Nucleotide sequence of a cDNA encoding starch-branching enzyme, or q-enzyme I, from rice endosperm.

The programmed regulation of carbohydrate-metabolizing enzymes is associated with an increase in the production of starch in rice endosperm (4, 5). The increase in starch-branching enzyme, or Q-enzyme, plays a critical role in the initiation and regulation of the biochemical process of starch accumulation in rice endosperm (4) (Table I). Q-enzyme catalyzes the synthesis of amylopectin by introducing the a-1,6-glycosylic linkage from a-1,4-glucan. QEI2 is the predominant form of the two isozymes, which are distinguishable immunologically from each other in developing rice endosperm (3). Figure 1 shows the nucleotide sequence of the rice endosperm QEI cloned from a Xgtl 1 cDNA library and the deduced amino acid sequence. The site of translation initiation is presumed to be at nucleotide sequence 30 of the sequence because (a) the rice endosperm QEI-40 cDNA is a full-length cDNA, or very close to it, and (b) the size (about 90 kD) of the in vitro translated product from QEI mRNA, as estimated from SDS-PAGE, is similar to a Mr of 93,258 (open reading frame of 820 amino acids) for the QEI protein predicted from its cDNA sequence. The nucleotide and predicted protein sequences of QEI of rice endosperm exhibit little similarity with those of Q-enzyme from bacteria, Escherichia coli (2) and Synechococcus (1). A leader sequence for amyloplast import remains to be determined. The amino terminal sequence of the native QEI could not be determined by microsequence analysis because the N-terminal amino acid of the native enzyme is modified. We observed that Q-enzyme, as well as ADP-glucose pyrophosphorylase, increases in activity at about the same time when active starch synthesis is occurring in rice endosperm, whereas the bulk of the other enzymes involved in carbohydrate metabolism accumulates during the earlier phase of the endosperm development (4). Therefore, it is postulated that QEI plays a key role in the efficient production of starch in developing rice endosperm.