The gene for the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)small subunit relocated to the plastid genome of tobacco directs the synthesis of small subunits that assemble into Rubisco

To assess the extent to which a nuclear gene for a chloroplast protein reta ined the ability to be expressed in its presumed preendosymbiotic location, we relocated the RbcS gene for the small subunit of ribulose-1,5-bisphosph ate carboxylase/oxygenase (Rubisco) to the tobacco plastid genome. Plastid RbcS transgenes, both with and without the transit presequence, were equipp ed with 3' hepta-histidine-encoding sequences and psbA promoter and termina tor elements. Both transgenes were transcribed abundantly, and their produc ts were translated into small subunit polypeptides that folded correctly an d assembled into the Rubisco hexadecamer. When present, either the transit presequence was not translated or the transit peptide was cleaved completel y. After assembly into Rubisco, transplastomic small subunits were relative ly stable. The hepta-histidine sequence fused to the C terminus of a single small subunit was sufficient for isolation of the whole Rubisco hexadecame r by Ni2+ chelation. Small subunits produced by the plastid transgenes were not abundant, never exceeding similar to1% of the total small subunits, an d they differed from cytoplasmically synthesized small subunits in their N- terminal modifications. The scarcity of transplastomic small subunits might be caused by inefficient translation or assembly.