In order to increase production of a useful protein by the chloroplast transformation technique, it seems to be necessary to determine the upper limit for the accumulation of a biologically active ...foreign protein in chloroplasts and then improve photosynthetic capacity and plant productivity. Here we show that the stromal fractions of tobacco chloroplasts could accommodate an additional 200-260 mg/ml of green fluorescent protein in the stroma without any inhibition of gas exchange under various light intensity and growth conditions. The minimum amount of fructose-1,6-/sedoheptulose-1,7-bisphosphatase (FBP/SBPase) limiting photosynthesis was then calculated. Analyses of the photosynthetic parameters and the metabolites of transformants into which FBP/SBPase was introduced with various types of promoter (PpsbA, Prrn, Prps2 and Prpsl2) indicated that a 2- to 3-fold increase in levels of FBPase and SBPase activity is sufficient to increase the final amount of dry matter by up to 1.8-fold relative to the wild-type plants. Their increases were equivalent to an increase of 1 mg/ml of the FBP/SBPase protein in chloroplasts and were calculated to represent 1% of the protein accumulated via chloroplast transformation. Consequently, 99% of the additional 200-260 mg/ml of protein expressed in the chloroplasts could be used for the production of useful proteins in the photosynthesis-elevated transplastomic plants having FBP/SBPase.
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