College of Plant Protection (Laboratory of Plant Immunity; Key Laboratory of Integrated Management of Crop Diseases and Pests), Nanjing Agricultural University, Nanjing 210095, China.
Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan.
Biomolecules. 2020 May 7;10(5):733. doi: 10.3390/biom10050733.
The bacterial flagellum is one of the best-studied surface-attached appendages in bacteria. Flagellarassembly in vivo is promoted by its own protein export apparatus, a type III secretion system (T3SS) in pathogenic bacteria. OH11 is a non-flagellated soil bacterium that utilizes type IV pilus (T4P)-driven twitching motility to prey upon nearby fungi for food. Interestingly, the strain OH11 encodes components homologous to the flagellar type III protein apparatus (FT3SS) on its genome, but it remains unknown whether this FT3SS-like system is functional. Here, we report that, despite the absence of flagella, the FT3SS homologous genes are responsible not only for the export of the heterologous flagellin in strain OH11 but also for twitching motility. Blocking the FT3SS-like system by in-frame deletion mutations in either or abolished the secretion of heterologous flagellin moleculesinto the culture medium, indicating that the FT3SS is functional in strain OH11. A deletion of , , , or inhibited T4P-driven twitching motility, whereas neither that of nor did, suggesting that FlhA, FlhB, FliI, and FliR may obtain a novel function to modulate the twitching motility. The flagellar FliI ATPase was required for the secretion of the major pilus subunit, PilA, suggesting that FliI would have evolved to act as a PilB-like pilus ATPase. These observations lead to a plausible hypothesis that the non-flagellated OH11 could preserve FT3SS-like genes for acquiring a distinct function to regulate twitching motility associated with its predatory behavior.
细菌鞭毛是细菌中研究得最好的表面附着附属物之一。细菌体内的鞭毛组装由其自身的蛋白质输出装置（III 型分泌系统，T3SS）促进。OH11 是一种非鞭毛土壤细菌，利用 IV 型菌毛（T4P）驱动的蠕动运动捕食附近的真菌作为食物。有趣的是，该菌株 OH11 基因组编码的成分与鞭毛 III 型蛋白装置（FT3SS）同源，但尚不清楚该 FT3SS 样系统是否具有功能。在这里，我们报告说，尽管没有鞭毛，但 FT3SS 同源基因不仅负责在 OH11 菌株中异源鞭毛蛋白的输出，而且还负责蠕动运动。通过在 或 中进行框内缺失突变来阻断 FT3SS 样系统，完全消除了异源鞭毛分子分泌到培养基中的能力，表明 FT3SS 在 OH11 菌株中是有功能的。 缺失 、 、 或 抑制了 T4P 驱动的蠕动运动，而缺失 或 则没有，这表明 FlhA、FlhB、FliI 和 FliR 可能获得了一种新的功能来调节蠕动运动。鞭毛 FliI ATP 酶是主要菌毛亚基 PilA 分泌所必需的，这表明 FliI 可能已经进化为一种类似于 PilB 的菌毛 ATP 酶。这些观察结果导致了一个合理的假设，即无鞭毛的 OH11 可以保留 FT3SS 样基因，以获得一种调节与捕食行为相关的蠕动运动的独特功能。
