The Function of Rice Starch Synthase I Expressed in Escherichia coli

Abstract

SSI accounts for 60% of the total SS activity in the soluble fraction in the developing rice endosperm. Rice SSI-deficient mutants were identified by using reverse genetics, and the chain-length analysis of the endosperm starch showed that SSI distinctly synthesizes DP 8-12 chains from short DP 6-7 chains emerging from the A chains and the branch points in the B₁ chains of amylopectin.¹⁾ In this study, to evaluate by in vitro study the function of recombinant SSI of rice (rSSI), the change in the chain-length distribution of glycogen or amylopectin in the activity band after rSSI enzymatic reactions in native-PAGE gel was examined. When glycogen was used as the substrate, the α-glucan produced in the rSSI activity band on the native-PAGE gel had specifically fewer DP 6 chains and more DP 8 chains than unmodified glycogen did. When rice amylopectin was used as the substrate, the α-glucan produced in the rSSI activity band showed the oscillation of a short turn in the range of DP ≤ 20 compared with the unmodified amylopectin; the chains with DP 6, 7, 10, 11, 13 and 17 decreased, while the extent of decrease was reduced in the chains with DP 8, 12 and 15. These results suggest that rSSI preferentially elongates the chains up to DP 8 from the short and long B₁ and B₂ chains with DP 6 or 7 from branch points to the non-reduced end as well as the A chains with DP 6 or 7 by adding one or two glucose moieties.