Complementation of the nuclear antisense rbcS-induced photosynthesis deficiency by introducing an rbcS gene into the tobacco plastid genome.

The small subunit (SS) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a nuclear gene-encoded protein that is imported into chloroplasts where it assembles with the large subunit (LS) after removal of the transit peptide to form Rubisco. We have explored the possibility that the severe deficiency in photosynthesis exhibited in nuclear transgenic tobacco (line alpha5) expressing antisense rbcS coding DNA that results in low SS and Rubisco protein content [Rodermel et al. (1988) Cell 55: 673] could be complemented by introducing a copy of the rbcS gene into its plastid genome through chloroplast transformation. Two independent lines of transplastomic plants were generated, in which the tobacco rbcS coding sequence, either with or without the transit sequence, was site-specifically integrated into the plastid genome. We found that compared with the antisense plants, expression of the plastid rbcS gene in the transplastomic plants resulted in very high mRNA abundance but no increased accumulation of the SS and Rubisco protein or improvement in plant growth and photosynthesis. Therefore, there is a limitation in efficient translation of the rbcS mRNA in the plastid or an incorrect processing and modification of the plastid-synthesized SS protein that might cause its rapid degradation.