Molecular biology and biochemistry of mimosine degradation by Rhizobium.

摘要

The rhizosphere of plants is a highly complex ecosystem due to a continuous supply of organic material from plants as root exudates. Mimosine, [β-N-(3-hydroxy-4-pyridone)-α-aminoproprionic acid], a toxic non-protein amino acid, is produced by members of the tree legume genus Leucaena. L. leucocephala is one such member that produces large amounts of mimosine, and secretes it into the rhizosphere as a component of its root exudates. Some rhizosphere bacteria, including some leucaena-nodulating rhizobia, can utilize mimosine as a source of carbon and nitrogen. The ability to degrade mimosine, although not necessary for successful nodulation or nitrogen fixation, does provide mimosine-degrading rhizobia with a competitive advantage in nodulation.; Rhizobium sp strain TAL1145 is a leucaena-nodulating Rhizobium that is also capable of degrading mimosine. Mimosine degradation by TAL1145 appears to involve two major steps. In the first step, mimosine is converted to 3-hydroxy-4-pyridone (HP) by the action of the mid (mimosine degradation) gene cluster. In the second major step, HP is degraded to ammonia, formate and pyruvate by the action of the pyd (pyridine degradation) gene cluster.; Utilizing two cosmid clones of TAL1145, we were able to demonstrate this two step process. Cosmid pUHR181 contains genes that can degrade mimosine to HP, but cannot degrade HP. Cosmid pUHR263 partially overlaps pUHR181, but contains additional genes necessary for the complete degradation of mimosine.; Utilizing transposon Tn3Hogus, we contructed mid and pyd mutants defective in mimosine degradation. The transposon insertion points of these mutants were identified and a restriction map of the entire 25 kb region was created.; We sequenced five genes in the pyd cluster and assigned putative functions to them on the basis of homology with reported genes of similar sequence. pydA encodes a unique dioxygenase of the catechol extra-diol family that is capable of aromatic ring fission, while pydB encodes a hydrolase that degrades the ring fission product to formate, pyruvate and ammonia. An ABC transporter required for HP uptake may be encoded by pydC, pydD, and pydE.; Like HP, many anthropogenic compounds are heterocyclic aromatics. It is therefore possible that the pyd genes may degrade other pollutants in the environment.