A comparison of the carbohydrate composition and kinetic properties of sucrose phosphate synthase (SPS) in transgenic tobacco (Nicotiana tabacum ) leaves expressing maize SPS protein with untransformed controls

Tobacco plants, transformed with a maize sucrose phosphate synthase (SPS) cDNA clone, had threefold increased SPS activity compared to wild-type tobacco. Measurement of SPS maximal activity and protein abundance using specific antibodies to the maize protein showed that the specific activity of the maize SPS protein was maintained when expressed in tobacco. Comparison of the kinetic properties of SPS in the transgenic lines compared to either wild-type maize or tobacco revealed that the heterologously expressed protein had reduced affinity for both substrates (fructose-6-phosphate and UDP-glucose) and reduced sensitivity to allosteric inhibition by inorganic phosphate. Moreover, the extent of light-induced activation was reduced in the transgenic lines, with smaller changes observed in the K-m for both F6P compared to maize and tobacco wild-type plants. Increased sucrose concentrations were observed in the transgenic lines at the end of the photoperiod and this was linearly related to SPS activity and associated with a parallel decrease in starch content. This suggests that SPS is a major control point for carbohydrate partitioning between starch and sucrose during photosynthesis.