pL-VP16 contains a wild-type lac-repressor coding region with the herpes simplex virus VP16 transcription activation domain fused in-frame in the C-terminal-coding region and the nuclear targeting signal of simian virus 40 large T antigen inserted at the N terminus. The T-DNAs of all these activator and control plasmids also contain a CaMV-35S-dihydrofolate reductase cassette encoding methotrexate resistance in transgenic plants.