Regulation of a Geminivirus Coat Protein Promoter by AL2 Protein (TrAP): Evidence for Activation and Derepression Mechanisms

Abstract

Tomato golden mosaic virus (TGMV) is a bipartite member of the subgroup III Geminiviridae. Like all geminiviruses, TGMV replicates in the nucleus 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 associated cellular transcription machinery. Previous studies in tobacco protoplasts andNicotiana benthamianaleaf discs have shown that the viralAL2gene product transactivates expression of the coat protein (CP) andBR1movement protein genes, and that activation occurs at the level of transcription. Because of its function and properties, we propose the name TrAP, transcriptional activator protein, for theAL2gene product. Using transgenes consisting of complete and truncated versions of theCPpromoter fused to the GUS reporter gene, we show in the studies presented here that TrAP is required forCPgene expression in both mesophyll and phloem tissues. Surprisingly, TrAP appears to induceCPexpression by different mechanisms in different cell types: it may activate theCPpromoter in mesophyll cells, and acts to derepress the 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 mediate expression and derepression in phloem and activation in mesophyll, suggesting that TrAP interacts with different components of the cellular transcription machinery to accomplishCPgene expression in different cell types, and underscoring the intricacy and complexity of virus–host interactions.