RELOCATION OF THE PLASTID RBCL GENE TO THE NUCLEUS YIELDS FUNCTIONAL RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE IN TOBACCO CHLOROPLASTS

The conserved plastid localization of rbcL suggests that biosynthesis of the large subunit of ribulose-1,5-bisphosphate carboxylase [Rubisco ; 3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39] in ch loroplasts is required to obtain functional enzyme. To examine the val idity of this hypothesis, we relocated the plastid rbcL gene to the nu cleus. First, we deleted the rbcL gene from the tobacco plastid genome by targeted insertion of a selectable aad4 gene encoding spectinomyci n resistance. The rbcL coding region was then inserted into an express ion cassette and introduced into the nuclear genome of these plants by Agrobacterium-mediated transformation. We report that the nuclear rbc L functionally complements the defective plastids when the Rubisco lar ge subunit is targeted to chloroplasts by a transit peptide. Therefore , the evolutionary process that relocates functional plastid genes to the nucleus has not yet occurred in the case of the rbcL gene. Targete d deletion of plastid genes, combined with their allotopic expression, will provide opportunities for studying the function of plastid enzym e complexes.