In the model organism Bacillus subtilis, a signaling protease produced in the forespore, SpoIVB, is essential for the activation of the sigma factor σK, which is produced in the mother cell as an inactive pro‐protein, pro‐σK. SpoIVB has a second function essential to sporulation, most likely during cortex synthesis. The cortex is composed of peptidoglycan (PG) and is essential for the spore's heat resistance and dormancy. Surprisingly, the genome of the intestinal pathogen Clostridioides difficile, in which σK is produced without a pro‐sequence, encodes two SpoIVB paralogs, SpoIVB1 and SpoIVB2. Here, we show that spoIVB1 is dispensable for sporulation, while a spoIVB2 in‐frame deletion mutant fails to produce heat‐resistant spores. The spoIVB2 mutant enters sporulation, undergoes asymmetric division, and completes engulfment of the forespore by the mother cell but fails to synthesize the spore cortex. We show that SpoIIP, a PG hydrolase and part of the engulfasome, the machinery essential for engulfment, is cleaved by SpoIVB2 into an inactive form. Within the engulfasome, the SpoIIP amidase activity generates the substrates for the SpoIID lytic transglycosylase. Thus, following engulfment completion, the cleavage and inactivation of SpoIIP by SpoIVB2 curtails the engulfasome hydrolytic activity, at a time when synthesis of the spore cortex peptidoglycan begins. SpoIVB2 is also required for normal late gene expression in the forespore by a currently unknown mechanism. Together, these observations suggest a role for SpoIVB2 in coordinating late morphological and gene expression events between the forespore and the mother cell.

中文翻译：

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艰难梭菌中孢子形成特征蛋白酶对吞噬肽聚糖水解酶的裂解


在模型生物枯草芽孢杆菌中，前孢子中产生的信号蛋白酶 SpoIVB 对于激活 σK 因子至关重要，σK 因子在母细胞中作为无活性的前蛋白 pro-σK 产生。 SpoIVB 具有对孢子形成至关重要的第二个功能，很可能是在皮质合成过程中。皮层由肽聚糖（PG）组成，对于孢子的耐热性和休眠至关重要。令人惊讶的是，肠道病原体艰难梭菌的基因组编码两个 SpoIVB 旁系同源物：SpoIVB1 和 SpoIVB2，其中 σK 在没有原序列的情况下产生。在这里，我们证明 spoIVB1 对于孢子形成是可有可无的，而 spoIVB2 框内缺失突变体无法产生耐热孢子。 spoIVB2突变体进入孢子形成，经历不对称分裂，并完成母细胞对前孢子的吞噬，但无法合成孢子皮层。我们发现，SpoIIP（一种 PG 水解酶）和吞噬体的一部分（吞噬所必需的机制）被 SpoIVB2 裂解为无活性形式。在吞噬体内，SpoIIP 酰胺酶活性产生 SpoIID 裂解转糖基酶的底物。因此，吞噬完成后，当孢子皮层肽聚糖开始合成时，SpoIVB2 对 SpoIIP 的裂解和失活会减少吞噬体的水解活性。 SpoIVB2 也是前孢子中正常晚期基因表达所必需的，其机制目前未知。总之，这些观察结果表明 SpoIVB2 在协调前孢子和母细胞之间的晚期形态和基因表达事件中发挥作用。