Lay summary
Copper in biological systems presents a formidable problem: it is essential for life, yet highly reactive and a potential source of cell damage. Tight control of copper is thus a cellular necessity. To meet this challenge, cells have evolved pumps to transport copper through the cell membrane, chaperones for intracellular routing, oxidases and reductases to change the oxidation state of copper, and regulators to control gene expression in response to copper. These systems are complemented by specific mechanisms for the insertion of copper into enzymes. Copper homeostasis has evolved early in evolution and some components have been conserved from bacteria to humans. This has allowed to apply knowledge across phyla and even involving human copper homeostatic diseases to elucidate the fundamental mechanism of cellular copper homeostasis. To discover new components which could be involved in the control of cellular copper, we search for new proteins which are up- or down-regulated by copper in the model bacterium Lactococcus lactis. Several novel proteins which are strongly induced by copper have already been identified by this approach. Among these are YtjD, a putative nitrate reductase, and LctO, a lactate oxidase. LctO probably serves to remove residual oxygen from the cells, which reduces cellular stress in the presence of excess copper. We also study of how copper enters bacteria, a process which not understood at all. To this end, an biosensor senses intracellular copper and produces ligh in response to it was constructed. This biosensor was used to isolate cells with defective copper entry. By this approach, we identified a novel regulator, YcfQ, which regulates the expression of YcfR, an outer membrane protein. YcfR in turn influences the permeability of the cell membrane to copper.