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.