Expression of active enzymes from an inducible tomato-mosaic-virus-based vector in cultured transgenic tobacco BY-2 cells

Previously, we reported on an inducible viral vector system for foreign protein production in cultured plant cells, in which the transcription of the recombinant viral vector RNA encoding a foreign protein is controlled by an estradiol-inducible promoter. In this study, we used the inducible virus vector system to test the efficiency of a modified tomato mosaic virus (ToMV) encoding the movement protein. The virus inducibly produced a foreign jellyfish green fluorescent protein encoded in the virus in transgenic tobacco BY-2 suspension-cultured cells as efficiently as modified ToMV without the movement protein. We then produced transgenic BY-2 cell lines ER-ToMV-MP-DHFR and ER-ToMV-MP-GUS, which encoded modified ToMV with Escherichia coli dihydrofolate reductase (DHFR) and beta-glucuronidase (GUS), respectively. After estradiol was added to the cell culture, DHFR and GUS activity was detected in the ER-ToMV-MP-DHFR and ER-ToMV-MP-GUS cells, respectively. In contrast, no DHFR or GUS activity was detected in untreated transgenic lines. Three days after induction, DHFR accumulation accounted for up to 15% of the total soluble proteins extracted from the cells, indicating that an inducible viral vector is an effective option for efficiently producing active enzymes in cultured plant cells.