EXPRESSION OF BRANCHING ENZYME-II OF MAIZE ENDOSPERM IN ESCHERICHIA-COLI

A cDNA clone encoding maize branching enzyme II (BEII) has been indepe ndently isolated from a maize endosperm cDNA library. The deduced prot ein sequence of maize BEII was compared with that of BE from diverse s ources. The gene encoding mature BEII of maize endosperm has been expr essed in E. coli using the T-7 promoter. The expressed BEII was purifi ed to near homogeneity so that amylolytic activity and bacterial BE co uld be completely eliminated from the BE preparation. The expressed en zyme showed very similar properties to those of BEII purified from dev eloping maize endosperm. This result confirmed our earlier report that BEII had a lower rate of branching amylose and the rate of branching amylopectin was twice that of branching amylose. This study also showe d a greater advantage of purifying BEII from the bacterial expression system than from developing maize endosperm. Most importantly, this st udy has established a useful tool to study the structure-function rela tionships of the maize BE using site-directed mutagenesis.