STRUCTURE AND EXPRESSION OF A DHURRINASE (BETA-GLUCOSIDASE) FROM SORGHUM

Sorghum (Sorghum bicolor L. Moench) has two isozymes of the cyanogenic beta-glucosidase dhurrinase: dhurrinase-1 (Dhr1) and dhurrinase-2 (Dh r2). A nearly full-length cDNA encoding dhurrinase was isolated from 4 -d-old etiolated seedlings and sequenced. The cDNA has a 1695-nucleoti de-long open reading frame, which codes for a 565-amino acid-long prec ursor and a 514-amine acid-long mature protein, respectively. Deduced amino acid sequence of the sorghum Dhr showed 70% identity with two ma ize (Zea mays) beta-glucosidase isozymes. Southern-blot data suggested that beta-glucosidase is encoded by a small multigene family in sorgh um. Northern-blot data indicated that the mRNA corresponding to the cl oned Dhr cDNA is present at high levels in the node and upper half of the mesocotyl in etiolated seedlings but at low levels in the root-onl y in the zone of elongation and the tip region. Light-grown seedling p arts had lower levels of Dhr mRNA than those of etiolated seedlings. I mmunoblot analysis performed using maize-anti-beta-glucosidase sera de tected two distinct dhurrinases (57 and 62 kD) in sorghum. The distrib ution of Dhr activity in different plant parts supports the mRNA and i mmunoreactive protein data, suggesting that the cloned cDNA correspond s to the Dhr1 (57 kD) isozyme and that the dhr1 gene shows organ-speci fic expression.