Copper-Dependent Iron Assimilation Pathway in the Model Photosynthetic EukaryoteChlamydomonas reinhardtii

Abstract

The unicellular green alga Chlamydomonas reinhardtii is a valuable model for studying metal metabolism in a photosynthetic background. A search of the Chlamydomonas expressed sequence tag database led to the identification of several components that form a copper-dependent iron assimilation pathway related to the high-affinity iron uptake pathway defined originally for Saccharomyces cerevisiae. They include a multicopper ferroxidase (encoded by Fox1), an iron permease (encoded by Ftr1), a copper chaperone (encoded by Atx1), and a copper-transporting ATPase. A cDNA, Fer1, encoding ferritin for iron storage also was identified. Expression analysis demonstrated that Fox1 and Ftr1 were coordinately induced by iron deficiency, as were Atx1 and Fer1, although to lesser extents. In addition, Fox1 abundance was regulated at the posttranscriptional level by copper availability. Each component exhibited sequence relationship with its yeast, mammalian, or plant counterparts to various degrees; Atx1 of C. reinhardtii is also functionally related with respect to copper chaperone and antioxidant activities. Fox1 is most highly related to the mammalian homologues hephaestin and ceruloplasmin; its occurrence and pattern of expression in Chlamydomonas indicate, for the first time, a role for copper in iron assimilation in a photosynthetic species. Nevertheless, growth of C. reinhardtii under copper-and iron-limiting conditions showed that, unlike the situation in yeast and mammals, where copper deficiency results in a secondary iron deficiency, copper-deficient Chlamydomonas cells do not exhibit symptoms of iron deficiency. We propose the existence of a copper-independent iron assimilation pathway in this organism.While iron is abundant in the environment, it is present in the insoluble ferric [Fe(III)] state, so that its bioavailability is low (16). Yet iron is an essential micronutrient for all organisms because it functions as a cofactor in enzymes that catalyze redox reactions in fundamental metabolic processes. Iron exhibits stable, redox-interchangeable ionic states with the potential to generate less stable electron-deficient intermediates during multielectron redox reactions involving oxygen chemistry (16). Therefore, organisms are challenged with the acquisition of sufficient iron to meet cellular metabolic requirements while avoiding uncontrolled intracellular chemistry. This is accomplished via the operation of iron homeostatic mechanisms. The essential features of iron metabolism include assimilation and distribution, storage and sequestration, and utilization and allocation. The assimilatory pathway can be further subdivided into reduction of insoluble ferric species to more soluble ferrous species and uptake into the cell, followed by intracellular transport and intraorganellar distribution. The storage and sequestration of iron involve loading of cellular proteins as well as compartmentalization into organelles like vacuoles and plastids, which in turn requires proteins for transport into and out of these compartm...

Ferritin is required for rapid remodeling of the photosynthetic apparatus and minimizes photo‐oxidative stress in response to iron availability in Chlamydomonas reinhardtii

“…This has also been described for the red algae Cyanidium caldarum (Nagasaka et al, 2002), which indicates that the unicellular organization and the more variable environment creates the need for a constitutively present iron buffer such as ferritin. Under iron deficiency, ferritin is induced at the protein level in Chlamydomonas, as expected from RNA expression studies (La Fontaine et al, 2002). This finding is in contrast to the mammalian and plant systems in which the synthesis of ferritin is repressed when the cells experience iron deficiency Thomson et al, 1999).…”

“…The ferr1 mRNA of Chlamydomonas was found to increase under iron-deficient conditions (La Fontaine et al, 2002). Recently, a second ferritin gene (ferr2) was identified in Chlamydomonas, but the gene model may be incomplete due to sequence gaps in the genome (Merchant et al, 2006).…”

“…Its ability to grow photo-heterotrophically or photo-autotrophically, the possibility of transforming all three genomes (Rochaix, 1995), all of which are sequenced (Maul et al, 2002;Merchant et al, 2007;Michaelis et al, 1990), and numerous molecular tools such as RNAi (Rohr et al, 2004) as well as proteomic approaches (Hippler et al, 2001) make Chlamydomonas a versatile model organism. Numerous studies have been published with respect to its iron homeostasis and consequences of iron deficiency (Allen et al, 2007;La Fontaine et al, 2002;Moseley et al, 2002;Naumann et al, 2005Naumann et al, , 2007, showing that Chlamydomonas reacts in a fine-tuned manner to changing iron availability. The adaptation reactions comprise induction of a copper-dependent high-affinity iron uptake system consisting of the ferroxidase FOX1 and the permease FTR1 (Herbik et al, 2002;La Fontaine et al, 2002), as well as secretion of putative iron-binding proteins such as FEA1 and FEA2 (Allen et al, 2007).…”

“…Numerous studies have been published with respect to its iron homeostasis and consequences of iron deficiency (Allen et al, 2007;La Fontaine et al, 2002;Moseley et al, 2002;Naumann et al, 2005Naumann et al, , 2007, showing that Chlamydomonas reacts in a fine-tuned manner to changing iron availability. The adaptation reactions comprise induction of a copper-dependent high-affinity iron uptake system consisting of the ferroxidase FOX1 and the permease FTR1 (Herbik et al, 2002;La Fontaine et al, 2002), as well as secretion of putative iron-binding proteins such as FEA1 and FEA2 (Allen et al, 2007). Furthermore, a distinct remodeling of photosystem I and its light-harvesting complex under photo-heterotrophic conditions has been described (Naumann et al, 2005).…”

Transition metal transport

“…It is noteworthy that also algae, such as C. reinhardtii, possess copper-independent iron uptake systems [73]. In analogy with the role of the ZIP protein AtIRT1 as the primary Fe(II) uptake system of A. thaliana, some of the ZIP proteins [70,77].…”