Isolation and sequencing of tomato fruit sucrose synthase cDNA.

Suc synthase (EC 2. 4. 1. 13) catalyzes the reversible reaction: Suc + UDP c-, UDP-Glc + Fru. In plants, its main physiological function is to cleave Suc and generate UDPGlc, which is used in such diverse pathways as starch synthesis, cell wall component synthesis, and energy generation (Huber and Akazawa, 1986; Sung et al., 1988). It has been demonstrated that Suc synthase is a key enzyme during the early stages of tomato fruit development. Its activity positively correlates with tomato fruit relative growth rate and starch accumulation in the early stages of fruit development (Robinson et al., 1988; Wang et al., 1993). It has been proposed that Suc synthase activity is a valuable biochemical marker of tomato fruit sink strength (Sun et al., 1992). To gain a better understanding of the role and regulation of Suc synthase in tomato fruit development, we isolated a Suc synthase cDNA clone, designated TOMSSF, from a cDNA library prepared from mRNA of tomato (Lycopersicon esculentum Mill. cv VF 36) pistils from flowers at anthesis (Gasser et al., 1989) (Table I). The cDNA is 2725 bp long and contains an open reading frame encoding a protein of 805 amino acids with an ATG at position 63 and the stop codon TGA at position 2478. The DNA sequence of the 5’ noncoding open reading frame and 3’ noncoding regions of TOMSSF has 97%, 97%, and 75% sequence similarity to the analogous regions of the potato Suc synthase cDNA clone isolated from a developing tuber cDNA library (Salanoubat and Belliard, 1987). Mismatches in the 5‘ noncoding region were found at positions -2 and -3 relative to the ATG initiation codon. The CTATAATGG sequence, within which the translation initiation codon resides, is unique compared with its counterparts in other plant Suc synthase sequences as well. The predicted amino acid sequence is very similar to the Suc synthase sequences of other plants. There is a C-rich region in the 3’ untranslated region (from 2569-2599 bp; C content in this region is 87%). Hydropathy analysis revealed that the deduced protein did not contain a hydrophobic region at the amino terminal and does not contain a signal sequence (Von