BIOCHEMICAL, PHYSIOLOGICAL, AND MOLECULAR CHARACTERIZATION OF SUCROSESYNTHASE FROM DAUCUS-CAROTA

Sucrose synthase (EC 2.4.1.13) from carrot (Daucus carota) is a tetram er with a molecular mass of 320 kD and subunits of 80 kD. The enzyme h as a pH optimum of 7.0 (cleavage direction). Maximal activities were m easured at 55 degrees. The K-m for Suc was estimated as 87 mM and for UDP as 0.39 mM. Fructose acts as a noncompetitive inhibitor with an in hibition constant of 17.2 mM. In contrast, glucose inhibits carrot suc rose synthase uncompetitively with an inhibition constant of 4.3 mM. c DNA clones encoding a single class of sucrose synthase polypeptide wer e isolated and sequenced. DNA gel blot analysis also indicated the occ urrence of only one to two genes. The deduced amino acid sequence of t he carrot enzyme is highly homologous to the sucrose synthase sequence s 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 enz yme is neither of the SS1 nor of the SS2 type. High enzyme activity wa s found in roots and petioles of developing carrot plants, with maxima l activity in roots at the transition of primary roots to tap roots. E nzyme activity was highly correlated with both polypeptide and transcr ipt levels, indicating that gene expression is regulated mainly at the mRNA level in the different tissues and organs of developing carrot p lants.