Résumé
Banana streak virus (BSV) belongs to the genus Badnavirus of plant pararetroviruses (family Caulimoviridae) that replicate by reverse transcription of pregenomic (pg)RNA and encapsidate circular non-covalently closed double stranded genomic DNA of ca. 7.4-8 kb to form episomal viruses. A complex of BSV species cause the streak disease transmitted by mealybugs in bananas and plantains worldwide. Gaining the knowledge on transcriptional regulation of episomal BSV is an important step forward in developing efficient strategies to fight against BSV. Schenk et al., 2001 have shown that the 1.3 and 2.1 kb fragments of the intergenic region of BSV Cavendish (BSCAV) and BSV Mysore (BSMYV) species, respectively, exhibited a promoter activity in a wide range of plant species including monocots, dicots, gymnosperm and fern. In 2005, Remans et al. have identified four elements in the intergenic region that are important for the activity of the BSCAV promoter. Here we mapped the pgRNA promoter of BSV Obino l'Ewai (BSOLV) species which is the most widespread BSV species on earth. Using transient expression in N. benthamiana of several deletion constructs of the intergenic region fused with the GFP reporter gene, we found that the BSOLV core promoter activity is regulated by mainly two elements. One element located downstream of a putative transcription start site (TSS) in the pgRNA leader region (5'UTR region with a strong secondary structure) has a negative effect decreasing the core promoter activity by 95%, whereas the second element located between positions - 450 and -180 upstream of the TSS enhances it by 90%. Interestingly, using Arabidopsis transgenic plants carriyng stably-integrated BSOLV promoter-reporter gene constructs, we observed the reporter gene expression in all types of the studied organs including roots, leaves, stem, reproductive organs, siliques and seeds, indicating that BSV is transcriptionally-active in any plant tissue. Besides, our work also suggests that infective BSV integrants characterised in the genome of Musa balbisiana subspecies (banana B genome), called endogenous (e)BSVs, can potentially give rise to BSV episomes in any plant tissue because their nucleic acid sequences (including promoter sequences) are over 99% identical to their counterpart episomal viruses (Chabannes et al., 2013). (Texte intégral)