Li H W, Lucy A P, Guo H S, Li W X, Ji L H, Wong S M, Ding S W
Molecular Virology Laboratory, Institute of Molecular Agrobiology, The National University of Singapore, 1 Research Link, Singapore 117604.
EMBO J. 1999 May 17;18(10):2683-91. doi: 10.1093/emboj/18.10.2683.
The 2b protein encoded by cucumber mosaic cucumovirus (Cmv2b) acts as an important virulence determinant by suppressing post-transcriptional gene silencing (PTGS), a natural plant defence mechanism against viruses. We report here that the tomato aspermy cucumovirus 2b protein (Tav2b), when expressed from the unrelated tobacco mosaic tobamovirus (TMV) RNA genome, activates strong host resistance responses to TMV in tobacco which are typical of the gene-for-gene disease resistance mechanism. Domain swapping between Cmv2b, which does not elicit these responses, and Tav2b, revealed functional domains in Tav2b critical for triggering virus resistance and hypersensitive cell death. Furthermore, substitution of two amino acids from Tav2b by those found at the same positions in Cmv2b, Lys21-->Val and Arg28-->Ser, abolished the ability to induce hypersensitive cell death and virus resistance. However, in Nicotiana benthamiana, a species related to tobacco, Tav2b functions as a virulence determinant and suppresses PTGS. Thus, a viral suppressor of the host gene silencing defence mechanism is the target of another independent host resistance mechanism. Our results provide new insights into the complex molecular strategies employed by viruses and their hosts for defence, counter-defence and counter counter-defence.
黄瓜花叶黄瓜病毒（Cmv2b）编码的2b蛋白通过抑制转录后基因沉默（PTGS）发挥重要的毒力决定因素作用，PTGS是植物抵御病毒的一种天然防御机制。我们在此报告，番茄不孕黄瓜病毒2b蛋白（Tav2b）从无关的烟草花叶烟草病毒（TMV）RNA基因组表达时，会在烟草中激活对TMV的强烈宿主抗性反应，这是典型的基因对基因抗病机制。不引发这些反应的Cmv2b与Tav2b之间的结构域交换，揭示了Tav2b中对触发病毒抗性和过敏细胞死亡至关重要的功能结构域。此外，将Tav2b的两个氨基酸替换为Cmv2b中相同位置的氨基酸，即赖氨酸21→缬氨酸和精氨酸28→丝氨酸，消除了诱导过敏细胞死亡和病毒抗性的能力。然而，在与烟草相关的本氏烟草中，Tav2b作为毒力决定因素发挥作用并抑制PTGS。因此，宿主基因沉默防御机制的一种病毒抑制因子是另一种独立宿主抗性机制的靶点。我们的结果为病毒及其宿主用于防御、反防御和反反防御的复杂分子策略提供了新的见解。
