Copper-dependent iron assimilation pathway in the model photosynthetic eukaryote chlamydomonas reinhardtii

The unicellular green alga <i>Chlamydomonas reinhardtii</i> is a valuable model for studying metal metabolism in a photosynthetic background. A search of the <i>Chlamydomonas </i>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 <i>Saccharomyces cerevisiae</i>. They include a multicopper ferroxidase (encoded by <i>Fox1</i>), an iron permease (encoded by <i>Ftr1</i>), a copper chaperone (encoded by <i>Atx1</i>), and a copper-transporting ATPase. A cDNA, <i>Fer1</i>, encoding ferritin for iron storage also was identified. Expression analysis demonstrated that <i>Fox1 </i>and <i>Ftr1 </i>were coordinately induced by iron deficiency, as were <i>Atx1 </i>and <i>Fer1</i>, 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. <i>reinhardtii </i>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 <i>Chlamydomonas </i>indicate, for the first time, a role for copper in iron assimilation in a photosynthetic species. Nevertheless, growth of C. <i>reinhardtii </i>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 <i>Chlamydomonas </i>cells do not exhibit symptoms of iron deficiency. We propose the existence of a copper-independent iron assimilation pathway in this organism.<br />