Comparison of Patterns of Accumulation of Ribulose Bisphosphate Carboxylase Antigen and Catalytic Activity and Measurement of Antigen Half-Life during the Cell Cycle of Chlorella sorokiniana

By use of specific immunochemical procedures, ribulose-1,5-bisphosphate carboxylase (RuBPCase), antigen and catalytic activity were shown to have coincident step-patterns of accumulation during the cell cycle of Chlorella sorokiniana. Pulse-chase studies, employing radioactive sulfate, were performed during the period of rapid accumulation of enzyme activity and during the period of constant enzyme activity in the cell cycle. No degradation of RuBPCase antigen could be detected during either of these cell cycle periods. Thus, the step-pattern of accumulation of RuBPCase activity resulted from periodic synthesis of an enzyme that was stable under steady-state cell cycle conditions. Although inhibition of protein synthesis by cycloheximide, at different times in the cell cycle in the light, resulted in rapid decay of RuBPCase activity, this loss in activity occurred without detectable loss in enzyme antigen. When synchronous cells were placed into the dark, to slow the rate of protein synthesis in the absence of cycloheximide, the levels of enzyme antigen and activity decreased by 30 and 50%, respectively, during the 10-hour dark period. Thus, in C. sorokiniana changes in RuBPCase activity do not necessarily reflect parallel changes in enzyme antigen, particularly when cell growth is perturbed by changes from steady-state cultural conditions.