Using reverse transcriptase PCR method and a bacterial fosmid library screening,an arsenite oxidase gene cluster were isolated from an arsenite-oxidizing bacterium Acidovorax sp.GW2.There are seven genes including aoxRSXABCD putatively encoding the transcriptional regulator AoxR of a two-component signal transduction system (68% identity),a periplasmic sensor histidine kinase AoxS (55% identity),a periplasmic binding protein AoxX (55% identity),arsenite oxidase AoxAB(74% and 71% identity,respectively),nitroreductase AoxC (46% identity) and cytochrome c AoxD (63% identity) respectively.According to the reverse transcriptase PCR experiments,aoxR and aoxS encoding for a two-component system proteins are co-transcribed and located in opposite to structural genes aoxABCD.aoxX and aoxRS are not in the same operon.Functional analyses through gene knock-out of aoxS,aoxX and aoxD showed that aoxS and aoxX are the essential genes in arsenite oxidation of GW2,and the loss of aoxD did not show significant effects on arsenite oxidation.