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

Abstract

Copper resistance system in the cyanobacterium Synechocystis sp. PCC 6803 comprises two operons, copMRS and copBAC, which are expressed in response to copper in the media. copBAC codes for a heavy-metal efflux–resistance nodulation and division (HME-RND) system, while copMRS codes for a protein of unknown function, CopM, and a two-component system CopRS, 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 (KD ∼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 CopRS, suggesting that CopBAC 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.

Contact killing and antimicrobial properties of copper

“…CopM could bind copper in the extracellular compartment, preventing copper accumulation in the periplasm, but its role outside the cell is not clearly known (Giner‐Lamia et al . , ).…”

Cloning of a copper resistance gene cluster from the cyanobacterium Synechocystis sp. PCC 6803 by recombineering recovery

“…These two operons are adjacent in pSYSX, a 106‐kb plasmid native to Synechocystis [13], and are transcribed in the same direction, while copMRS genes are also present on the chromosome. The pcopMRS operon encodes the CopR/CopS copper‐responsive two‐component regulatory system [12] and also CopM, a protein that has recently been shown to bind copper and contribute to copper resistance in Synechocystis [14]. The copBAC genes encode proteins that comprise a member of the heavy metal efflux‐resistance, nodulation and division (HME‐RND) family [15].…”

Homology- and coevolution-consistent structural models of bacterial copper-tolerance protein CopM support a ‘metal sponge’ function and suggest regions for metal-dependent protein-protein interactions

“…[4][5][6][7][8][9] Recently, a novel protein dubbed CopM was identified in the copMRS operon of a copperresistance regulon of Synechocystis. 10,11 The CopM protein is of periplasmic and extracelullar localizations, 11 and plays an important role in resistance according to the effects observed in knock out strains. 11 In vitro, it can tightly bind one equivalent of Cu(I) or, less tightly, of Cu(II).…”

“…10,11 The CopM protein is of periplasmic and extracelullar localizations, 11 and plays an important role in resistance according to the effects observed in knock out strains. 11 In vitro, it can tightly bind one equivalent of Cu(I) or, less tightly, of Cu(II). 11 However, its amino acid sequence includes several histidine and methionine residues that could allow it to bind more metal equivalents.…”

“…11 In vitro, it can tightly bind one equivalent of Cu(I) or, less tightly, of Cu(II). 11 However, its amino acid sequence includes several histidine and methionine residues that could allow it to bind more metal equivalents. 12 Indeed, other periplasmic copper-tolerance proteins that function as metal sponges, like E. coli PcoE, retain one metal ion upon purification but accept more on titration.…”