Allelic analysis of the maize amylose-extender locus suggests that independent genes encode starch-branching enzymes LLa and LLb

Starch branching enzymes (SEE) catalyze the formation of alpha-1,6-glucan linkages in the biosynthesis of starch. Three distinct SEE isoforms have been identified in maize (Zea mays L.) endosperm, SBEI, IIa, and IIb. Independent genes have been identified that encode maize SBEI and IIb; however, it has remained controversial as to whether SBEIIa and IIb result from posttranscriptional processes acting on the product of a single gene or whether they are encoded by separate genes. To investigate this question, we analyzed 16 isogenic lines carrying independent alleles of the maize amylose-extender (ae) locus, the structural gene for SBEIIb. We show that 22 d after pollination ae-BI endosperm expressed little Sbe2b (ae)-hybridizing transcript, and as expected, ae-B1 endosperm also lacked detectable SBEIIb enzymatic activity. Significantly, we show that ae-BI endosperm contained SBEIIa enzymatic activity, strongly supporting the hypothesis that endosperm SBEIIa and IIb are encoded by separate genes. Furthermore, we show that in addition to encoding the predominant SbeSb-hybridizing message expressed in endosperm, the ae gene also encodes the major Sbe2b-like transcript expressed in developing embryos and tassels.