Two barley alpha-amylase gene families are regulated differently in aleurone cells

Isolation of a full- or nearly full-length cDNA clone for the type B isozyme of barley alpha-amylase is described. The coding region has a high GC content and an unusual codon bias similar to that described for the type A isozyme cDNA previously characterized by our laboratory. The theoretical secondary structure for the 5' end of this type B mRNA, as well as that for the two previously identified type A transcripts, is unusual in that the AUG initiation codon is enclosed in each instance by a very stable hairpin loop. Results of primer extension experiments utilizing probes for the 5' coding regions of both types of mRNA are consistent with some of the features of these predicted structures. Primer extension experiments utilizing a probe specific for the type B 5' coding region identified only one type of extension products, and these had 5' untranslated region sequences identical to that of the type B cDNA. Thus there appears to be only one major type of type B gene transcribed. Hybridization experiments utilizing 5' probes specific for type A and for the type B mRNA demonstrate different effects of the hormone, gibberellic acid. The type A mRNAs are present in relatively large amounts in unstimulated aleurone cells and increase about 20-fold after stimulation with gibberellic acid. In contrast, the type B mRNA is present at very low levels in unstimulated cells, but increases at least 100-fold after cells are exposed to the hormone.