A single limit dextrinase gene is expressed both in the developing endosperm and in germinated grains of barley

The single gene encoding limit dextrinase (pullulan 6-glucanohydrolase; EC 3.2.1.41) in barley (Hordeom vulgare) has 26 introns that range in size fro m 93 to 822 base pairs. The mature polypeptide encoded by the gene has 884 amino acid residues and a calculated molecular mass of 97,417 D. Limit dext rinase mRNA is abundant in gibberellic acid-treated aleurone layers and in germinated grain. Gibberellic acid response elements were found in the prom oter region of the gene. These observations suggest that the enzyme partici pates in starch hydrolysis during endosperm mobilization in germinated grai n. The mRNA encoding the enzyme is present at lower levels in the developin g endosperm of immature grain, a location consistent with a role for limit dextrinase in starch synthesis. Enzyme activity was also detected in develo ping grain. The limit dextrinase has a presequence typical of transit pepti des that target nascent polypeptides to amyloplasts, but this would not be expected to direct secretion of the mature enzyme from aleurone cells in ge rminated grain. It remains to be discovered how the enzyme is released from the aleurone and whether another enzyme, possibly of the isoamylase group, might be equally important for starch hydrolysis in germinated grain.