A plant basal in vitro system supporting accurate transcription of both RNA polymerase II- and III-dependent genes: supplement of green leaf component(s) drives accurate transcription of a light-responsive rbcS gene.

An in vitro transcription initiation system has been developed from nuclei of rapidly growing, non-green tobacco (Nicotiana tabacum) cultured (BY-2) cells. Conditions for nuclear extraction and in vitro transcription reaction have been optimized with a tobacco beta-1,3-glucanase gene, a constitutively expressed gene in BY-2 cells. The in vitro system supports accurate transcription of RNA polymerase II-dependent promoters from not only plant genes (tobacco beta-1,3-glucanase gene, cauliflower mosaic virus 35S promoter) but also animal genes (adenovirus 2 major late promoter, simian virus 40 early major promoter). In addition, this system drives accurate transcription of an RNA polymerase III-dependent Arabidopsis thaliana U6 snRNA gene. As BY-2 cells do not differentiate in response to light or any other stimuli, they would provide a basal transcription system which lacks tissue-specific and light-responsive nuclear signals as well as chloroplast-derived signals. Consequently, the BY-2 cell-free system is unable to transcribe the tomato gene encoding the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcS3C) whose expression is tissue-specific and light-inducible. However, the transcription of rbcS3C was obtained by supplementing the BY-2 system with a nuclear extract of light-grown tomato seedlings. The promoter regions necessary for rbcS transcription was mapped in vitro using a series of 5' deletion mutants. The 351 bp upstream sequence is essential and the further upstream region from -351 to -441 enhances its transcription. The in vitro basal system will be useful to identify specific signals from both the nucleus and chloroplast in green leaves and other organs/tissues.