Regulation of C4-phosphoenolpyruvate carboxylase activity by ambient CO2.

Light activation of phosphoenolpyruvate carboxylase from the leaves of the C4 plant Setaria verticillata (L.) is more pronounced at low CO2 levels. The 2-fold activation observed at physiological ambient CO2 becomes 3.64-fold at 5 µL/L and completely abolished above 700 µL/L. When the stomata close under the influence of abscisic acid at 330 µL/L CO2, the extent of light activation is high (3.59-fold), probably because the increased diffusive resistance keeps the internal CO2 at much lower levels. Under darkness. CO2 and absicisic acid do not affect the extractable phosphoenolpyruvate carboxylase activity. Internal CO2 levels may determine phosphoenolpyruvate concentratio in the cytoplasm through the control of its utilization by phosphoenolpyruvate carboxylase. We have recently proposed (Samaras et al. 1988) that photosynthetically produced phosphoenolpyruvate could be an activator of the enzyme. It is therefore suggested that CO2 indirectly affects the activation state of phosphoenolpyruvate carboxylase by controlling the levels of phosphoenolpyruvate which may act as an activator.

Effects of temperature and photosynthetic inhibitors on light activation of C4-phosphoenolpyruvate carboxylase.

Phosphoenolpyruvate carboxylase from leaves of the C4 plant Setaria verticillata (L.) Beauv. is activated by light; day levels of activity are reached after 30 minutes of illumination. Photoactivation is prevented by inhibitors of photosynthetic electron flow or of photophosphorylation and by D,L-glyceraldehyde, which inhibits the reductive pentose phosphate pathway.Although the extractable activity in the dark is not affected by temperature the photoactivation is prevented when both illumination and extraction are done under low temperature (5 C). High temperature (30 C) during either illumination or extraction is needed for activation. Once the enzyme is photoactivated at 30 C, a transfer of the leaves to 5 C does not abolish the extra activity.The results suggest that both unimpaired electron flow and photophosphorylation are prerequisites for the activation of phosphoenolpyruvate carboxylase. Low temperature apparently suppresses either the transport to the cytoplasm of a photosynthetic intermediate or the activating reaction itself. The inclusion of phosphoenolpyruvate in the extraction medium increases the night activity.On the basis of the available information, it is suggested that phosphoenolpyruvate could be the activator in vivo. In that case, the activation of phosphoenolpyruvate carboxylase would depend on internal CO2 level and prior photoactivation of both pyruvate, orthophosphate, dikinase and NADP malate dehydrogenase.