ROLE AND REGULATION OF SUCROSE-PHOSPHATE SYNTHASE IN HIGHER-PLANTS

Sucrose-phosphate synthase (SPS; E.C. 2.4.1.14) is the plant enzyme th ought to play a major role in sucrose biosynthesis. In photosynthetic and nonphotosynthetic tissues, SPS is regulated by metabolites and by reversible protein phosphorylation. In leaves, phosphorylation modulat es SPS activity in response to light/dark signals and end-product accu mulation. SPS is phosphorylated on multiple seryl residues in vivo and the major regulatory phosphorylation site involved is Ser158 in spina ch leaves and Ser162 in maize leaves. Regulation of the enzymatic acti vity of SPS appears to involve calcium, metabolites, and novel ''coars e'' control of the protein phosphatase that activates SPS. Activation of SPS also occurs during osmotic stress of leaf tissue in darkness, w hich may function to facilitate sucrose formation for osmoregulation. Manipulation of SPS expression in vivo confirms the role of this enzym e in the control of sucrose biosynthesis.