ABSTRACTThe acidophilic proteobacteriumAcidithiobacillus ferrooxidansis involved in the industrial biorecovery of copper. It is found in acidic environments in biofilms and is important in the biogeochemical cycling of metals and nutrients. Its genome contains a cluster of four genes,glyQ,glysS,gph, andact, that are predicted to encode the α and β subunits of glycine tRNA synthetase, a phosphatase, and an acyltransferase, respectively (GenBank accession no. DQ149607).act, cloned and expressed inEscherichia coli, produces acyl homoserine lactones (AHLs) principally of chain length C14according to gas chromatography and mass spectrometry measurements. The AHLs have biological activity as shown by in vivo studies using the reporter strainSinorhizobium melilotiRm41 SinI−. Reverse transcription-PCR (RT-PCR) experiments indicate that the four genes are expressed as a single transcript, demonstrating that they constitute an operon. According to semiquantitative RT-PCR results,actis expressed more highly whenA. ferrooxidansis grown in medium containing iron than when it is grown in medium containing sulfur. Since AHLs are important intercellular signaling molecules used by many bacteria to monitor their population density in quorum-sensing control of gene expression, this result suggests thatA. ferrooxidanshas two quorum-sensing systems, one based on Act, as described herein, and the other based on a Lux-like quorum-sensing system, reported previously. The latter system was shown to be upregulated inA. ferrooxidansgrown in sulfur medium, suggesting that the two quorum-sensing systems respond to different environmental signals that may be related to their abilities to colonize and use different solid sulfur- and iron-containing minerals.
