Deletion analysis of the spinach curly top virus coat protein promoter
Spinach curly top virus (SCTV) belongs to the Curtovirus genus of the family Geminiviridae and is an emergent plant pathogen. As a recently characterized virus species, little is known about the pathogenesis of this virus. In an effort to increase our understanding of SCTV pathogenesis we are characterizing the coat protein (CP) promoter of SCTV. A series of deletions of the SCTV CP promoter were generated and cloned as translational fusions with the β -glucuronidase gene (GUS). The level of GUS expression was measured which is a reflection of promoter activity. When sequences within 541 bp upstream of the CP coding region were deleted, activity of the CP promoter decreased seven-fold, while further deletion of sequences within 415 bp upstream of the CP coding region abolished activity. Therefore sequences necessary for activity of the CP promoter lie within 541 bp upstream of the CP coding region and an element(s) required for activity may lie between -541 and -415 of the CP translation start site. Also, the CP promoter was found to be active, independent of C2 in the phloem but was inactive in the mesophyll. The relevance of this finding is discussed with regard to the tissue tropism of this virus. To further characterize the CP promoter, we used bioinformatics tools to predict cis-acting elements within this region and transcription factors that could bind to it. Among the cis-acting elements predicted were TATA, CAAT and G-boxes. Several transcription factors binding sites were found including the ones from the E2F and TCP families. The relevance of these to viral pathogenesis is discussed.