reactions and utilizes the products of the light reactions of photosynthesis, ATP and NADPH to fix atmospheric The photosynthetic carbon reduction (Calvin) cycle is CO 2 into carbon skeletons which are then used directly the primary pathway for carbon fixation and the for starch and sucrose biosynthesis ( Fig. 1 ) ( Woodrow enzyme sedoheptulose-1,7-bisphosphatase functions and Berry, 1988; Geiger and Servaites, 1995; Quick and in the regenerative phase of this cycle where it Neuhaus, 1997).

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... cycle can be considered to have catalyses the dephosphorylation of sedoheptulosethree stages, the first of these being carboxylation of the 1,7-bisphosphate. This enzyme is unique to the CO 2 acceptor molecule ribulose-1,5-bisphosphate (RubP) Calvin cycle and has no counterpart in non-photoby the enzyme ribulose-1,5-bisphosphate carboxylase synthetic organisms. The isolation and sequence oxygenase (Rubisco), resulting in the formation of analysis of an SBPase clone has led to a number 3-phosphoglycerate. This is followed by the reduction of investigations which have yielded interesting phase which produces the triose phosphates, glyceraland novel information on this enzyme and in this dehyde and dihydroxyacetone phosphate, consuming ATP paper the biochemistry and molecular biology of and NADPH in the process. The final phase of the cycle SBPase are reviewed. Some recent exciting developis the regenerative stage when triose phosphates are used ments are also reported, including the analysis of to produce the CO 2 acceptor molecule RubP. The carbtransgenic plants with reduced levels of SBPase which oxylation and reduction steps of the cycle can be considhas shown that SBPase is a key regulator of carbon ered together as the linear assimilatory phase, the products flux and mutagenesis studies which have resulted of which are available for allocation either to RubP in the identification of the redox active cysteines regeneration within the cycle or to the starch and sucrose responsible for the regulation by light of SBPase catabiosynthetic pathways. It is extremely important that a lytic activity. balance is maintained between the amount of carbon leaving the cycle and that needed to maintain RubP levels Key words: Gene expression, photosynthesis, primary for the continued functioning of the cycle. In order to carbon metabolism, redox regulation, SBPase, structure, achieve this balance the catalytic activities of certain transgenic plants. enzymes within the cycle are highly regulated. In particular, a number of the enzymes, including SBPase, are