Figure 1.

Set-point homeostasis models and speciation in bacteria. A, transition metal homeostasis (top) is orchestrated by a panel of metal-specific sensors that prevent metal starvation or toxicity by regulating the expression of proteins involved in the uptake, efflux, storage, or allocation of metals in cells (3, 4, 11–14). Transcriptional response curves are shown for a pair of sensors that detect a specific metal (e.g. Zn^II). These dual sensors collaboratively control metal bioavailability in a concentration range that is compatible with cellular physiology (gray box). H₂S/RSS homeostasis (bottom) is achieved by a single RSS sensor that transcriptionally regulates the expression of enzymes involved in the biogenesis, clearance, transport, and assimilation of H₂S/RSS (16–20). The transcriptional response of an RSS sensor detects a concentration range (gray box) that prevents cellular toxicity, while maintaining access to H₂S/RSS that is physiologically beneficial at lower concentrations. B, metal speciation (top) of first row, late d-block transition metals is defined by the metallome, a descriptor of all oxidation states and coordination complexes in the cell, ranging from exchange-labile small-molecule metal complexes to protein cofactors (shown in cartoon form). Reactive sulfur speciation (bottom) is defined by all inorganic and organic small molecules that harbor sulfur atoms in oxidation states more positive than –2 (see key) (27) and are collectively termed reactive sulfur species (RSS).