Isolation of an actin promoter for strong expression of transgenes in the orchid genus Dendrobium

We isolated and functionally characterized a Dendrobium Actin1 (DmACT1) promoter that drives strong gene expression in the orchid genus Dendrobium. A genomic fragment containing the region 3227 bp upstream of the coding region of DmACT1 was obtained by inverse PCR. Detailed comparison of the full-length cDNA and genomic sequences revealed that DmACT1 has a 1374 bp first intron in the 5' UTR. However, the 5' flanking sequences upstream of the coding region showed no obvious sequence similarities compared to those of known promoters, including plant actin promoters. Serial deletion constructs of the 5' flanking region from the translation initiation codon were fused to the coding sequence of a GUS/luciferase fusion reporter to identify the regulatory elements necessary for promoter activity. Transient assays in the flowers of Dendrobium revealed that the 5' UTR-intron greatly enhanced promoter activity. Moreover, the DmACT1 promoter with its 5' UTR-intron yielded approximately 10-fold higher reporter activity than the rice Act1 promoter-intron. Our data suggest that the DmACT1 promoter with its 5' UTR-intron is a useful tool for strong expression of transgenes in Dendrobium orchids.