FUNCTIONAL-ORGANIZATION OF THE CASSAVA VEIN MOSAIC-VIRUS (CSVMV) PROMOTER

Cassava vein mosaic virus (CsVMV) is a pararetrovirus that infects cas sava plants in Brazil. A promoter fragment isolated from CsVMV, compri sing nucleotides -443 to +72, was previously shown to direct strong co nstitutive gene expression in transgenic plants. Here we report the fu nctional architecture of the CsVMV promoter fragment. A series of prom oter deletion mutants were fused to the coding sequence of uidA report er gene and the chimeric genes were introduced into transgenic tobacco plants. Promoter activity was monitored by histochemical and quantita tive assays of beta-glucuronidase activity (GUS). We found that the pr omoter fragment is made up of different regions that confer distinct t issue-specific expression of the gene. The region encompassing nucleot ides -222 to -173 contains cis elements that control promoter expressi on in green tissues and root tips. Our results indicate that a consens us asl element and a GATA motif located within this region are essenti al for promoter expression in those tissues. Expression from the CsVMV promoter in vascular elements is directed by the region encompassing nucleotides -178 to -63. Elements located between nucleotides -149 and -63 are also required to activate promoter expression in green tissue s suggesting a combinatorial mode of regulation. Within the latter reg ion, a 43 bp fragment extending from nucleotide -141 to -99 was shown to interact with a protein factor extracted from nuclei of tobacco see dlings. This fragment showed no sequence homology with other pararetro virus promoters and hence may contain CsVMV-specific regulatory cis el ements.