Ni interferes in the Cu‐regulated transcriptional switch petJ/petE in Synechocystis sp. PCC 6803

Plastocyanin ( petE ) plays an essential role in photosynthesis as an electron carrier between cytochrome b 6 f and photosystem I, and in some cyanobacteria it can be replaced by the haem‐containing protein, cytochrome c 6 ( petJ ). In Synechocystis sp. PCC 6803, transcription of petE and petJ is activated and repressed, respectively, by Cu. Here, we show that Ni can act similarly to Cu in inducing petE and repressing petJ , thus leading to a partial switch between cytochrome c 6 and plastocyanin. Transcription of these genes is only altered by Ni in Cu‐depleted medium, and none of the Ni‐dependent transcription factors described in Synechocystis , NrsR and InrS seem to be involved in this regulation. Finally, we show that plastocyanin is essential for growth under conditions of excess Ni.