Promoter activity associated with the intergenic regions of banana bunchy top virus DNA-1 to -6 in transgenic tobacco and banana cells

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-B promoter was strong and, in some lines, comparable to the CaMV 35S promoter. Detectable promoter activity associated with the BBN 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.