CHARACTERIZATION OF DAPB, A GENE REQUIRED BY PSEUDOMONAS-SYRINGAE PV TABACI BR2.024 FOR LYSINE AND TABTOXININE-BETA-LACTAM BIOSYNTHESIS

The dapB gene, which encodes L-2,3-dihydrodipicolinate reductase, the second enzyme of the lysine branch of the aspartic amino acid family, was cloned and sequenced from a tabtoxin-producing bacterium, Pseudomo nas syringae pv. tabaci BR2.024, The deduced amino acid sequence share d 60 to 90% identity to known dapB gene products from gram-negative ba cteria and 19 to 21% identity to the dapB products from gram-positive bacteria. The consensus sequence for the NAD(P)H binding site [(V/I)(A /G)(V/I)XGXXGXXG)] and the proposed substrate binding site (HHRHK) wer e conserved in the polypeptide. A BR2.024 dapB mutant is a diaminopime late auxotroph and tabtoxin negative, The addition of a mixture of L-, L-, D,D-, and meso-diaminopimelate to defined media restored growth bu t not tabtoxin production. Cloned DNA fragments containing the parenta l dapB gene restored the ability to grow in defined media and tabtoxin production to the dapB mutant, These results indicate that the dapB g ene is required for both lysine and tabtoxin biosynthesis, thus provid ing the first genetic evidence that the biosynthesis of tabtoxin proce eds in part along the lysine biosynthetic pathway. These data also sug gest that L-2,3,4,5-tetrahydrodipicolinate is a common intermediate fo r both lysine and tabtoxin biosynthesis.