Characterization of the Sinorhizobium meliloti genes encoding a functional dihydrodipicolinate synthase (dapA) and dihydrodipicolinate reductase (dapB)

In bacteria, the known pathways for diaminopimelate (DAP) and lysine biosynthesis share two key enzymes, dihydrodipicolinate synthase and dihydrodipicolinate reductase, encoded by the dapA and dapB genes, respectively. In rhizobia, these genes have not yet been genetically characterized. In this work, by sequence analysis, we identified two divergent open reading frames on the 140-MDa plasmid pRmeGR4b of Sinorhizobium meliloti strain GR4. Termed dapA and dapB, these encode products which show significant sequence similarities to DapA and DapB proteins, respectively. Escherichia coli DAP auxotrophs (dapA and dapB mutants) could be complemented with the pRmeGR4b dapA and dapB genes, indicating that these genes code for functional dihydrodipicolinate synthase and dihydrodipicolinate reductase, respectively. Reverse-transcriptase PCR analyses and beta-galactosidase assays using transcriptional dapA-lacZ and dapB-lacZ fusions suggest that these genes are constitutively expressed in S. meliloti. The dapA and dapB genes are not widely distributed in S. meliloti and appear to be specific for strains carrying pRmeGR4b-type plasmids.