<i>FEA1</i> , <i>FEA2</i> , and <i>FRE1</i> , Encoding Two Homologous Secreted Proteins and a Candidate Ferrireductase, Are Expressed Coordinately with <i>FOX1</i> and <i>FTR1</i> in Iron-Deficient <i>Chlamydomonas reinhardtii</i>

<jats:title>ABSTRACT</jats:title> <jats:p> Previously, we had identified FOX1 and FTR1 as iron deficiency-inducible components of a high-affinity copper-dependent iron uptake pathway in <jats:italic>Chlamydomonas</jats:italic> . In this work, we survey the version 3.0 draft genome to identify a ferrireductase, FRE1, and two ZIP family proteins, IRT1 and IRT2, as candidate ferrous transporters based on their increased expression in iron-deficient versus iron-replete cells. In a parallel proteomic approach, we identified FEA1 and FEA2 as the major proteins secreted by iron-deficient <jats:italic>Chlamydomonas reinhardtii</jats:italic> . The recovery of FEA1 and FEA2 from the medium of <jats:italic>Chlamydomonas</jats:italic> strain CC425 cultures is strictly correlated with iron nutrition status, and the accumulation of the corresponding mRNAs parallels that of the <jats:italic>Chlamydomonas FOX1</jats:italic> and <jats:italic>FTR1</jats:italic> mRNAs, although the magnitude of regulation is more dramatic for the <jats:italic>FEA</jats:italic> genes. Like the <jats:italic>FOX1</jats:italic> and <jats:italic>FTR1</jats:italic> genes, the <jats:italic>FEA</jats:italic> genes do not respond to copper, zinc, or manganese deficiency. The 5′ flanking untranscribed sequences from the <jats:italic>FEA1</jats:italic> , <jats:italic>FTR1</jats:italic> , and <jats:italic>FOX1</jats:italic> genes confer iron deficiency-dependent expression of <jats:italic>ARS2</jats:italic> , suggesting that the iron assimilation pathway is under transcriptional control by iron nutrition. Genetic analysis suggests that the secreted proteins FEA1 and FEA2 facilitate high-affinity iron uptake, perhaps by concentrating iron in the vicinity of the cell. Homologues of <jats:italic>FEA1</jats:italic> and <jats:italic>FRE1</jats:italic> were identified previously as high-CO <jats:sub>2</jats:sub> -responsive genes, <jats:italic>HCR1</jats:italic> and <jats:italic>HCR2</jats:italic> , in <jats:italic>Chlorococcum littorale</jats:italic> , suggesting that components of the iron assimilation pathway are responsive to carbon nutrition. These iron response components are placed in a proposed iron assimilation pathway for <jats:italic>Chlamydomonas</jats:italic> . </jats:p>