Conservation in wheat high-molecular-weight glutenin gene promoter sequences: comparisons among loci and among alleles of the GLU-B1-1 locus

Abstract

The high-molecular-weight glutenin (HMW) genes and encoded subunits are known to be critical for wheat quality characteristics and are among the best-studied cereal research subjects. Two lines of experiments were undertaken to further understand the structure and high expression levels of the HMW-glutenin gene promoters. Cross hybridizations of clones of the paralogous x-type and y-type HMW-glutenin genes to a complete set of six genes from a single cultivar showed that each type hybridizes best within that type. The extent of hybridization was relatively restricted to the coding and immediate flanking DNA sequences. Additional DNA sequences were determined for four published members of the HMW-glutenin gene family (encoding subunits Ax2*, Bx7, Dx5, and Dy10) and showed that the flanking DNA of the examined genes diverge at approximately −1200 bp 5′ to the start codon and 200–400 bp 3′ to the stop codon. These divergence sites may indicate the boundaries of sequences important in gene expression. In addition, promoter sequences were determined for alleles of the Bx gene (Glu-B1-1), a gene reported to show higher levels of expression than other HMW-glutenin genes and with variation among cultivars. The sequences of Bx promoters from three cultivars and one wild tetraploid wheat indicated that all Bx alleles had few differences and contained a duplicated portion of the promoter sequence “cereal-box” previously suspected as a factor in higher levels of expression. Thus, the “cereal-box” duplication preceeded the origin of hexaploid wheat, and provides no evidence to explain the variations in Bx subunit synthesis levels. One active Bx allele contained a 185-bp insertion that evidently resulted from a transposition event.