CYTOSOLIC FRUCTOSE-1,6-BISPHOSPHATASE - A KEY ENZYME IN THE SUCROSE BIOSYNTHETIC-PATHWAY

Fructose-1,6 bisphosphatase (FBPase) is a ubiquitous enzyme controllin g a key reaction. In non-photosynthetic tissues, it regulates the rate of gluconeogenesis. In photosynthetic tissues, two FBPase isozymes (c hloroplastic and cytosolic) play key roles in carbon assimilation and metabolism. The cytosolic FBPase is one of the regulatory enzymes in t he sucrose biosynthetic pathway - its activity is regulated by both fi ne and coarse control mechanisms. Kinetic and allosteric properties of the plant cytosolic FBPase are remarkably similar to the mammalian an d yeast FBPase, but differ greatly from those of the chloroplastic FBP ase. Cytosolic FBPase is relatively conserved among various organisms both at amino acid and nucleotide sequence levels. There is slightly h igher similarity between mammalian FBPase and plant cytosolic FBPase t han there is between the two plant FBPases. Expression of plant cytoso lic FBPase gene is developmentally regulated and appears to be coordin ated with the expression of Rubisco and other carbon metabolism enzyme s. Similar to the gluconeogenic FBPase, relatively rapid end product r epression of FBPase gene occurs in plant. However, unlike the gluconeo genic FBPase, a concurrent decline in plant FBPase activity does not o ccur in response to increased end product levels. The physiological si gnificance of FBPase gene repression, therefore, remains unclear in pl ants. Both expression and activity of the cytosolic FBPase are regulat ed by environmental factors such as light and drought conditions. Ligh t-dependent modulation of FBPase activity in plants appears to involve some type of posttranslational modification. In addition to elucidati ng the exact nature of the presumed posttranslational modification, cl oning of genomic and upstream sequences is needed before we fully unde rstand the molecular regulation of the cytosolic FBPase in plants. Use of transgenic plants with altered rates of FBPase activity offers pot ential for enhanced crop productivity.