Poster: Plant Pathogen/Symbiont Interactions

Abs # 735: Rhizobium genes required for meta-cleavage of aromatics in leucaena

Presenter:	Awaya, Jonathan D., awayaj@hawaii.edu
Authors	Awaya, Jonathan D. (A)   Fox, Paul M. (A)   Borthakur, Dulal  (A)
Affiliations:	(A): Dept. Molecular Biosciences and Bioengineering, University of Hawaii

Mimosine and 3-hydroxy-4-pyridone (HP) are toxic aromatic compounds found in the tree legume Leucaena leucocephala and which are also exuded into the rhizosphere. We have identified and isolated genes from Rhizobium sp. strain TAL1145 that encode enzymes involved in the degradation pathway for mimosine and HP. Mimosine degradation involves at least two major steps: (1) mimosine is degraded into HP; (2) HP is degraded in subsequent steps. As in the degradation of many aromatic compounds, the degradation of HP involves the major enzymes, dioxygenase and hydrolase, that are encoded by the pydA and pydB genes, respectively. The dioxygenase opens the aromatic ring by adding two atoms of molecular oxygen in a meta-cleavage. Hydrolase adds water across the double bonds of the product of the meta-cleavage, producing the final products pyruvate and formate. pydA is expressed constitutively in Rhizobium TAL1145 but its expression level increases 3.5-4.5 times in the presence of mimosine. The sizes of the dioxygenase and hydrolase proteins were confirmed to be 37kDa and 30kDa, respectively. Elucidation of the mimosine and HP degradation pathways will be useful for genetic engineering of leucaena and rhizosphere bacteria for bioremediation of toxins as well as investigating plant-microbe interactions.