Genome analysis of Chlamydomonas reinhardtii reveals the existence of multiple, compartmentalized iron-sulfur protein assembly machineries of different evolutionary origins

The unicellular green alga Chlamydomonas reinhardtii is used extensively as a model to study eukaryotic photosynthesis, flagellar functions, and more recently the production of hydrogen as biofuel. Two of these processes, photosynthesis and hydrogen production, are highly dependent on iron–sulfur (Fe–S) enzymes. To understand how Fe–S proteins are assembled in Chlamydomonas, we have analyzed its recently sequenced genome for orthologs of genes involved in Fe–S cluster assembly. We found a total of 32 open reading frames, most single copies, that are thought to constitute a mitochondrial assembly pathway, mitochondrial export machinery, a cytosolic assembly pathway, and components for Fe–S cluster assembly in the chloroplast. The chloroplast proteins are also expected to play a role in the assembly of the H-cluster in [FeFe]-hydrogenases, together with the recently identified HydEF and HydG proteins. Comparison with the higher plant model Arabidopsis indicated a strong degree of conservation of Fe–S cofactor assembly pathways in the green lineage, the pathways being derived from different origins during the evolution of the photosynthetic eukaryote. As a haploid, unicellular organism with available forward and reverse genetic tools, Chlamydomonas provides an excellent model system to study Fe–S cluster assembly and its regulation in photosynthetic eukaryotes.