Characterization of cis-acting elements affecting strength and phloem specificity of the coconut foliar decay virus promoter.

During replication in its host plant, coconut foliar decay virus (CFDV) remains restricted to the phloem tissue. Previous in vivo studies on subgenomic CFDV DNA had provided evidence for the phloem specificity of the CFDV promoter. Here, new promoter constructs are described which are distinguished by the presence or absence of various cis-acting signals and which gave rise to a 16-fold higher reporter gene (beta-glucuronidase) activity (reaching 30% of the cauliflower mosaic virus 35S promoter) in tobacco protoplasts, while the phloem specificity in transgenic tobacco plants was conserved. Surprisingly, the CFDV stem-loop structure dramatically influenced transcriptional efficiency. From these studies and sequence comparisons with other phloem-specific promoters, cis-signals involved in CFDV promoter strength and tissue specificity were identified.