FER1 and FER2 Encoding Two Ferritin Complexes in Chlamydomonas reinhardtii Chloroplasts Are Regulated by Iron

Abstract

Two unlinked genes FER1 and FER2 encoding ferritin subunits were identified in the Chlamydomonas genome. An improved FER2 gene model, built on the basis of manual sequencing and incorporation of unplaced reads, indicated 49% identity between the ferritin subunits. Both FER1 and FER2 transcripts are increased in abundance as iron nutrition is decreased but the pattern for each gene is distinct. Using subunitspecific antibodies, we monitored expression at the protein level. In response to low iron, ferritin1 subunits and the ferritin1 complex are increased in parallel to the increase in FER1 mRNA. Nevertheless, the iron content of the ferritin1 complex is decreased. This suggests that increased expression results in increased capacity for iron binding in the chloroplast of iron-limited cells, which supports a role for ferritin1 as an iron buffer. On the other hand, ferritin2 abundance is decreased in iron-deprived cells, indicative of the operation of iron-nutrition-responsive regulation at the translational or post-translational level for FER2. Both ferritin subunits are plastid localized but ferritin1 is quantitatively recovered in soluble extracts of cells while ferritin2 is found in the particulate fraction. Partial purification of the ferritin1 complex indicates that the two ferritins are associated in distinct complexes and do not coassemble. The ratio of ferritin1 to ferritin2 is 70:1 in iron-replete cells, suggestive of a more dominant role of ferritin1 in iron homeostasis. The Volvox genome contains orthologs of each FER gene, indicating that the duplication of FER genes and potential diversification of function occurred prior to the divergence of species in the Volvocales.A LTHOUGH iron is abundant on earth, its bioavailability is limited in the aerobic world because of the insolubility of ferric salts, and iron can be a limiting nutrient for most forms of life. A third of the agricultural land and the same fraction of the ocean are considered iron deficient (reviewed by Boyd et al. 2007). Therefore, iron nutrition is a key component of global productivity. Organisms have evolved multiple and varied pathways for assimilating iron in its various chemical forms and at a range of concentrations in the nutrient environment (Staiger 2002; reviewed in Curie and Briat 2003;Hentze et al. 2004). Even though iron can be toxic to cells as a consequence of its propensity for participating in redox chemistry, cells do not generally excrete iron because it is a limiting nutrient (Liochev and Fridovich 1999). Instead, cells tend to store intracellular iron in a less reactive and hence nontoxic form. Because of the importance of iron for life, these pathways of uptake and storage are subject to layers of homeostatic regulation.We have developed Chlamydomonas as a model organism for understanding trace metal nutrition in plants, especially in the context of chloroplast function and photosynthesis (Merchant et al. 2006). As a microorganism, Chlamydomonas lends itself to nutritional studies because of the ease wit...

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