A putative merR family transcription factor Slr0701 regulates mercury inducible expression of MerA in the cyanobacterium Synechocystis sp. PCC6803

In cyanobacteria, genes conferring mercury resistance are not organized as mer‐operon, unlike in other bacterial phyla. Synechocystis contains only a putative MerR regulator, Slr0701, and a mercury reductase, MerA, located aside from each other in the genome. The slr0701‐mutant showed reduction in photosynthetic activity and reduced tolerance to mercury compared to the wild‐type. The incubation of wild‐type cells with HgCl2 resulted in the upregulation of slr0701 and slr1849 genes whereas mercury‐induced expression was not observed in the slr0701‐mutant. Slr0701 binds to a conserved cis‐regulatory element located in the upstream of slr1849 and slr0701 ORFs. The same element was also identified in the upstream of other cyanobacterial homologs. Slr0701 binds to cis‐regulatory element with faster association and slower dissociation rates in the presence of HgCl2. Although these genes were constitutively expressed, the addition of HgCl2 enhanced their promoter activity suggesting that mercury‐bound Slr0701 triggers induced expression of these genes. The enhanced promoter activity could be attributed to the observed secondary structural changes in Slr0701 in the presence of HgCl2. For the first time, we demonstrated the mechanism of merA regulation in a cyanobacterium, Synechocystis. Although merA and merR genes are distantly located on the cyanobacterial genome and distinct from other bacterial mer‐operons, the transcriptional regulatory mechanism is conserved. Cyanobacteria contain only two mercury responsive genes, merR and merA, unlike other bacterial phyla in which at least five structural genes form an operon. In cyanobacterial genomes they are located far apart from each other. The manuscript demonstrates mercury‐induced merA gene regulatory mechanism by a putative transcriptional regulator, Slr0701 in Synechocystis sp. PCC6803 for the first time.