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.