The genus Azotobacter, belonging to the Pseudomonadaceae family, is characterized by the formation of cysts, which are metabolically dormant cells produced under adverse conditions and able to resist desiccation. Although this developmental process has served as a model for the study of cell differentiation in Gram-negative bacteria, the molecular basis of its regulation is still poorly understood. Here, we report that the ubiquitous second messenger cyclic dimeric GMP (c-di-GMP) is critical for the formation of cysts in Azotobacter vinelandii. Upon encystment induction, the levels of c-di-GMP increased, reaching a peak within the first 6 h. In the absence of the diguanylate cyclase MucR, however, the levels of this second messenger remained low throughout the developmental process. A. vinelandii cysts are surrounded by two alginate layers with variable proportions of guluronic residues, which are introduced into the final alginate chain by extracellular mannuronic C-5 epimerases of the AlgE1 to AlgE7 family. Unlike in Pseudomonas aeruginosa, MucR was not required for alginate polymerization in A. vinelandii. Conversely, MucR was necessary for the expression of extracellular alginate C-5 epimerases; therefore, the MucR-deficient strain produced cyst-like structures devoid of the alginate capsule and unable to resist desiccation. Expression of mucR was partially dependent on the response regulator AlgR, which binds to two sites in the mucR promoter, enhancing mucR transcription. Together, these results indicate that the developmental process of A. vinelandii is controlled through a signaling module that involves activation by the response regulator AlgR and c-di-GMP accumulation that depends on MucR.

中文翻译：

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细胞外甘露聚糖 C-5 差向异构酶的循环 di-GMP 介导的调节对于 vinelandii 中的囊肿形成至关重要

固氮杆菌属属于假单胞菌科，其特征是形成包囊，这是在不利条件下产生的代谢休眠细胞，能够抵抗干燥。尽管这一发育过程已成为研究革兰氏阴性菌细胞分化的模型，但对其调控的分子基础仍知之甚少。在这里，我们报告普遍存在的第二信使环状二聚体 GMP (c-di-GMP) 对Azotobacter vinelandii中包囊的形成至关重要. 在包囊诱导后，c-di-GMP 水平增加，在前 6 小时内达到峰值。然而，在没有二鸟苷酸环化酶 MucR 的情况下，这个第二信使的水平在整个发育过程中仍然很低。A. vinelandii囊肿被两个具有不同比例的古洛糖醛酸残基的藻酸盐层包围，它们通过 AlgE1 至 AlgE7 家族的细胞外甘露糖醛 C-5 差向异构酶引入最终的藻酸盐链。与铜绿假单胞菌不同， A. vinelandii中的藻酸盐聚合不需要 MucR. 相反，MucR 是细胞外藻酸盐 C-5 差向异构酶表达所必需的。因此，MucR 缺陷菌株产生了没有藻酸盐胶囊且无法抵抗干燥的囊状结构。mucR的表达部分依赖于反应调节因子 AlgR，它与mucR启动子中的两个位点结合，增强mucR转录。总之，这些结果表明A. vinelandii的发育过程是通过一个信号模块控制的，该模块涉及响应调节器 AlgR 的激活和依赖于 MucR 的 c-di-GMP 积累。