ISOLATION OF A PROTEIN CONTAINING COVALENTLY-LINKED LARGE AND SMALL SUBUNITS OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE FROM BOTRYOCOCCUS-BRAUNII/

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and a 66-kD protein were co-purified from solubilized microsomal preparations of t he green alga Botryococcus braunii by Green A agarose, sucrose density gradient, MonoQ, and gel filtration. The 66-kD protein remained intac t after 6 M urea treatment and sodium dodecyl sulfate-polyacrylamide g el electrophoresis. It could be detected in the soluble fraction of th e cell-free extract but appeared to be more abundant in the microsomal preparations. It cross-reacted with antibodies raised against Rubisco holoenzyme, large and small subunits, indicating that the 66-kD prote in contains both the large and the small subunits of Rubisco. The N-te rminal amino acid sequence of this protein and that of a proteolytic f ragment showed high homology with the mature Rubisco small subunits, a nd the sequence of another proteolytic fragment showed high homology w ith that of the Rubisco large subunit. It is concluded that the 66-kD protein is produced by cross-linking of large and small subunits of Ru bisco in the cell.