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The Mungbean Yellow Mosaic Begomovirus Transcriptional Activator Protein Transactivates the Viral Promoter-Driven Transgene and Causes Toxicity in Transgenic Tobacco Plants

The Begomovirus transcriptional activator protein (TrAP/AC2/C2) is a multifunctional protein which activates the viral late gene promoters, suppresses gene silencing, and determines pathogenicity. To study TrAP-mediated transactivation of a stably integrated gene, we generated transgenic tobacco plants with a Mungbean yellow mosaic virus (MYMV) AV1 late gene promoter-driven reporter gene and supertransformed them with the MYMV TrAP gene driven by a strong 35S promoter. We obtained a single supertransformed plant with an intact 35S-TrAP gene that activated the reporter gene 2.5-fold. However, 10 of the 11 supertransformed plants did not have the TrAP region of the T-DNA, suggesting the likely toxicity of TrAP in plants. Upon transformation of wild-type tobacco plants with the TrAP gene, six of the seven transgenic plants obtained had truncated T-DNAs which lacked TrAP. One plant, which had the intact TrAP gene, did not express TrAP. The apparent toxic effect of the TrAP transgene was abolished by mutations in its nuclear-localization signal or zinc-finger domain and by deletion of its activation domain. Therefore, all three domains of TrAP, which are required for transactivation and suppression of gene silencing, also are needed for its toxic effect.