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 enzyme, D-ribulose-1,5-bisphosphate carboxylase-oxygenase (RubisCO), with more efficient forms, such as those found in some algae, for example. This important challenge has been frustrated by failure of all previous attempts to substitute a fully functional, foreign RubisCO (efficient or inefficient) into higher plants. This failure could be caused by incompatibility between the plastid-encoded large subunits and the nucleus-encoded small subunits or by inability of the foreign RubisCO subunits to fold or assemble efficiently in the plastid. Mismatch between the regulatory requirements of the foreign RubisCO and conditions in the chloroplast also might render the substituted enzyme inactive but, previously, it has not been possible to test this. To answer the general question of whether a foreign RubisCO can support photosynthesis in a plant, we used plastid transformation to replace RubisCO in tobacco with the simple homodimeric form of the enzyme from the α-proteobacterium, Rhodospirillum rubrum, which has no small subunits and no special assembly requirements. The transplastomic plants so obtained are fully autotrophic and reproductive but require CO₂ supplementation, consistent with the kinetic properties of the bacterial RubisCO. This establishes that the activity of a RubisCO from a very different phylogeny can be integrated into chloroplast photosynthetic metabolism without prohibitive problems.