Molecular cloning and characterization of two novel isoforms of the small subunit of ADPglucose pyrophosphorylase from sweet potato

Two cDNA clones encoding two different ADPglucose pyrophosphorylase (AGPase) polypeptides designated IbAGP-sTL1 (sTL1) and IbAGP-sTL2 (sTL2) were isolated from sweet potato tuberous root and leaf libraries. The two are 84.1% and 90.6% identical at the nucleotide and amino acid sequence level, respectively. They showed higher homology with previously identified small AGPase subunit polypeptides than with large subunits, indicating that they belong to the class of small AGPase subunit polypeptides. Although both isoforms were expressed in the same organs of sweet potato, including tuberous root, leaf and stem tissues, the steady-state level of sTL1 transcripts was always higher than that of sTL2. Throughout the various developmental stages of leaves examined both isoforms were actively expressed and no significant changes in mRNA level were detected. In leaves, the level of sTL1 mRNA was increased enormously by treatment with exogenous sucrose and moderately by growth under constant light, whereas the transcript level of sTL2 remained almost unaffected, indicating that sTL1 but not sTL2 is a sucrose-inducible and light-responsive gene of starch biosynthesis. sTL1 is the only sucrose-inducible gene encoding a small AGPase subunit so far characterized from higher plants. Genomic Southern analysis suggests that the two isoforms originate from different loci in the sweet potato genome.