Abstract
Mrs3 and Mrs4 are mitochondrial inner membrane proteins that deliver an unidentified cytosolic iron species into the matrix for use in iron-sulfur cluster (ISC) and heme biosynthesis. The Mrs3/4 double-deletion strain (ΔΔ) grew slowly in iron-deficient glycerol/ethanol medium but recovered to wild-type (WT) rates in iron-sufficient medium. ΔΔ cells grown under both iron-deficient and iron-sufficient respiring conditions acquired large amounts of iron relative to WT cells, indicating iron homeostatic dysregulation regardless of nutrient iron status. Biophysical spectroscopy (including Mössbauer, electron paramagnetic resonance, and electronic absorption) and bioanalytical methods (liquid chromatography with online inductively coupled plasma mass spectrometry detection) were used to characterize these phenotypes. Anaerobically isolated mitochondria contained a labile iron pool composed of a nonheme high-spin Fe(II) complex with primarily O and N donor ligands, called Fe(580). Fe(580) likely serves as feedstock for ISC and heme biosynthesis. Mitochondria from respiring ΔΔ cells grown under iron-deficient conditions were devoid of Fe(580), ISCs, and hemes; most iron was present as Fe(III) nanoparticles. O(2) likely penetrates the matrix of slow-growing poorly respiring iron-deficient ΔΔ cells and reacts with Fe(580) to form nanoparticles, thereby inhibiting ISC and heme biosynthesis. Mitochondria from iron-sufficient ΔΔ cells contained ISCs, hemes, and Fe(580) at concentrations comparable to those of WT mitochondria. The matrix of these mutant cells was probably sufficiently anaerobic to protect Fe(580) from degradation by O(2). An ∼1100 Da manganese complex, an ∼1200 Da zinc complex, and an ∼5000 Da copper species were also present in ΔΔ and WT mitochondrial flow-through solutions. No lower-mass copper complex was evident.