Rapid degradation of unassembled ribulose 1,5-bisphosphate carboxylase small subunits in chloroplasts.

We have detected a proteolytic mechanism in chloroplasts that selectively and rapidly degrades the imported small subunit of ribulose 1,5-bisphosphate carboxylase when pools of the chloroplast-synthesized large subunit are depleted. This degradation system is constitutively present and appears to be responsible for precise stoichiometric accumulation of the two subunits of the enzyme. We believe similar proteolytic mechanisms participate in regulating the accumulation of other photosynthetic proteins during chloroplast biogenesis.