PLANT PHYSIOLOGY , Vol 108, Issue 1 75-83, Copyright © 1995 by American Society of Plant Biologists
|
BIOCHEMISTRY AND ENZYMOLOGY |
Biochemical, Physiological, and Molecular Characterization of Sucrose Synthase from Daucus carota
V. Sebkova, C. Unger, M. Hardegger and A. Sturm
Friedrich Miescher-Institut, Postfach 2543, CH-4002 Basel, Switzerland
Sucrose synthase (EC 2.4.1.13) from carrot (Daucus carota) is a tetramer
with a molecular mass of 320 kD and subunits of 80 kD. The enzyme has a pH
optimum of 7.0 (cleavage direction). Maximal activities were measured at
55[deg]C. The Km for Suc was estimated as 87 mM and for UDP as 0.39 mM.
Fructose acts as a noncompetitive inhibitor with an inhibition constant of
17.2 mM. In contrast, glucose inhibits carrot sucrose synthase
uncompetitively with an inhibition constant of 4.3 mM. cDNA clones encoding
a single class of sucrose synthase polypeptide were isolated and sequenced.
DNA gel blot analysis also indicated the occurrence of only one to two
genes. The deduced amino acid sequence of the carrot enzyme is highly
homologous to the sucrose synthase sequences of tomato, potato, and bean. A
comparison of the cDNA-derived amino acid sequence with the SS1-and
SS2-type sucrose synthase sequences of the monocot plants maize, rice, and
barley showed that the carrot enzyme is neither of the SS1 nor of the SS2
type. High enzyme activity was found in roots and petioles of developing
carrot plants, with maximal activity in roots at the transition of primary
roots to tap roots. Enzyme activity was highly correlated with both
polypeptide and transcript levels, indicating that gene expression is
regulated mainly at the mRNA level in the different tissues and organs of
developing carrot plants.
