A Common Repression Strategy Directs Late Viral Gene Expression in Two Geminiviruses
Tomato Golden Mosaic Virus (TGMV) has become a model geminivirus for studying transcriptional regulation in plants. The promoter of the gene required for expression of the Coat Protein (CP) has been shown to be activated by a second viral protein, AL2, in mesophyll tissue and to be de-repressed by AL2 in phloem. The promoter for the BR1 movement protein also requires AL2 as an activator in mesophyll tissue but appears to be constitutively expressed in phloem tissue as determined by promoter-reporter gene assays containing core sequences required for BR1 promoter activity. Interestingly, similar results were obtained for the CP promoter of Spinach Curly Top Virus (SCTV), which demonstrated phloem specific expression as observed for the TGMV CP promoter. However no expression was ever observed in tissues outside the phloem. The phloem-specific expression was repressed by sequences within the SCTV viral genome and was reversible in the presence of the SCTV C2 protein. Based on these results we speculate that repression in phloem is a common strategy for the temporal regulation of the CP gene in geminiviruses and that mesophyll is an acquired phenotype seen only in the bipartite begomoviruses.