In a state of flux: new insight into the transport processes that maintain bacterial metal homeostasis

Abstract

A new study by Nies et al. (J Bacteriol 206:e00080-24, 2024, https://doi.org/10.1128/jb.00080-24) provides a rich, quantitative data set of zinc accumulation by cells of Cupriavidus metallidurans, including of mutant bacterial strains lacking import or efflux genes, and comparison of zinc accumulation by cells previously starved of metal with those of zinc-replete cells. The data surprisingly demonstrate the concomitant activity of both active metal import and metal efflux systems. They present a flow equilibrium model to describe zinc homeostasis in bacteria.

Keywords: metal homeostasis; metal sensing; metal transporters; zinc.

Fig 1

The flow equilibrium model of zinc homeostasis in Cupriavidus metallidurans. (A) Simplistic, stepwise model of bacterial metal homeostasis. Cells under zinc-deficient conditions (left) express zinc-import systems (green) on their membrane (blue), whose activity increases the cytosolic concentration of zinc (gray circles), whereas cells under zinc-excess conditions (right) express zinc-efflux systems (red), whose activity decreases the cytosolic zinc concentration. Cells under metal-replete conditions (center) represent a steady state, where neither import nor export systems are needed. Note that in panels A and B, only a single type of importer and exporter system is shown for simplicity, but in fact, each organism tends to possess a range of different transporters for each function for each metal. (B) New flow equilibrium model of metal homeostasis, based on data from Nies et al. (17). Both import and export systems are expressed under metal-deficient (left), metal excess (right), and metal-replete conditions (center) but with the relative abundance of the import vs efflux systems under each condition determining the net inflow or outflow of metal ions. In this model, a futile cycle could be established, as can clearly be seen under metal-replete conditions. The expression of ZupT and ZntR from C. metallidurans is illustrated, but it should be noted that this organism possesses numerous zinc importers and zinc exporters, some (but not all) of which are regulated by zinc availability. (C) The expression of plasma membrane (top) zinc-import (e.g., ZupT) and zinc-efflux (e.g., ZntA) systems is regulated by the metal-sensing transcriptional regulators, Zur (pink) and ZntR (dark blue), respectively, through binding of zinc (gray circles) and DNA (bottom). Zur is a metal-dependent co-repressor, which binds to the operator upstream to repress expression of the downstream zupT gene under metal-replete conditions (inset, center-left) and dissociates from DNA when zinc dissociates from Zur (inset, left). ZntR is a metal-dependent activator, which is bound to the operator upstream and represses transcription of the downstream zntA gene in its apo-form (inset, center-right) and then activates expression of the downstream zntA gene by the unwinding of the operator DNA when it becomes bound to zinc (inset, right). (D) Projected oscillation of intracellular zinc concentrations (solid black line, y-axis not shown) over time (x-axis not shown). The zinc affinities of the low- (Zur) and high-zinc (ZntR) sensing transcriptional regulators are very close, differing by approximately one order of magnitude (dashed lines), which are taken to define the boundaries of zinc-replete conditions. When zinc concentration falls below the Zur affinity (first red arrow), it will induce transcription of the zupT gene. However, transcription, translation, membrane insertion, and protein folding take time, so the concentration will fall significantly below the boundary demarcated by the affinity of Zur before functional ZupT can act to resolve the zinc deficiency. Likewise, once zinc concentration rises back across the Zur affinity concentration (second red arrow), zupT expression will be deactivated by demetalation of Zur, but the functional ZupT proteins in the membrane will continue to import zinc, raising the zinc concentration further until the ZupT proteins are eventually degraded by unknown mechanisms. These residual import proteins will ultimately cause the concentration to rise beyond the ZntR affinity concentration, leading to induction of the zntA gene (third red arrow), with the high-zinc response following the reciprocal of the low-zinc response until efflux crosses (fourth red arrow) and overshoots the ZntR affinity concentration. Thus, the long-lived nature of the transport proteins relative to their transcripts results in a predicted continuous oscillation of intracellular zinc concentration, even in conditions of stable exogenous zinc concentrations. Note that numerous other import and efflux systems are present in addition to ZupT and ZntA, each of which is regulated by a variety of signals, further complicating our view of overall zinc homeostasis. Created with BioRender.com.