Small subunit contacts in ribulose-1,5-bisphosphate carboxylase

The arrangement of subunits of ribulosebisphosphate carboxylase in solution has been studied by exposing the enzyme to the cross-linking agents tetranitromethane, dimethyl suberimidate, and dimethyl adipimidate, and the cleavable cross-linking agent, methyl 4-mercaptobutyrimidate followed by gel electrophoresis in the presence of dodecyl sulfate. All these agents caused the formation of dimers of the enzyme's small subunit, independently of protein concentration. In addition, trimers and tetramers of small subunit were detected in the mercaptobutyrimidate-treated enzyme. The data show that small subunits are closely paired in the native enzyme and may be in layers of four, or a ring of eight.