Light-dependent assembly of ribulose-1,5-bisphosphate carboxylase

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Abstract
Ribulose-1,5-bisphosphate carboxylase [RuP2Case; 3-phospho-D-glycerate carboxyl-lyase (dimerizing)] is composed of 8 small subunits (MW 14,000) and 8 large subunits (MW 55,000). Newly synthesized large subunits are associated with 2 complexes having sedimentation coefficients of 7 and 29 S. Assembly of RuP2Case occurs in isolated intact chloroplasts [Pisum sativum cv. Progress no. 9] in the light, but not in the dark. When extracts of chloroplasts are treated with ATP or GTP, RuP2Case assembly is accelerated while the 29S large subunit complex is maintained. In the presence of Mg2+, ATP brings about almost complete dissociation of the 29S complex; GTP and a nonhydrolyzable analog of ATP are without effect. The existence was indicated of a complex set of reactions involving nucleotides, Mg2+ and several putative intermediates in RuP2Case assembly. These reactions may at least partly account for the light dependence of RuP2Case assembly. In particular, ATP and GTP promote the assembly of large subunits into RuP2Case.

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