INDEPENDENT GENETIC-CONTROL OF MAIZE STARCH-BRANCHING ENZYMES IIA ANDIIB - ISOLATION AND CHARACTERIZATION OF A SBE2A CDNA

In maize (Zea mays L.) three isoforms of starch-branching enzyme (SBEI , SBEIIa, and SBEIIb) are involved in the synthesis of amylopectin, th e branched component of starch. To isolate a cDNA encoding SBEIIa, deg enerate oligonucleotides based on domains highly conserved in Sbe2 fam ily members were used to amplify Sbe2-family cDNA from tissues lacking SBEIIb activity. The predicted amino acid sequence of a Sbe2a cDNA ma tches the N-terminal sequence of SBEIIa protein purified from maize en dosperm. The size of the mature protein deduced from the cDNA also mat ches that of SBEIIa. Features of the predicted protein are most simila r to members of the SBEII family; however, it differs from maize SBEII b in having a 49-amino acid N-terminal extension and a region of subst antial sequence divergence. Sbe2a mRNA levels are 10-fold higher in em bryonic than in endosperm tissue, and are much lower than Sbe2b in bot h tissues; Unlike Sbe2b, Sbe2a-hybridizing mRNA accumulates in leaf an d other vegetative tissues, consistent with the known distribution of SBEIIa and SBEIIb activities.