DIVERGENT FRUCTOKINASE GENES ARE DIFFERENTIALLY EXPRESSED IN TOMATO

Two cDNA clones (Frk1 and Frk2) encoding fructokinase (EC 2.7.1.4) wer e isolated from tomato (Lycopersicon esculentum). The Frk2 cDNA encode d a deduced protein of 328 amino acids that was more than 90% identica l with a previously characterized potato (Solanum tuberosum) fructokin ase. In contrast, the Frk1 cDNA encoded a deduced protein of 347 amino acids that shared only 55% amino acid identity with Frk2. Both deduce d proteins possessed an ATP-binding motif and putative substrate recog nition site sequences identified in bacterial fructokinases. The Frk1 cDNA was expressed in a mutant yeast (Saccharomyces cerevisiae) line, which lacks the ability to phosphorylate glucose and fructose and is u nable to grow on glucose or fructose. Mutant cells expressing Frk1 wer e complemented to grow on fructose but not glucose, indicating that Fr k1 phosphorylates fructose but not glucose, and this activity was veri fied in extracts of transformed yeast. The mRNA corresponding to Frk2 accumulated to high levels in young, developing tomato fruit, whereas the Frk1 mRNA accumulated to higher levels late in fruit development. The results indicate that fructokinase in tomato is encoded by two div ergent genes, which exhibit a differential pattern of expression durin g fruit development.