A Coordinated Decline in the Synthesis of Subunits of Ribulosebisphosphate Carboxylase in Aging Wheat Leaves. I. Analysis of Isolated Protein, Subunits and Ribosomes

Abstract

In wheat plants, the rate of synthesis of ribulosebisphosphate carboxylase (EC 4.1.1.39) protein, in the second leaf approaching full expansion, is at about the average rate for all proteins. In the days following full expansion of the leaf, the rate of synthesis of the carboxylase falls sharply relative to the average rate for all protein. According to analyses of the subunits in the carboxylase holoprotein, there was little difference in the rates of decline of synthesis between the large subunit, made on plastid ribosomes, and the small subunit which is made on cytoplasmic ribosomes. Fractions smaller than the carboxylase holoprotein and reactive to antibodies to the carboxylase proteins were identified in leaf extracts. This fraction was present in greatest amount in young, expanding leaves. Carboxylase subunits in the low-molecular-weight fractions were not labelled rapidly after pulse-labelling of expanded leaves, and there was no evidence of a continuing high rate of synthesis of non-assembled carboxylase subunits in expanded leaves. The cumulative evidence is of a coordinated decline with leaf age in the synthesis of each of the subunits of the enzyme.