| Sumario: | A gene cluster composed of nine open reading frames (ORFs) involved in Ni2+, Co2+, and Zn2+ sensing and tolerance in the cyanobacterium Synechocystis sp. Strain PCC 6803 has been identified. The cluster includes an Ni2+ response operon and a Co2+ response system, as well as a Zn2+ response system previously described. Expression of the Ni2+ response operon (nrs) was induced in the presence of Ni2+ and Co2+. Reduced Ni2+ tolerance was observed following disruption of two ORFs of the operon (nrsA and nrsD). We also show that the nrsD gene encodes a putative Ni2+ permease whose carboxy-terminal region is a metal binding domain. The Co2+ response system is composed of two divergently transcribed genes, corR and corT, mutants of which showed decreased Co2+ tolerance. Additionally, corR mutants showed an absence of Co2+-dependent induction of corT, indicating that CorR is a transcriptional activator of corT. To our knowledge, CorR is the first Co2+-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 Ni2+, Co2+, and Zn2+.
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