ABSTRACTThe ability of microorganisms to perform dissimilatory (per)chlorate reduction is, for most species, known to be oxygen sensitive. Consequently, bioremediation processes for the removal of oxochlorates will be disturbed if oxygen is present. We measured the expression of chlorite dismutase and chlorate reductase in the presence of different terminal electron acceptors in the chlorate reducerIdeonella dechloratans. Enzyme activity assays and mRNA analyses by real-time quantitative reverse transcription (qRT)-PCR were performed on cell extracts from cells grown aerobically with and without chlorate and on cells grown anaerobically in the presence of chlorate. Our results showed that both chlorite dismutase and chlorate reductase are expressed during aerobic growth. However, transfer to anaerobic conditions with chlorate resulted in significantly enhanced enzyme activities and mRNA levels for both enzymes. Absence of oxygen was necessary for the induction to occur, since chlorate addition under aerobic conditions produced neither increased enzyme activities nor higher relative levels of mRNA. For chlorite dismutase, the observed increase in activity was on the same order of magnitude as the increase in the relative mRNA level, indicating gene regulation at the transcriptional level. However, chlorate reductase showed about 200 times higher enzyme activity in anaerobically induced cells, whereas the increase in mRNA was only about 10-fold, suggesting additional mechanisms influence the enzyme activity.