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.