Expression of T7-based constructs in tobacco cells

Bacteriophage T7 promoter and RNA polymerase (T7-Pol) are widely used for recombinant protein expression in bacteria. In plants, there exists conflicting results regarding the efficacy of protein expression from T7-Pol-derived mRNAs. To reconcile these contradictory observations, the expression of green fluorescent protein (GFP) from T7 constructs was evaluated in tobacco protoplasts. T7 constructs transcribed by a nuclearly targeted T7-Pol did not express GFP in plant protoplasts, however T7-Pol lacking a nuclear targeting signal was able to translate cytosolically transcribed mRNAs, but only if the messages contained a viral translation enhancer. GFP expression was further evaluated at the plant level by using agroinfiltration-mediated transient expression system. Unlike for cytosolic expression, nuclear T7 transcripts containing a viral translation enhancer element did not express GFP, and modifications designed to stabilize and facilitate export of T7 transcripts to the cytosol did not improve the expression. We conclude that expression of nuclear T7 constructs is not feasible in tobacco cells, but cytosolic transcription provides an alternative means to over-express RNAs directly in the cytosol.