Abstract
Isoamylase (ISA) is known to play an essential role in amylopectin biosynthesis. Recently, Hussain et al. (2003) characterized three ISA isoforms from potato tuber, and showed that all higher plants have three distinct ISA isoforms encoded by three gene families.
cDNAs of OsISA1, OsISA2 and OsISA3 ORF were identified by RT-PCR. Their predicted amino acid sequences showed high homologies to the corresponding ISA families of higher plants. OsISA1 was mainly expressed in endosperm, while OsISA3 was mainly expressed in leaves. OsISA2 was considered to be inactive on its own, but present in both tissue. The ISA preparation purified from rice endosperm contained only a single OsISA1 band, whereas that from potato tuber contained ISA1 and ISA2 bands. These results suggest that OsISA1 is a homo-oligomeric enzyme whereas potato ISA is a multimeric enzyme composed of ISA1 and ISA2.
