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標題: Tzs蛋白質不同功能域缺失對農桿菌致病基因表現、細菌生長與致病性之影響
Deletions of various domains of the Tzs protein affect Agrobacterium tumefaciens virulence gene expressions, bacterial growth, and virulence
作者: Jin-Yi Dong
關鍵字: 農桿菌
teans zeatin
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摘要: The Agrobacterium tumefaciens is a plant pathogen and causes crown gall disease due to the transfer and integration of the T-DNA (transfer-DNA) from the Ti plasmid (tumor-inducing plasmid) and expression of auxin and cytokinin biosynthesis genes in infected plant cells. The nopaline-type A. tumefaciens has a Tzs (trans-zeatin synthesizing) gene in the Ti plasmid. The Tzs gene shares high sequence similarity with the IPT gene in the T-DNA region of the Ti plasmid. The Tzs gene expression is regulated by the VirA/VirG two-component system and is induced by the phenolic compounds, such as acetosyringone (AS). The Tzs gene encodes a cytokinin biosynthetic prenyl transferase. Recent studies demonstrated that the tzs deletion mutant decreased its virulence and transient transformation efficiency on Arabidopsis roots, suggesting that Tzs might be involved in step(s) prior to T-DNA integrations. Interestingly, the tzs deletion mutant contains higher Vir (virulence) gene expressions and Vir protein accumulations during AS inductions. In order to elucidate further the potential role(s) of Tzs and cytokinin (trans-zeatin) in A. tumefaciens, we generated several A. tumefaciens mutants expressing the Tzs protein with the nonsense muttations, N-terminal, C-terminal deletions, or amino acid substitution mutations. The amounts of secreted trans-zeatin inall the tzs mutants generated in this study were not detected by the HPLC (High Performance Liquid Chromatography) analysis. The VirA and VirB promoter activities, transient formation and tumorigenesis efficiencies were decreased in the tzs mutants with amino acid substitutions, C-termianl, or N-terminal deletion. The decreased percentages of bacterial growth rates, transient transformation and turmorgenesis efficiencies in the various tzs mutants were in positively correlation with the deletion length of the Tzs gene. Additionally, the Tzs promoter activities were significantly decreased in the virA and virG mutants. The Tzs promoter activities were increased in the virB, virB3, and virC mutants; whereas the promoter activity was decreased in the virE mutant, suggesting the loss of functions of the VirB, VirC, and VirE affect the Tzs promoter activity. The Tzs protein accumulation levels were lower in the chvG, chvI, and chvG/exoR mutans, suggesting the Tzs protein expression is regulated by the ChvG/ChvI two-component system. In summary, the data shown in this study demonstrated that deletions of various domains of the Tzs protein affect the Tzs protein functions, may therefore affect bacterial growth, Vir gene expressions, and virulence of Agrobacterium tumefaciens.
農桿菌為一種存在於土壤的植物病原菌,感染植物時會將菌體內腫瘤誘導質體(Ti plasmid)上特定的T-DNA片段(transferred-DNA)跨界送入宿主植物細胞內,同時T-DNA嵌入植物染色體後,誘發植物細胞不正常的大量合成植物生長素與細胞分裂素,導致植物產生冠癭腫瘤。農桿菌可依使用碳氮營養來源種類不同分為數個品系,其中在nopaline品系的農桿菌,其Ti質體上具備Tzs (trans-zeatin synthesizing)基因,可編碼產生反玉米素生合成酶。過去研究已知該基因與植物細胞分裂素之催化酶-ipt (isopentenyl transferase)基因序列有高度相似性;另Tzs基因於農桿菌體內會持續低量表現,可被植物傷口分泌的酚類化合物(acetosyringone,AS)誘導大量表現,並受農桿菌中的VirA/VirG 雙分子調控系統所調控,農桿菌中其他相關致病(Vir)基因也受此系統所調控,並參與農桿菌感染植物之過程。Tzs已知可與第四型分泌系統(T4SS)組成蛋白VirB5結合,並運送至細胞膜上。已知酸性(pH 5.5)低溫(19℃)培養條件下,外添加AS處理可提高tzs突變株Vir啟動子活性,卻因大量累積致病(Vir)蛋白質而造成生長速率遲緩,導致短暫表現T-DNA能力與腫瘤形成效率亦低於野生株,推Tzs基因可能參與農桿菌感染過程的早期。本研究嘗試分析上述突變株性狀是否因Tzs基因功能片段缺失、缺乏Tzs蛋白質功能或缺乏細胞分裂素所致,因此藉由Tzs蛋白質結構分析建構了tzs提早終止突變株、三株胺基酸替換突變株、N端及C端缺失突變株並檢測相關性狀;結果顯示所有突變株內皆無反玉米素生成。tzs提早終止突變株、胺基酸替換突變株與野生株相比VirA、VirB啟動子活性、短暫表現T-DNA效率、腫瘤形成效率皆下降。而Tzs基因功能缺失株中,短暫表現T-DNA之效率、腫瘤形成效率及生長速率會隨tzs缺失片段提升而影響加劇。另為了釐清Vir基因與Tzs相互影響之關係,因此藉由各式vir突變株,檢測其Tzs啟動子活性,發現virA與virG突變株中Tzs啟動子活性極低。而virB、virB3與virC突變株中Tzs啟動子活性會增強,但在virE突變株中則會降低,可知確實VirA/VirG為Tzs基因調控上游,且當virB、virC、virE基因功能喪失時,會影響Tzs啟動子活性。除此之外在酸性環境外添加AS處理下,nopaline品系農桿菌有較高的細胞分裂素合成,而農桿菌的酸性調控主要是藉由ChvG/ChvI雙分子系統調控,並在chvG、chvI、chvG/exoR突變株中發現Tzs蛋白質累積量皆比野生株低,由此得知Tzs蛋白質累積量也會受此雙分子系統調控。綜合上述可知VirA/VirG 雙分子調控系統為調控Tzs基因之上游,另ChvG/ChvI雙分子系統也會參與其中,且Tzs基因功能不同區域缺失會使Tzs蛋白質功能受影響,使菌體生長較差且會影響Vir基因表現,導致其致病力受影響,進而影響農桿菌感染過程。
文章公開時間: 2018-06-15
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