Fig. 3. Iron cofactor assembly pathways in Arabidopsis. Overview of the biosynthesis pathways for FeS clusters and haem and their localisation in a typical plant cell. Iron is represented by red spheres, sulphur by yellow spheres. Please note that the concentration of ‘free’ iron or ‘free’ sulphur in cells is close to zero, as these elements will be chelated or form part of a larger molecule to avoid toxicity. Similarly, FeS clusters do not exist in free form, and are only stable within a protein fold. Mono-iron and di-iron cofactors are not depicted, but occur in all cell compartments. See the main text for more details. ATM3, ABC Transporter of the Mitochondria 3; ER, Endoplasmic reticulum; FC, Ferrochelatase; ISC, Iron-Sulphur Cluster; NFU, NifU-like protein; SUF, Sulfur mobilization.