The complex global response to copper in the multicellular bacterium Myxococcus xanthus

Bacteria must adapt to fluctuations in their environment to survive. One of the most interesting challenges they must cope with is changes in metal concentrations. Many metals are essential for viability, since they act as cofactors of indispensable enzymes. But on the other hand, they are potentially toxic because they generate reactive oxygen species or displace other metals from proteins, turning them inactive. This dual effect of metals forces cells to maintain homeostasis by using a variety of systems to import and export them. These systems are usually inducible, and their expression is regulated by metal sensors and signal-transduction mechanisms, one of which is mediated by extracytoplasmic function (ECF) sigma factors. In this review we have focused on the metal-responsive ECF sigma factors, several of which are activated by iron depletion (FecI, FpvI, and PvdS), while others are activated by excess of metals such as nickel and cobalt (CnrH), copper (CarQ and CorE), or cadmium and zinc (CorE2). We focus particularly on their physiological roles, mechanisms of action and signal-transduction pathways.