Plastome-encoded bacterial ribulose-1,5-bisphosphate carboxylase/oxygenase(RubisCO) supports photosynthesis and growth in tobacco

The efficiency with which crop plants use their resources of light, water, and fertilizer nitrogen could be enhanced by replacing their CO2-fixing enz yme, D-ribulose-1,5-bisphosphate carboxylase-oxygenase (RubisCO), with more efficient forms, such as those found in some algae, for example. This impo rtant challenge has been frustrated by failure of all previous attempts to substitute a fully functional, foreign RubisCO (efficient or inefficient) i nto higher plants. This failure could be caused by incompatibility between the plastid-encoded large subunits and the nucleus-encoded small subunits o r by inability of the foreign RubisCO subunits to fold or assemble efficien tly in the plastid. Mismatch between the regulatory requirements of the for eign RubisCO and conditions in the chloroplast also might render the substi tuted enzyme inactive but, previously, it has not been possible to test thi s. To answer the general question of whether a foreign RubisCO can support photosynthesis in a plant, we used plastid transformation to replace RubisC O in tobacco with the simple homodimeric form of the enzyme from the alpha -proteobacterium, Rhodospirillum rubrum, which has no small subunits and no special assembly requirements. The transplastomic plants so obtained are f ully autotrophic and reproductive but require CO2 supplementation, consiste nt with the kinetic properties of the bacterial RubisCO. This establishes t hat the activity of a RubisCO from a very different phylogeny can be integr ated into chloroplast photosynthetic metabolism without prohibitive problem s.