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-A resolution and found that the enzyme contained two kinds of S, SI and SII, 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 SI, although histidine was at that position in SII. There were other residue differences. Thus, spinach Rubisco has a L8SI4SII4 subunit structure. The orderly disposition of the heterogeneous small subunits in the Rubisco holoenzyme provides accounts of a multigene family of S in plants.