Orderly disposition of heterogeneous small subunits in D-ribulose-1,5-bisphosphate carboxylase/oxygenase from spinach

We determined the crystal structure of spinach ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) by x-ray diffraction at 1.8-Angstrom resolution and found that the enzyme contained two kinds of S, S-I and S-II, present in equal number and disposed in an orderly way within the Rubisco holoenzyme. The electron density maps suggested that leucine was at residue 56 in S-I, although histidine was at that position in S-II. There were other residue differences. Thus, spinach Rubisco has a L(8)S(4)(I)S(II)4 subunit structure. The orderly disposition of the heterogeneous small subunits in the Rubisco holoenzyme provides accounts of a multigene family of S in plants.