A minor form of starch branching enzyme in potato (Solanum tuberosum L.) tubers has a major effect on starch structure: cloning and characterisation of multiple forms of SBE A

Full length cDNAs encoding a second starch branching enzyme (SBE A) isoform have been isolated from potato tubers. The predicted protein has a molecul ar mass of 101 kDa including a transit peptide of 48 amino acids. Multiple forms of the SBE A gene exist which differ mainly in the length of a polygl utamic acid repeat at the C-terminus of the protein. Expression of the matu re protein in Escherichia coli demonstrates that the gene encodes an active SBE. Northern analysis demonstrates that SBE A mRNA is expressed at very l ow levels in tubers but is the predominant isoform in leaves. This expressi on pattern was confirmed by Western analysis using isoform specific polyclo nal antibodies raised against E. coli expressed SBE A. SBE A protein is fou nd predominantly in the soluble phase of tuber extracts, indicating a strom al location within the plastid. Transgenic potato plants expressing an anti sense SBE A RNA were generated in which almost complete reductions in SBE A were observed. SBE activity in the leaves of these plants was severely red uced, but tuber activity was largely unaffected. Even so, the composition a nd structure of tuber starch from these plants was greatly altered. The pro portion of linear chains was not significantly increased but the average ch ain length of amylopectin was greater, resulting in an increase in apparent amylose content as judged by iodine binding. In addition, the starch had m uch higher levels of phosphorous.