Microbial nickel metalloregulation: NikRs for nickel ions.

Nickel is a required co-factor for several microbial enzymes; however, because of its potential toxicity, nickel import and homeostasis must be tightly controlled. Recent biophysical and biochemical studies have revealed that NikR proteins are a new type of metalloregulatory protein that utilize allostery and coordination geometry to sense nickel ions and regulate transcription of genes involved in nickel import and processing. Nickel import into bacteria occurs through either ABC-type transporters (NikABCDE) or HoxN type permeases (NixA). Recent structural evidence suggests that nickel is transported through NikABCDE as a metallophore (akin to a siderophore). Nickel storage is accomplished via the HPN protein, a histidine-rich protein similar to metallothionein.