Regulation of the atrazine degradative pathway in Pseudomonas

Abstract

In recent times, the use of the s-triazine herbicide atrazine has become a major concern, due to increasing evidence of severe ecotoxicological effects. The development of strategies for bioremediation of contaminated soils and waters requires the isolation and development of strains that harbor an appropriate catabolic pathway, are competitive in the wild and display the degradative phenotype under field conditions. A limitation to the use of bioremediation for the decontamination of atrazine-polluted sites is the fact that the presence of preferential nitrogen sources, such as those used for fertilization of agricultural soils often inhibits the degradative pathway, resulting in low degradation rates. We have characterized this phenomenon in the model strain Pseudomonas sp. ADP. In this organism, atrazine degradation is nitrogen-repressed both in cultures and in soil. Nitrogen status is sensed from intracellular pools of metabolites. We have used this knowledge to develop a mutant unable to assimilate nitrate that displays an efficient degradative phenotype in nitrate-amended soil. The inhibitory effect of nitrogen operates at the level of gene expression. One of the targets of this regulation is the cyanuric acid degradative operon atzDEF, wich is coordinately activated by nitrogen limitation and the presence of cyanuric acid. A complex regulatory circuit involving at least two regulators and two forms of RNA polymerase is responsible for both responses. The atrazine degradative pathway has proven an attractive model for both basic and applied studies on biodegradation.