Minisymposium 2: Plant Biology in the Tropics

Abs # 12002: Rhizobium genes required for meta-cleavage of aromatics in leucaena

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

Mimosine is a toxic aromatic compound found in the tree legume Leucaena leucocephala (leucaena) and which is also exuded into the rhizosphere. The leucaena-nodulating Rhizobium sp. strain TAL1145 degrades mimosine and utilizes it as a source of carbon and nitrogen. We have identified and isolated genes from Rhizobium sp. strain TAL1145 that encode enzymes involved in the degradation pathway for mimosine. Mimosine degradation involves at least two major steps: first, mimosine is degraded into 3-hydroxy-4-pyridone (HP); and then, HP is degraded through a meta-cleavage pathway. The first step of mimosine degradation involves five genes; midA, midB, midC, midD and midR, of which the first three genes encode ABC-transporter proteins involved in mimosine uptake while midD encodes an aminotransferase required for degrading mimosine into HP, and midR is a regulatory gene encoding a LysR-type transcriptional activator. The degradation of HP involves two major enzymes of meta-cleavage pathway, 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, while hydrolase adds water across the double bonds of the product of the meta-cleavage, producing the final products pyruvate, formate and ammonia. Genes downstream of pydA and pydB encode an ABC transport system that is specific for the uptake of HP. mid and pyd genes are specific for the Mid+ strains of leucaena Rhizobium and are absent in strains of other Rhizobium, Sinorhizobium and Bradyrhizobium spp. Elucidation of the mimosine degradation pathway will be useful for genetic engineering of leucaena and rhizosphere bacteria for bioremediation of toxins as well as investigating plant-microbe interactions.