Metabolic utilization of ⁵⁷Fe‐labeled coprogen in Neurospora crassa An in vivo Mössbauer study: An in vivo Mössbauer study

Mössbauer spectra of whole cells of Neurospora crassa arg‐5 ota aga (a siderophore‐free mutant) show that the siderophore coprogen is accumulated inside the cell as an entity. ⁵⁷Fe from ⁵⁷Fe‐labeled coprogen is slowly removed from the complex (45% in 27 h). The rate of removal depends on the degree of iron starvation of the cells. The distribution of ⁵⁵Fe from [⁵⁵Fe]coprogen in vacuoles, membranes, and cytoplasm has been also determined. From this it is clear that coprogen is accumulated in the cytoplasm. In addition to its role as a siderophore, coprogen serves as an iron‐storage compound. No holoferritins could be detected. We therefore conclude that this type of iron‐storage protein is lacking in N. crassa. Metabolized iron was found predominantly to exist as an envelope of Fe(II) high‐spin (δ= 1.2–1.3 mm s^‐1; ΔE_Q= 3.0–3.1 mm s^‐1 at 4.2 K) and fast‐relaxing Fe(III) high‐spin species (δ∼ 0.25 mm s^‐1 and 0.45 mm s^‐1; ΔE_Q= 0.6 mm s^‐1 and 0.55 mm s^‐1, respectively, at 4.2 K). An assignment of these major iron metabolites is difficult. The Mössbauer data of the Fe(II) species do not fit those reported for heme, cytochromes and ferredoxins. We therefore assume that this iron metabolite represents a novel internal iron compound. One of the Fe(III) species becomes the dominant component of the cell spectra after 65 h of metabolization and might correspond to an iron‐storage compound with iron oxide cores similar to bacterioferritin. After 27 h of growth in mycelia supplied with ⁵⁷Fe‐labeled coprogen, the siderophore ferricrocin was observed in the cell spectra. This is unexpected, since N. crassa arg‐5 ota aga is unable to synthesize ornithine. We assume that ferricrocin is synthesized by the use of coprogen degradation products.