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Plant Physiology 97:1323-1328 (1991)
© 1991 American Society of Plant Biologists

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Metabolism and Enzymology

Bifunctional Protein in Carrot Contains Both Aspartokinase and Homoserine Dehydrogenase Activities 1

Barbara J. Wilson, Ann C. Gray and Benjamin F. Matthews

U.S. Department of Agriculture, Agricultural Research Service, Beltsville, Maryland 20705, Plant Molecular Biology Laboratory, Beltsville, Maryland 20705

We have purified homoserine dehydrogenase to homogeneity and subjected polypeptide fragments derived from digests of the protein to amino acid sequencing. The amino acid sequence of homoserine dehydrogenase from carrot (Daucus carota) indicates that in carrot both aspartokinase and homoserine dehydrogenase activities reside on the same protein. Additional evidence that aspartokinase and homoserine dehydrogenase reside on a bifunctional protein is provided by coelution of activities during purification steps and by enzyme-specific gel staining techniques. Highly purified fractions containing aspartokinase activity were stained for aspartokinase activity, homoserine dehydrogenase activity, and protein. These gels confirmed that aspartokinase activity and homoserine dehydrogenase activity were present on the same protein. This arrangement of aspartokinase and homoserine dehydrogenase activities residing on the same protein is also found in Escherichia coli, which has two bifunctional enzymes, aspartokinase I-homoserine dehydrogenase I and aspartokinase II-homoserine dehydrogenase II. The amino acid sequence of the major form of homoserine dehydrogenase from carrot cell suspension cultures most closely resembles that of the E. coli ThrA gene product aspartokinase I-homoserine dehydrogenase I.

1 Supported in part by U.S. Department of Agriculture Competitive Research Grant Office (No. 87-CRCR-1-2284).






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