Laboratório Nacional de Biociências, Centro Nacional de Pesquisa em Energia e Materiais, CEP 13083-970, Campinas, São Paulo, Brazil.
Plant Physiol. 2013 Sep;163(1):232-42. doi: 10.1104/pp.113.224642. Epub 2013 Jul 29.
Transcription activator-like (TAL) effectors from Xanthomonas species pathogens act as transcription factors in plant cells; however, how TAL effectors activate host transcription is unknown. We found previously that TAL effectors of the citrus canker pathogen Xanthomonas citri, known as PthAs, bind the carboxyl-terminal domain of the sweet orange (Citrus sinensis) RNA polymerase II (Pol II) and inhibit the activity of CsCYP, a cyclophilin associated with the carboxyl-terminal domain of the citrus RNA Pol II that functions as a negative regulator of cell growth. Here, we show that PthA4 specifically interacted with the sweet orange MAF1 (CsMAF1) protein, an RNA polymerase III (Pol III) repressor that controls ribosome biogenesis and cell growth in yeast (Saccharomyces cerevisiae) and human. CsMAF1 bound the human RNA Pol III and rescued the yeast maf1 mutant by repressing tRNA(His) transcription. The expression of PthA4 in the maf1 mutant slightly restored tRNA(His) synthesis, indicating that PthA4 counteracts CsMAF1 activity. In addition, we show that sweet orange RNA interference plants with reduced CsMAF1 levels displayed a dramatic increase in tRNA transcription and a marked phenotype of cell proliferation during canker formation. Conversely, CsMAF1 overexpression was detrimental to seedling growth, inhibited tRNA synthesis, and attenuated canker development. Furthermore, we found that PthA4 is required to elicit cankers in sweet orange leaves and that depletion of CsMAF1 in X. citri-infected tissues correlates with the development of hyperplastic lesions and the presence of PthA4. Considering that CsMAF1 and CsCYP function as canker suppressors in sweet orange, our data indicate that TAL effectors from X. citri target negative regulators of RNA Pol II and Pol III to coordinately increase the transcription of host genes involved in ribosome biogenesis and cell proliferation.
黄单胞菌属病原菌中的转录激活子样效应物（TAL）在植物细胞中作为转录因子发挥作用；然而，TAL 效应物如何激活宿主转录尚不清楚。我们之前发现，柑橘溃疡病菌（Xanthomonas citri）的 TAL 效应物 PthAs 与甜橙（Citrus sinensis）RNA 聚合酶 II（Pol II）的羧基末端结构域结合，并抑制 CsCYP 的活性，后者与柑橘 RNA Pol II 的羧基末端结构域相关联，作为细胞生长的负调控因子。在这里，我们表明 PthA4 特异性地与甜橙 MAF1（CsMAF1）蛋白相互作用，后者是一种 RNA 聚合酶 III（Pol III）抑制剂，在酵母（酿酒酵母）和人类中控制核糖体生物发生和细胞生长。CsMAF1 结合人类 RNA Pol III，并通过抑制 tRNA（His）转录来拯救 maf1 突变体酵母。在 maf1 突变体中表达 PthA4 略微恢复了 tRNA（His）合成，表明 PthA4 拮抗 CsMAF1 活性。此外，我们表明，甜橙 RNA 干扰植物中 CsMAF1 水平降低会导致 tRNA 转录显著增加，并在溃疡形成过程中表现出明显的细胞增殖表型。相反，CsMAF1 过表达对幼苗生长有害，抑制 tRNA 合成并减弱溃疡发展。此外，我们发现 PthA4 是在甜橙叶片中引发溃疡所必需的，并且在 X. citri 感染组织中 CsMAF1 的耗竭与过度增生病变的发育和 PthA4 的存在相关。考虑到 CsMAF1 和 CsCYP 在甜橙中作为溃疡抑制因子发挥作用，我们的数据表明，来自 X. citri 的 TAL 效应物靶向 RNA Pol II 和 Pol III 的负调节剂，以协调增加参与核糖体生物发生和细胞增殖的宿主基因的转录。
