Figure 5. Effect of genomic knockouts on cellular metal concentrations.

(a) ICP-MS measurement of Fe concentration for combinations of ferritin or metal exporter knockouts (−). With the master regulator fur protein present, iron homeostasis is maintained unless multiple ferritin homologs are knocked out (Fe-stor = ftnA, bfr and dps). (b) Genetic Fe-sensor measurement of intracellular free Fe²⁺ concentration shows effect of knockouts in the same strains despite homeostasis of total Fe in A. (c) ICP-MS of total Fe concentration for additional mutants with fur KO, which increases total Fe level. Over-expressing ftnA (red bars), however, did not increase iron level. (d) ICP-MS of zinc concentration for additional mutants with fur KO. Knockout of zntA alone dramatically increases cellular Zn levels, especially when expressing ferritins capable of binding zinc. zntA plays a crucial role in Fe export in the absence of the iron exporters and regulator fur, resulting in dramatically decreased Fe levels in contrast to its knockout. zntA knockout in combination with either fieF or rcnA KO further increases intracellular Fe sequestration. Comparison of total Fe content (a) vs. free Fe²⁺ via the genetic sensor (b) illustrates the interplay between metal transport and storage in cells.