Kanayama Y, Granot D, Dai N, Petreikov M, Schaffer A, Powell A, Bennett AB
Plant Physiol. 1998 May;117(1):85-90. doi: 10.1104/pp.117.1.85.
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.
先前已表明，番茄（Lycopersicon esculentum L.）中存在两个编码果糖激酶的不同基因Frk1和Frk2，现在对它们的空间表达以及所编码蛋白质的酶学特性进行了进一步表征。外源性糖可协同诱导Frk1和Frk2的mRNA水平，这表明二者都属于不断增加的糖调节基因类别。然而，原位杂交表明，Frk1和Frk2的表达在空间上存在差异，Frk2的mRNA主要定位于储存淀粉的果实果皮细胞中，而Frk1的mRNA则普遍分布于果皮组织中。为了评估Frk1和Frk2基因产物的生化特性，将每个cDNA在己糖磷酸化缺陷且无法在葡萄糖或果糖（Fru）上生长的突变酵母（Saccharomyces cerevisiae）系中进行表达。在酵母中表达的Frk1和Frk2蛋白均赋予了在Fru上生长的能力，并在体外表现出果糖激酶活性。尽管Frk1和Frk2都利用Fru作为底物，但只有Frk2的活性在高Fru浓度下受到抑制。这些结果表明，Frk2可通过其对底物抑制的敏感性及其表达的时空模式与Frk1区分开来，这表明它在专门用于淀粉储存的植物细胞中起主要作用。
