LIGHT DARK MODULATION OF PHOSPHOENOLPYRUVATE CARBOXYLASE IN C-3 AND C-4 SPECIES/

In this report, the effects of light on the activity and allosteric pr operties of phosphoenolpyruvate (PEP) carboxylase were examined in new ly matured leaves of several C-3 and C-4 species. Illumination of prev iously darkened leaves increased the enzyme activity 1.1 to 1.3 fold i n C-3 species and 1.4 to 2.3 fold in C-4 species, when assayed under s uboptimal conditions (pH 7) without allosteric effecters. The sensitiv ities of PEP carboxylase to the allosteric effecters malate and glucos e-6-phosphate were markedly different between C-3 and C-4 species. In the presence of 5 mM malate, the activity of the enzyme extracted from illuminated leaves was 3 to 10 fold higher than that from darkened le aves in C-4 species due to reduced malate inhibition of the enzyme fro m illuminated leaves, whereas it increased only slightly in C-3 specie s. The Ki(malate) for the enzyme increased about 3 fold by illuminatio n in C-4 species, but increased only slightly in C-3 species. Also, th e addition of the positive effector glucose-6-phosphate provided much greater protection against malate inhibition of the enzyme from C-4 sp ecies than C-3 species. Feeding nitrate to excised leaves of nitrogen deficient plants enhanced the degree of light activation of PEP carbox ylase in the C-4 species maize, but had little or no effect in the C-3 species wheat. These results suggest that post-translational modifica tion by light affects the activity and allosteric properties of PEP ca rboxylase to a much greater extend in C-4 than in C-3 species.