Promoter regions associated with each of the six ssDNA components of banana bunchy top virus (BBTV) have been characterized. DNA segments incorporating the intergenic regions of BBTV DNA-1 to -6 were isolated and fused to the uidA (beta-glucuronidase) reporter gene to assess promoter activity. In tobacco cell suspensions, the BBTV DNA-2 and -6 promoters generated levels of GUS expression 2-fold greater and similar to the 800 bp CaMV 35S promoter, respectively. Deletion analysis of the BBTV DNA-6 promoter suggested all the necessary promoter elements required for strong expression were located within 239 nucleotides upstream of the translational start codon. In transgenic tobacco plants, the BBTV-derived promoters generally provided a weak, tissue-specific GUS expression pattern restricted to phloem-associated cells. However, in callus derived from tobacco leaf tissue, GUS expression directed by the BBTV DNA-6 promoter was strong and, in some lines, comparable to the CaMV 35S promoter. Detectable promoter activity associated with the BBTV promoters in banana embryogenic cells was only observed using a sensitive green fluorescent protein (GFP) reporter. Promoters derived from BBTV DNA-4 and -5 generated the highest levels of transient activity, which were greater than that of the maize ubi-1 promoter. In transgenic banana plants, the activity of the BBTV DNA-6 promoter was restricted to the phloem of leaves and roots, stomata and root meristems.


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