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N-acyl-homoserine lactones (AHLs) serve as quorum-sensing signals,which control a number of bacterial processes in many proteobacteria.Here we report the effects of exogenous short-chain AHL on the denitrifying process of Paraeoccus denitrificans,which are capable of aerobic and anaerobic growth by utilizing nitrate.The denitrification activity of these cells was monitored by measuring denitrification products (including nitrate,nitrite,and nitrous oxide),and the individual messenger ribonucleic acid (mRNA) levels of nitrate,nitrite,nitric oxide and nitrous oxide reductases.The results indicated that 2 μmol/L C6-homoserine lactone (HSL) has little effect on cell density under either anaerobic or aerobic culture conditions,and the nitrate reduction activity appeared slightly affected by N-hexanoyl-DL-homoserine lactone (C6-HSL).However,exogenous C6-HSL significantly affected the transcription of nitrite reductase and nitric oxide reductase genes in P.denitrificans regardless of the presence of oxygen,and N2O accumulation activity in P.denitrificans was suppressed by C6-HSL under aerobic condition.In contrast,exogenous C6-HSL stimulated the production of N2O under anaerobic condition,suggesting that the regulation of denitrification by quorum sensing may be important in N2O release.