Structure and organization of two divergent α-amylase genes from barley

Abstract

We have isolated several α-amylase genomic clones from an Eco RI library of barley DNA in λ-Charon 32. Five of these clones exhibit unique restriction maps and differences in their abilities to hybridize with two previously characterized α-amylase cDNA probes representing two different loci, α-Amy 1 (high pI) and α-Amy 2 (low pI) on barley chromosomes 6 and 1, respectively. Stringent hybridizations indicate that four of the five genomic clones contain α-Amy 1 sequences and one contains α-Amy 2 sequences. The regions containing α-amylase genes from one representative genomic clone of each group have been sub-cloned, mapped and sequenced. S1-nuclease protection experiments indicate that the two α-amylase genes contained in these clones are functional in aleurone tissue. Transcription start sites in these genes were determined by primer extension using specific synthetic oligonucleotide primers.

The DNA sequences of the two α-amylase genes, including promoter regions, are divergent, as are the predicted amino acid sequences of the mature proteins and the N-terminal “leader” peptides. The α-Amy 1 gene contains two introns while the α-Amy 2 gene has three introns. In the coding region, each gene shows 7–10% sequence divergence with respect to the previously characterized cDNA clones of the same gene type. Therefore, differences in nucleotide sequences can account for some of the isozyme variations seen between the sub-families of α-amylases and among members of the same subfamily. Although the nucleotide sequences of the promoter regions of α-Amy 1 and α-Amy 2 genes show little homology, both contain pairs of inverted repeat elements which could constitute regulatory sites.