CopM is a novel copper‐binding protein involved in copper resistance in S ynechocystis sp. PCC 6803

Copper resistance system in the cyanobacterium S ynechocystis sp. PCC 6803 comprises two operons, cop MRS and cop BAC , which are expressed in response to copper in the media. cop BAC codes for a heavy‐metal efflux–resistance nodulation and division ( HME ‐ RND ) system, while cop MRS codes for a protein of unknown function, CopM, and a two‐component system Cop RS , which controls the expression of these two operons. Here, we report that CopM is a periplasmic protein able to bind Cu(I) with high affinity ( K D ~3 × 10 −16 ). Mutants lacking copM showed a sensitive copper phenotype similar to mutants affected in copB , but lower than mutants of the two‐component system Cop RS , suggesting that Cop BAC and CopM constitute two independent resistance mechanisms. Moreover, constitutive expression of copM is able to partially suppress the copper sensitivity of the copR mutant strain, pointing out that CopM per se is able to confer copper resistance. Furthermore, constitutive expression of copM was able to reduce total cellular copper content of the copR mutant to the levels determined in the wild‐type ( WT ) strain. Finally, CopM was localized not only in the periplasm but also in the extracellular space, suggesting that CopM can also prevent copper accumulation probably by direct copper binding outside the cell.