Two whole-cell bioluminescent reporters were constructed by fusing the reporter genes luxAB with the Co(2+) and Zn(2+) inducible coaT promoter or the Ni(2+)-inducible nrsBACD promoter, respectively, in the genome of Synechocystis sp. PCC 6803. The obtained reporters, designated coaLux and nrsLux, respectively, responded quantitatively to metal ions. After 3 h incubation at 40 micromol m(-2) s(-1) visible light, the detection range of coaLux was 0.3-6 microM for Co(2+) and 1-3 microM for Zn(2+). Incubation in darkness increased the detection range by about four times. The nrsLux reporter was specific to Ni(2+), with a detection range of 0.2-6 microM. However, its activity was inhibited by Zn(2+) with a half maximal inhibitory concentration c. 6 microM, and totally inhibited by darkness. This is the first whole-cell Ni(2+)-specific reporter with a clear dose-signal relationship. In a soil-like mixture of different chemical and oil industry wastes, the coaLux reporter strain detected about 90% of the zinc content of the sample. This study demonstrates the potential for development of a rapid, simple and economical field assay for nickel, cobalt and zinc detection using the coaLux and nrsLux reporters.

Although the biogeochemistry of the two environmentally hazardous compounds arsenic and sulfide has been extensively investigated, the biological interference of these two toxic but potentially energy-rich compounds has only been hypothesized and indirectly proven. Here we provide direct evidence for the first time that in the photosynthetic model organism Synechocystis sp. strain PCC6803 the two metabolic pathways are linked by coregulated genes that are involved in arsenic transport, sulfide oxidation, and probably in sulfide-based alternative photosynthesis. Although Synechocystis sp. strain PCC6803 is an obligate photoautotrophic cyanobacterium that grows via oxygenic photosynthesis, we discovered that specific genes are activated in the presence of sulfide or arsenite to exploit the energy potentials of these chemicals. These genes form an operon that we termed suoRSCT, located on a transposable element of type IS4 on the plasmid pSYSM of the cyanobacterium. suoS (sll5036) encodes a light-dependent, type I sulfide:quinone oxidoreductase. The suoR (sll5035) gene downstream of suoS encodes a regulatory protein that belongs to the ArsR-type repressors that are normally involved in arsenic resistance. We found that this repressor has dual specificity, resulting in 200-fold induction of the operon upon either arsenite or sulfide exposure. The suoT gene encodes a transmembrane protein similar to chromate transporters but in fact functioning as an arsenite importer at permissive concentrations. We propose that the proteins encoded by the suoRSCT operon might have played an important role under anaerobic, reducing conditions on primordial Earth and that the operon was acquired by the cyanobacterium via horizontal gene transfer.

Aiming at developing cyanobacterial-based biosensors for heavy metal detection, expression of heavy metal inducible genes of the cyanobacterium Synechocystis PCC 6803 was investigated by quantitative RT-PCR upon 15 minutes exposure to biologically relevant concentrations of Co2+, Zn2+, Ni2+, Cd2+, Cr6+, As3+ and As5+. The ziaA gene, which encodes a Zn2+-transporting P-type ATPase showed a markedly increased mRNA level after incubation with Cd2+ and arsenic ions, besides the expected induction by Zn2+ ions. The Co2+ efflux system-encoding gene coaT was strongly induced by Co2+ and Zn2+ ions, moderately induced by As3+ ions, and induced at a relatively low level by Cd2+ and As5+ ions. Expression of nrsB, which encodes a part of a putative Ni2+ efflux system was highly induced by Ni2+ salts and at a low extent by Co2+ and Zn2+ salts. The arsB gene, which encodes a putative arsenite-specific efflux pump was highly induced by As3+ and As5+ ions, while other metal salts provoked insignificant transcript level increase. The transcript of chrA, in spite of the high sequence similarity of its protein product with several bacterial chromate transporters, shows no induction upon Cr6+ salt exposure. We conclude that due to the largely unspecific heavy metal response of the studied genes only nrsB and arsB are potential candidates for biosensing applications for detection of Ni2+ and arsenic pollutants, respectively.