the PCR cycle. As indicated in Figure 3, the synthesis of sucrose results in the formation of Piin the cy-
tosol. [Although not indicated in the diagram, the inorganic pyrophosphate (PPi) released by reaction 1
can also be converted to Piby the action of an inorganic pyrophosphate [10].] Because this production of
Picould result in a large drain of triose-P from the chloroplast, the synthesis of sucrose in the cytosol (or
perhaps more correctly the synthesis of cytosolic Pi) must be coordinated with ongoing photosynthetic
rates in the chloroplast. Sucrose synthesis is, therefore, a tightly regulated metabolic reaction in the pho-
tosynthetic cell [10].
At present, there appear to be at least two different strategies for regulation of sucrose production in
green plant cells: one mechanism involving regulation of cytosolic FBPase [10,11], which provides hex-
ose phosphates for UDPG and sucrose formation, and the other involving regulation of the SPS enzyme
itself [12,13].
REGULATION OF CYTOSOLIC FBPASE ACTIVITY The cytosolic FBPase reaction (reaction 1,
Figure 3) is the first irreversible step in carbon flow to sucrose and is subject to strong inhibition by a spe-
cific metabolite, fructose 1,2-bisphosphate (F-2,6-bis-P). Formation of F-2,6-bis-P in the cytosol is con-
trolled by a specific Fru-6-P,2-kinase, and degradation is controlled by a Fru-2,6-bis-P phosphatase. The
total concentration of F-2,6-bis-P is, therefore, a net result of the combined activities of these two en-
zymes. The amount of F-2,6-bis-P can, therefore, control the flow of carbon to sucrose by modulating the
activity of the FBPase [10,11].
470 PATTANAGUL ET AL.Figure 2 Chemical structure of sucrose and related raffinose family oligosaccharides.