Microbial Degradation of Chlorinated Aromatic Compounds
The aerobic microbial degradation of various chloroaromatics usually occurs via chlorocatechols as central intermediates. These are further degraded through the modified ortho-cleavage pathway. Dechlorination takes place during cycloisomerization of chloromuconates and reduction of chloromaleylacetates. In contrast, the degradation of haloaromatics via meta-cleavage was thought to be impossible due to toxicity of cleavage intermediates, whereas the meta route is more effective for methylaromatics. Recently, Pseudomonas putida strain GJ31 was shown to be able to degrade toluene and chlorobenzene simultaneously. Strain GJ31 rapidly degrades chlorobenzene via 3-chlorocatechol using the meta-cleavage pathway without any apparent toxic effects. An unusual chlorocatechol 2,3-dioxygenase oxidizes 3-chlorocatechol. Stoichiometric displacement of chloride then leads to the production of 2-hydroxymuconate, which is a common metabolite of the meta-cleavage pathway. In contrast to other catechol 2,3-dioxgenases, which are subject to inactivation when exposed to 3-chlorocatechol, the chlorocatechol 2,3-dioxygenase is resistant. The gene encoding the chlorocatechol 2,3-dioxygenase (cbzE) of strain GJ31 was cloned and sequenced. CbzE was most similar to catechol 2,3-dioxygenases of the 2.C subfamily of type 1 extradiol dioxygenases. Hybrid enzymes, which were made of CbzE and the 3-methylcatechol 2,3-dioxygenase of strain P. putida UCC2, showed that the resistance of CbzE to suicide inactivation and the substrate specificity were mainly determined by the C-terminal region of the protein. Establishing whether the meta-cleavage pathway of strain GJ31 can function as pathway segment in the construction of novel chloroaromatics-degraders is a future task. Organisms such as P. putida strain GJ31 and its derivatives may be useful for effective treatment of waste streams containing various methyl- and chloroaromatics.
KeywordsPseudomonas Putida Environmental Microbiology Chlorine Substituent Hybrid Enzyme Extradiol Dioxygenases
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