MOLECULAR-FORMS AND KINETIC-PROPERTIES OF PYRUVATE, P-I DIKINASE FROM2 POPULATIONS OF BARNYARD GRASS (ECHINOCHLOA-CRUS-GALLI) FROM SITES OF CONTRASTING CLIMATES

Plants from two populations of the C-4 barnyard grass (Echinochloa cru s-galli (L.) Beauv.) from Quebec (QUE) and Mississippi (MISS) were acc limated under controlled conditions to 26/20 and 14/8 degrees C day/ni ght. The apparent energy of activation (E(a)), K-m for pyruvate, V-max /K-m ratios, K-cat (substrate turnover number) and specific activity o f pyruvate, P-i dikinase (PPDK, EC 2.7.9.1) were analysed from partial ly purified Sephadex G-25 extracts of PPDK from leaves and from highly purified PPDK. PPDK from both populations consisted of one isomorph w ith the same electrophoretic mobility in polyacrylamide gels and simil ar molecular weights for the native enzyme (385 kDa) and for the subun it of the tetramer (94.8 kDa). No significant differences were observe d for any of the kinetic properties of partially purified or purified PPDK or for the specific activity per mg protein of purified PPDK extr acted from plants of the two populations and acclimated to the two the rmoperiods. Net photosynthetic rates (Ps) were positively correlated w ith PPDK activity levels (E) but E/Ps ratios were lower than 1.0, rang ing from 0.43 to 0.67. Results indicate that differences in activity l evels, thermal properties and in the kinetics of light activation and dark inactivation of PPDK extracted from cold-acclimated MISS and QUE plants, as reported in earlier studies, are due to causes other than k inetic properties or electrophoretic characteristics of PPDK.