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Tomato Fructokinases Exhibit Differential Expression and
Substrate Regulation1
Yoshinori Kanayama2,
David Granot,
Nir Dai,
Marina Petreikov,
Arthur Schaffer,
Ann Powell, and
Alan B. Bennett*
Mann Laboratory, Department of Vegetable Crops, University of
California, Davis, California 95616 (Y.K., A.P., A.B.B.); and Institute
of Field and Garden Crops, The Volcani Center, Bet Dagan 50250, Israel
(D.G., N.D., M.P., A.S.)
Two
divergent genes encoding fructokinase, Frk1 and
Frk2, have been previously shown to be expressed in
tomato (Lycopersicon esculentum L.) and have now been
further characterized with regard to their spatial expression and the
enzymic properties of the encoded proteins. Frk1 and
Frk2 mRNA levels were coordinately induced by exogenous
sugar, indicating that both belong to the growing class of
sugar-regulated genes. However, in situ hybridization indicated that
Frk1 and Frk2 were expressed in a
spatially distinct manner, with Frk2 mRNA primarily
localized in cells of the fruit pericarp, which store starch, and
Frk1 mRNA distributed ubiquitously in pericarp tissue.
To evaluate the biochemical characteristics of the products of the
Frk1 and Frk2 genes, each cDNA was
expressed in a mutant yeast (Saccharomyces cerevisiae)
line defective in hexose phosphorylation and unable to grow on glucose
or fructose (Fru). Both Frk1 and Frk2 proteins expressed in yeast
conferred the ability to grow on Fru and exhibited fructokinase
activity in vitro. Although both Frk1 and Frk2 both utilized Fru as a
substrate, only Frk2 activity was inhibited at high Fru concentrations.
These results indicate that Frk2 can be distinguished from Frk1 by its sensitivity to substrate inhibition and by its temporal and spatial pattern of expression, which suggests that it plays a primary role in
plant cells specialized for starch storage.
1
This research was supported by grants from the
Binational Agricultural Research and Development Fund (no. US-2451-94)
and the University of California BIO-STAR program (no. S96-17).
2
Present address: Faculty of Agriculture, Tohoku
University, 1-1 Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai 981, Japan.
*
Corresponding author; e-mail, abbennett{at}ucdavis.edu; fax
1-530-752-4554.
Plant Physiol. (1998) 117: 85-90
Copyright Clearance Center: 0032-0889/98/117/0085/06
© 1998 American Society of Plant Physiologists
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