Purification and Characterization of Catabolic Mannopine Cyclase Encoded by the Agrobacterium tumefaciens

Catabolic mannopine (MOP) cyclase encoded by certain Agrobacterium Ti and Ri plasmids lactonizes MOP to agropine (AGR). The enzyme, purified to homogeneity from a recombinant clone, has a molecular mass of 45 kDa as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and size exclusion chro-matography. The enzyme catalyzed the lactonization of MOP to AGR without the need for any cofactors. The enzyme also converted AGR to MOP with the lactonizing activity being predominant over the reverse reaction. MOP cyclase is specific for imine conjugates of D-hexose and L-glutamine and was not inhibited by sugars or amino acids. The enzyme lactonized deoxyfructosyl glutamine, a natural intermediate of MOP synthesis and catabolism, to a product indistinguishable from chrysopine, a newly discovered crown gall opine. The enzyme also lactonized N-1-(1,2-dideoxy-D-mannityl)-L-glutamine, indicating that a hydroxyl group at carbon atom 2 of the sugar moiety is not required for the enzymatic reaction. The molecular basis of crown gall tumors caused by Agrobac-terium tumefaciens is the transfer, integration, and expression of a specific segment of the Ti plasmid, called T-DNA, into plant cells (1, 11). When integrated into the plant cell genome, the T-DNA expresses enzymes that direct the production o