CHANGES IN PHOSPHOENOLPYRUVATE CARBOXYLASE AND RIBULOSE-BIPHOSPHATE CARBOXYLASE ACTIVITIES DURING PHOTOHETEROTROPHIC GROWTH OF NICOTIANA-TABACUM (CV-XANTHI) CELL-SUSPENSIONS

The capacities (in vitro* * in vitro experiments related in this report are enzymatic capacities measurements performed with cell-free preparations studied in optimal conditions of pH and in excess of enzyme substrate. measurements) of phosphoenolpyruvate carboxylase (PEPCase) and ribulose-biphosphate carboxylase (RuBPCase) have been studied in photoheterotrophically growing cell suspensions of Nicotiana tabacum L. (cv Xanthi). The most significant observation was the different behaviour of the two carboxylases. The PEPCase capacity expressed on a g dry wt. basis rises from an initial day O value of 200-300 μmol of CO2 fixed to 600-800 μmol during the exponential phase and then returns to the initial value during the post-exponential and stationary phases. The RuBPCase capacity was nearly constant (90-140 μmol of CO2 fixed) throughout the growth cycle. The peak of respiration and the high PEPCase capacity during the exponential growth phase were concomitant with an active process of soluble protein synthesis. After electrophoresis of cell-free extracts, two bands of PEPCase activity (forms I and II with Rm values of 0.23 and 0.38 respectively)_and one band of RuBPCase activity (Rm of 0.27) were localized on polyacrylamide gels. The densitometric profiles of soluble proteins showed a marked enrichment of PEPCase form II band during the active phase of cell division as compared to that of cells in the stationary phase of growth. Moreover, the 2-fold increase of the specific activity (μmol CO2 fixed/h and/mg of soluble proteins) of the PEPCase during the exponential phase suggested that the enhancement of the catalytic power of this enzyme could be due to de novo protein synthesis. The biosynthesis pattern of PEPCase was essentially that of peak enzyme in contrast with RuBPCase which showed only slight variations over the growth cycle. © 1978.