Elucidation of the lysine biosynthesis pathway in plants focusing on the genetic model Arabidopsis thaliana

André O. Hudson and Thomas Leustek

Biotechnology Center, Rutgers University, New Brunswick, NJ

The exact pathway by which plants synthesize the essential amino acid lysine is unknown. Yet, the nutritional value of the major crop plants is limited by the low lysine content. Strategies to improve the lysine content must begin with the characterization of the biosynthetic pathway. The published literature indicates that in plants, the lysine pathway proceeds via the intermediate diaminopimelate (DAP), as it does in bacteria, but only two of the 7 possible enzyme activities have been identified. The availability of the complete genome sequence of Arabidopsis thaliana offers the chance to characterize the genes for lysine biosynthesis. Homologs of bacterial lysine genes have been identified for dihydrodipicolinate synthase, dihydrodipicolinate reductase, diaminopimelate epimerase and diaminopimelate decarboxylase. Homologs could not be identified for tetrahydrodipicolinate acylase, acyldiaminopimelate aminotransferase and acyl-diaminopimelate deacylase, or meso-diaminopimelate dehydrogenase, suggesting that Arabidopsis uses novel genes to carry out the conversion of tetrahydropicolinate to LL-diaminopimelate. In an ongoing effort each of the Arabidopsis genes will be functionally characterized. Three genes were identified with homology to dihydrodipicolinate reductase. The cDNA for one of them was able to functionally complement the diaminopimelate auxotrophy of a dapB strain of Escherichia coli. A single gene was identified with homology to diaminopimelate epimerase. A cDNA derived from the gene was used to express the protein as a recombinant enzyme in E. coli. The protein was partially purified and demonstrated to have diaminopimelate epimerase activity in vitro. By systematically characterizing the genes in the lysine pathway, we hope to elucidate the anabolism of this essential amino acid in the model organism Arabidopsis thaliana.