The major beta-amylase isoforms of wheat leaves correspond to one of two ubiquitously expressed beta-amylase genes

Leaves of wheat (Triticum aestivum L., cv. Star) exhibit five distinguishab le isoforms of a beta-amylase (EC 3.2.1.2) considered to represent the tiss ue-'ubiquitous' type of exohydrolase common to all cereals. The object of t his study was to determine whether the multiple leaf isoforms originate fro m different genes or reflect post-translational processing of an isoform fi rst expressed in juvenile leaf tissue. Two different cDNAs encoding for bet a-amylase were isolated from leaves and each produced an active beta-amylas e protein upon heterologous expression in Escherichia coli. Transcripts of these two genes were detected in tissues of wheat leaves, roots, flowers an d seeds. However, only one of the two heterologously expressed beta-amylase s appeared to correspond to the beta-amylase isoforms detectable in non-end osperm wheat tissues. It exhibited specific sequence identities with, and e lectrophoretic mobility under non-denaturing conditions similar to, the ini tially expressed leaf beta-amylase isoform. As does the initially in vivo e xpressed leaf isoform, the heterologously expressed beta-amylase was conver ted by a beta-amylase-free wheat leaf extract into secondary isoforms which closely resemble beta-amylase isoforms appearing in vivo upon the maturati on of leaf tissue. The molecular masses and the N-terminal amino acid seque nces of the heterologously expressed beta-amylase, its secondary conversion products and the extractable leaf beta-amylases indicate that at least the major components of wheat leaf beta-amylase polymorphism reflect C-termina l proteolytic processing of a single beta-amylase translation product. (C) Elsevier, Paris.