Regulation Of Cr(?)Detoxification By Bacillus Thuringiensis 407

The widespread use of chromium in diverse industrial processes has converted it into a serious contaminant of air,soil and water.Cr?VI?may cause losses in plant yields,which is mainly due to the change of soil fertility,microbial activity and diversity in the field.Studies on the potential for the biological control of insect pests with Bt,therefore,could be expanded to include the detoxification Cr???from agricultural fields.Investigation on the mechanisms of Cr???detoxification by Bt 407 will establish the fundation of remediation of chromium contaminants using bacteria.In this study,the growth curve of Bt 407 Cry-and its mutations?407-?plcR,407-?inhA2,407-?AmpbE,407-?sigmaE and 407-?sigmaK?and the Cr?VI?and total Cr concentration at t-3?3 h before t0?,t-2?2 h before t0?,t-0?1 h before t0?,t0?time of the strain enter into the stationary phase?,t1?1 h after t0?,t2?2 h after t0?,t3?3 h after t0?,t4?4 h after t0?were determined.The results showed that the pleiotropic transcription activator PlcR negatively regulates the reducation of Cr?VI?at t-3 and t-1,positively affects the reducation of Cr???at t2;MpbE negatively regulates the reducation of Cr???from t-3 to t3;InhA2 and SigmaK' negatively regulates the reducation of Cr???from t₃ to t4;SigmaE have no influence on the the reducation of Cr???from t-3 to t4.The observation that total Cr resisted was similar to initial concentration of Cr?VI?suggests that the essential detoxification mechanism of the Cr?VI?was dependent on Bt reduction,but not bioabsorbing.Besides,PlcR and MpbE negatively regulates the germination of spore.Cr???resistant genes of Bt 407 Cry" were obtained by bioinformatics as follows:an azoR gene encoding azo reductase was 97%homology to chrR of Pseudomonas putida and 57%homology to yieF of Escherichia coli;an efx gene encoding efflux protein was in the downstream;and an chrA gene encoding ChrA chromium transporter was 81%,75%,73%and 59%homology to those of Pseudomonas aeruginosa,Shewanella oneidensis,Alcaligenes eutrophus and Ochrobactrum tritici.The mechanisms of Cr???detoxification by Bt 407 Cry-could be as follows:azo reductase?encoded by azoR?reduce Cr?VI?to Cr???,which is negatively regulated by PlcR,InhA2,MpbE and SigmaK genes.The efflux of chromium is conferred by ChrA protein,as a antiporter.In a word,the resistant of the Cr???by Bt 407 Cry-is a complicated network regulated by azoR,efx,chrA,plcR,inhA2,mpbE and sigmaK genes.In the last part,the purpose of our study was to assess the Cr???-resistance and Cr???-reducing abilities of an indigenous soil isolate of Bt and determine the factors governing reduction.Towards this end a novel dichromate-reducing Bt BRC-ZYR2,characterized with insecticidal crystal proteins?ICPs?was isolated from a uranium deposit.The isolate contains five ICP genes?crylBa,crylBb,crylBe/1Bf,cry9Ca and cry9Da?and exhibits a high level of Cr?VI?resistance with MICs of 150 mg/L at pH 7.0 and 30 ?,and 500 mg/L under optimal conditions?pH 9.0 and 40 ??.The observation that total Cr resisted was similar to initial concentration of Cr?VI?under the optimal condition suggests that the essential removal mechanism of the Cr?VI?was dependent on Bt reduction.Under optimal conditions,the initial Cr?VI?concentrations from 25 to 75 mg/L significantly decreased in 24 h after incubation.Addition of Mn2+,Co2+,Mo2+ and Cu2+ activated Bt mediated Cr?VI?reduction,while Zn2+,Ni2+ and glucose were found to be inhibitory.Our results indicate that this isolate could be a promising biopesticide with the potential for both insect biocontrol and chromium bioremediation in the field.