BIODEGRADATION OF TRIBUTYL PHOSPHATE BY Rhodopseudomonas palustris UNDER PHOTOSYNTHETIC CONDITIONS - INVOLVEMENT OF A PUTATIVE CYTOCHROME P450

摘要

Tributyl phosphate (TBP) is an organophosphorous compound widely used in nuclear fuel processing and also in a number of other chemical industries generating many wastes. Althought this compound is bacteriostatic, some strains are resistant or even able to degrade it in some cases. Rhodopseudomonas palustris (R. palustris) is one of these few bacterial strains able to degrade TBP under both aerobic and anaerobic conditions. Indeed, under anaerobic photosynthetic conditions, up to 80% of the TBP 2 mM initially present in the medium is degraded in three weeks growth, which ranks as one of the best degradation yield reported for a pure strain. The aim of this work was to identify the enzymes involved in this degradation. We chose to focus our studies on R. palustris CGA009 because its genome is now completely sequenced. As cytochromes P450 correspond to a family of enzymes involved in the oxidative metabolism of a high number of xenobiotic compounds by microorganisms as well as higher animals and plants, they are expected to initiate the TBP degradation. To provide an evidence of the participation of a cytochrome P450 system during the TBP degradation, R. palustris CGA009 was initially grown in the presence of several cytochromes P450 regulators. As a result, its ability to degrade TBP was decreased by the presence of miconazole, 3-methylcholanthrene and carbon monooxide, all known to be inhibitors of P450. In addition, TBP degradation is increased by amobarbital and atrazine, both inducers of cytochromes P450. These results suggest a correlation between TBP degradation and cytochrome P450 activation. Moreover the gene 5939 encoding for a putative cytochrome P450 in the R. palustris CGA009 genome was deleted. This inactivation is concomitant with a dramatic decrease of the TBP degradation (40% less). Thus, these results show that a cytochrome P450, encoding by the 5939 gene, is involved in the TBP degradation in R. palustris CGA009.