REGULATION OF A GEMINIVIRUS COAT PROTEIN PROMOTER BY AL2 PROTEIN (TRAP) - EVIDENCE FOR ACTIVATION AND DEREPRESSION MECHANISMS

Tomato golden mosaic virus (TGMV) is a bipartite member of the subgrou p III Geminiviridae. Like all geminiviruses, TGMV replicates in the nu cleus of susceptible cells by rolling circle replication (RCR). Double -stranded replicative form DNA generated during RCR serves as template for the transcription of viral genes by RNA polymerase II and the ass ociated cellular transcription machinery. Previous studies in tobacco protoplasts and Nicotiana benthamiana leaf discs have shown that the v iral AL2 gene product transactivates expression of the coat protein (C P) and BR1 movement protein genes, and that activation occurs at the l evel of transcription. Because of its function and properties, we prop ose the name TrAP, transcriptional activator protein, for the AL2 gene product. Using transgenes consisting of complete and truncated versio ns of the CP promoter fused to the GUS reporter gene, we show in the s tudies presented here that TrAP is required for CP gene expression in both mesophyll and phloem tissues. Surprisingly, TrAP appears to induc e CP expression by different mechanisms in different cell types: it ma y activate the CP promoter in mesophyll cells, and acts to derepress t he promoter in phloem tissue. In addition, TrAP is clearly capable of inducing the expression of responsive chromosomal promoters and could, in principle, activate host genes. Distinct viral sequence elements m ediate expression and derepression in phloem and activation in mesophy ll, suggesting that TrAP interacts with different components of the ce llular transcription machinery to accomplish CP gene expression in dif ferent cell types, and underscoring the intricacy and complexity of vi rus-host interactions. (C) 1997 Academic Press.