Mitochondria are indispensable for cell viability; however, major mitochondrial functions including citric acid cycle and oxidative phosphorylation are dispensable. Most known essential mitochondrial proteins are involved in preprotein import and assembly, while the only known essential biosynthetic process performed by mitochondria is the biogenesis of iron–sulfur clusters (ISC). The components of the mitochondrial ISC‐assembly machinery are derived from the prokaryotic ISC‐assembly machinery. We have identified an essential mitochondrial matrix protein, Isd11 (YER048w‐a), that is found in eukaryotes only. Isd11 is required for biogenesis of cellular Fe/S proteins and thus is a novel subunit of the mitochondrial ISC‐assembly machinery. It forms a complex with the cysteine desulfurase Nfs1 and is required for formation of an Fe/S cluster on the Isu scaffold proteins. We conclude that Isd11 is an indispensable eukaryotic component of the mitochondrial machinery for biogenesis of Fe/S proteins.

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Background: Ascorbate biosynthesis in plants occurs mainly via the L-galactose pathway. Results: Chlamydomonas reinhardtii VTC2 encodes a GDP-L-galactose phosphorylase whose transcript levels are induced in response to oxidative stress concurrent with increased ascorbate accumulation. Conclusion: Increased oxidative stress in C. reinhardtii results in an enzymatic and non-enzymatic antioxidant response. Significance: First characterization of C. reinhardtii ascorbate biosynthesis and recycling pathways.

Iron-sulfur (Fe/S) clusters are important cofactors of numerous proteins involved in electron transfer, metabolic and regulatory processes. In eukaryotic cells, known Fe/S proteins are located within mitochondria, the nucleus and the cytosol. Over the past years the molecular basis of Fe/S cluster synthesis and incorporation into apoproteins in a living cell has started to become elucidated. Biogenesis of these simple inorganic cofactors is surprisingly complex and, in eukaryotes such as Saccharomyces cerevisiae, is accomplished by three distinct proteinaceous machineries. The "iron-sulfur cluster (ISC) assembly machinery" of mitochondria was inherited from the bacterial ancestor of mitochondria. ISC components are conserved in eukaryotes from yeast to man. The key principle of biosynthesis is the assembly of the Fe/S cluster on a scaffold protein before it is transferred to target apoproteins. Cytosolic and nuclear Fe/S protein maturation also requires the function of the mitochondrial ISC assembly system. It is believed that mitochondria contribute a still unknown compound to biogenesis outside the organelle. This compound is exported by the mitochondrial "ISC export machinery" and utilised by the "cytosolic iron-sulfur protein assembly (CIA) machinery". Components of these two latter systems are also highly conserved in eukaryotes. Defects in the mitochondrial ISC assembly and export systems, but not in the CIA machinery have a strong impact on cellular iron uptake and intracellular iron distribution showing that mitochondria are crucial for both cellular Fe/S protein assembly and iron homeostasis.

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