Characterization of the activity of heavy metal-responsive promoters in the cyanobacterium Synechocystis PCC 6803

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.