ABSTRACT A gene cluster composed of nine open reading frames (ORFs) involved in Ni 2+ , Co 2+ , and Zn 2+ sensing and tolerance in the cyanobacterium Synechocystis sp. strain PCC 6803 has been identified. The cluster includes an Ni 2+ response operon and a Co 2+ response system, as well as a Zn 2+ response system previously described. Expression of the Ni 2+ response operon ( nrs ) was induced in the presence of Ni 2+ and Co 2+ . Reduced Ni 2+ tolerance was observed following disruption of two ORFs of the operon ( nrsA and nrsD ). We also show that the nrsD gene encodes a putative Ni 2+ permease whose carboxy-terminal region is a metal binding domain. The Co 2+ response system is composed of two divergently transcribed genes, corR and corT , mutants of which showed decreased Co 2+ tolerance. Additionally, corR mutants showed an absence of Co 2+ -dependent induction of corT , indicating that CorR is a transcriptional activator of corT . To our knowledge, CorR is the first Co 2+ -sensing transcription factor described. Our data suggest that this region of the Synechocystis sp. strain PCC 6803 genome is involved in sensing and homeostasis of Ni 2+ , Co 2+ , and Zn 2+ .