Studies on the assembly of large subunits of ribulose bisphosphate carboxylase in isolated pea chloroplasts.

Abstract

Ribulose bisphosphate carboxylase [RuBPCase] consists of cytoplasmically synthesized small subunits and chloroplast-synthesized large subunits. Large subunits of RuBPCase synthesized in vivo or in organello can be recovered from intact chloroplasts in the form of 2 different complexes with sedimentation coefficients of 7S and 29S. About 1/3-1/2 of the large subunits synthesized in isolated chloroplasts are found in the 7S complex, the remainder being found in the 29S complex. Upon prolonged illumination of the chloroplasts, newly synthesized large subunits accumulate in the 18S RuBPCase molecule and disappear from both the 7S and the 29S large subunit complexes. The 29S complex undergoes an in vitro dissociation reaction and is not as stable as RuBPCase. Apparently, the 7S large subunit complex is a chloroplast product, the 29S large subunit complex is labeled in vivo, each of these two complexes can account quantitatively for all the large subunits assembled into RuBPCase in organello, and excess large subunits are degraded in chloroplasts.