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

Leaves of wheat (Triticum aestivum L.), cv. Star) exhibit five distinguishable isoforms of a β-amylase (EC 3.2.1.2) considered to represent the tissue-‘ubiquitous’ type of exohydrolase common to all cereals. The object of this study was to determine whether the multiple leaf isoforms originate from different genes or reflect post-translational processing of an isoform first expressed in juvenile leaf tissue. Two different cDNAs encoding for β-amylase were isolated from leaves and each produced an active β-amylase protein upon heterologous expression in Escherichia coli. Transcripts of these two genes were detected in tissues of wheat leaves, roots, flowers and seeds. However, only one of the two heterologously expressed β-amylases appeared to correspond to the β-amylase isoforms detectable in non-endosperm wheat tissues. It exhibited specific sequence identities with, and electrophoretic mobility under non-denaturing conditions similar to, the initially expressed leaf β-amylase isoforms. As does the initially in vivo expressed leaf isoform, the heterologously expressed β-amylase was converted by a β-amylase-free wheat leaf extract into secondary isoforms which closely resemble β-amylase isoforms appearing in vivo upon the maturation of leaf tissue. The molecular masses and the N-terminal amino acid sequences of the heterologously expressed β-amylase, its secondary conversion products and the extractable leaf β-amylases indicate that at least the major components of wheat leaf β-amylase polymorphism reflect C-terminal proteolytic processing of a single β-amylase translation product.