Abstract : Metals are ubiquitous on Earth and have become necessary for living organisms like bacteria. However, there is a subtle balance between the life required amount and toxicity resulting from an overload of metal ions. The intracellular level of these ions must be tightly regulated. A strategy often used by cells is to regulate uptake and efflux of metal ions by expressing speci c proteins involved in this traffic. In the bacterium Escherichia coli, nickel and cobalt toxicity is counterbalanced by action of the RcnA efflux pump that permits excess Ni2+ and Co2+ ions to be excluded out of cells. Its expression is under the control of the nickel and cobalt inducible RcnR metalloregulator that represses rcnA transcription in absence of metals. Recently, we identi ed a new gene that we called rcnB (formerly yohN ) involved in this system. This gene is a part of the rcnAB operon and as rcnA, is involved in nickel and cobalt homeostasis. But, unlike rcnA, a mutant strain lacking rcnB is more resistant to nickel and cobalt. This phenotype is associated to a lower accumulation of these ions in the cells. rcnB gene encodes a small monomeric protein of 10 kDa which is found in the periplasmic fraction. Until now, databases do not contain any described homologs. RcnB displayed any Ni2+ or Co2+ binding capacity. On the other hand, RcnB can bind both Cu+ and Cu2+ ions in vitro. It is equally true in vivo and a link with copper homeostasis was found based on the copper resistance systems of E. coli. This is of particular interest because copper confers particular redox properties to proteins which could be directly related to the biological function of the RcnB protein. Further studies are required to precise the role of RcnB but this work provides the identifi cation and characterization of the first member of a new family of accessory proteins tied to metal ions efflux pumps.