Genomic analysis of the Bacillus megaterium Mes11: New insights into nitroreductase genes associated with the degradation of mesotrione

Mesotrione is among the most recently marketed herbicides belonging to the triketone family and applied on maize crops. A Bacillus megaterium Mes11 strain isolated from an agricultural soil was able to completely transform 1 mM of mesotrione, alone or in a mixture with 1 mM of nicosulfuron and/or S-metolachlor herbicides, in 5 h. Its complete genome contains one circular chromosome (5,191,612 bp) and nine plasmids (from 45,372 to 169,684 bp). Genome analysis revealed that 10 genes encoded oxygen-insensitive nitroreductases of which only one (NfrA2) was shown to be involved in the first step of mesotrione transformation « in cellulo ». Indeed, quantitative reverse-transcription PCR analysis demonstrated that the NfrA2 expression was concomitant with mesotrione transformation, showing a maximum 6-fold increase after a 4 h exposure to this single molecule. As opposed to nicosulfuron, the presence of S-metolachlor led to a significant 12 to 24-fold increase of NfrA2 expression after the same time of exposure, in the binary and ternary mixtures, respectively. These results highlight, for the first time, the physiological role of NfrA2 in mesotrione transformation « in cellulo » and the environmental role of B. megaterium Mes11 for bioremediation of mesotrione-contaminated ecosystems.