似性。主要差异在于梭菌阶段细胞的形成以及梭菌属中颗粒质的积累。在这两个属中，隔膜形成后（借助σ(H)和磷酸化的Spo0A），一系列σ因子按σ(F)、σ(E)、σ(G)和σ(K)的顺序被激活。其中，σ(F)和σ(G)在前芽孢内部具有活性，并受抗σ因子和抗抗σ因子调控，而σ(E)和σ(K)（母细胞特异性σ因子）以前体蛋白形式合成，并通过蛋白水解被激活。这些σ因子中的每一个都允许特定一组基因和操纵子进行转录，从而导致成功形成芽孢所需的阶段特异性蛋白的协调表达。芽孢杆菌属的枯草芽孢杆菌和梭菌属的丙酮丁醇梭菌这两种模式生物中，各自操纵子的遗传组织和σ因子的表达模式非常相似。然而，在芽孢形成的起始阶段发现了一个主要的调控差异。基因组测序表明，除了必需的转录因子Spo0A外，梭菌属不包含所谓磷酸化信号转导途径的成分。这可能反映了对不同环境信号的识别，因为对于梭菌属来说，营养限制不是芽孢形成的先决条件。与芽孢杆菌属不同 的是，梭菌属的sigH基因以低水平组成型表达，在芽孢形成开始时没有增加。丙酮丁醇梭菌中的spo0A基因也组成型表达，但Spo0A的合成仅发生在指数生长早期和中期，这表明存在转录后或共翻译调控。突变研究揭示了丙酮丁醇梭菌spo0A基因上游双回文区域的重要调控功能，其中一部分与Spo0A结合位点重叠。除了控制芽孢形成基因外，磷酸化的梭菌属Spo0A还参与丙酮和丁醇合成的调控。
