A wheat group 1 Lea intron enhances β-glucuronidase gene expression in cereal cells

The Group 1 late embryogenesis abundant (Lea) genes, typified by the wheat Em gene, are expressed to very high levels during the final third of embryo maturation. The structure of Group 1 Lea genes is highly homologous and all contain one small intron at a conserved location in the coding region. Due to the general correlation between the presence of an intron and enhanced expression, it was of interest to determine if the wheat Em intron contributes to high level expression. The intron from the wheat Em gene was embedded in the NH2-terminal region of the β-glucuronidase (GUS) reporter gene coding region such that enzyme activity is dependent on splicing and expression was analyzed in both in vivo and in vitro expression systems. The results show that the Em intron is efficiently spliced (>80%) in transfected rice protoplasts using the normal wheat donor and acceptor sites. However, normalized enzyme activity directed by the intron-containing plasmid is only slightly higher than from an analogous non-intron-containing plasmid. It is further demonstrated that intron-associated enhancement of enzyme activity can be accounted for by differences in spliced GUS mRNA levels and that three novel amino acid codons inserted into the GUS messenger RNA (mRNA) as a result of splicing do not contribute to enhanced expression. We conclude that although this Group 1 Lea intron is able to enhance expression, it does not contribute significantly to high level expression during embryo maturation.