Making sense of nuclear localization: a zinc-finger protein encoded by a cytoplasmically replicating plant RNA virus acts a transcription factor: a novel function for a member of large family of viral proteins.

Recent studies have uncovered numerous nucleus-localized proteins encoded by plant RNA viruses. Whereas for some of these viruses nuclear (or, more specifically, nucleolar) passage of the proteins is needed for the virus movement within the plant or suppression of host defense, the nuclear function of these proteins remains largely unknown. Recently, the situation has been clarified for one group of plant RNA viruses, the Carlaviruses. Being positive-stranded RNA viruses, carlaviruses multiply exclusively in the cytoplasm. Chrysanthemum virus B (CVB, a carlavirus) encodes a zinc-finger protein p12 targeted to the nucleus in a nuclear localization signal-dependent manner. In a recent work, we demonstrated that p12 directly interacts with chromatin and plant promoters, thus, acts as a eukaryotic transcription factor (TF) and activates expression of a host TF involved in regulation of cell size and proliferation to favor virus infection. Therefore our studies identified a novel nuclear stage of in CVB infection involving modulation of host gene expression and plant development. Whereas it is well established that any RNA virus actively replicating in the cell causes changes in the transcriptome, our study expanded this view by showing that some positive-stranded RNA viruses can directly manipulate host transcription by encoding eukaryotic TFs.