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

Abstract 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.