Cloning and characterization of two fructokinases from maize

Two cDNA clones (ZmFRK1 and ZmFRK2) encoding fructokinases (FRKs, E.C. 2.7.1.4) were isolated from maize (Zea mays L.). They encode polypeptides of 323 and 335 amino acids, respectively. Both of the deduced proteins contain ATP binding motif and putative substrate recognition sequences. They share 73.7% sequence identity, and have high sequence homology with previously isolated FRKs from potato, sugar beet, tomato, and Arabidopsis. The in vitro enzymatic studies of the recombinant ZmFrk1 and ZmFrk2 proteins showed that they both had fructose specific kinase activities, but ZmFrk1 had much higher turnover rate than ZmFrk2. Both ZmFrk1 and ZmFrk2 were inhibited by the Substrate fructose although ZmFrk1 was less sensitive. The mRNAs of both ZmFRK1 and ZmFRK2 accumulated to high levels in roots and stems, but were barely detectable in leaves. The expression pattern of the two genes was different during seed development. The transcript of ZmFRK1 only appeared in later stages of development, whereas the ZmFRK2 mRNA was expressed in earlier time. Taken together, our results imply that ZmFRK1 and ZmFRK2 play distinct biological roles during seed development. (C) 2003 Elsevier Ireland Ltd. All rights reserved.