The understanding of the processes underlying organellar function and inheritance requires the identification and characterization of the molecular components involved. We pursued a genomic approach to define the complements of genes required for respiratory growth and inheritance of mitochondria with normal morphology in yeast. With the systematic screening of a deletion mutant library covering the nonessential genes of Saccharomyces cerevisiae the numbers of genes known to be required for respiratory function and establishment of wild-type-like mitochondrial structure have been more than doubled. In addition to the identification of novel components, the systematic screen revealed unprecedented mitochondrial phenotypes that have never been observed by conventional screens. These data provide a comprehensive picture of the cellular processes and molecular components required for mitochondrial function and structure in a simple eukaryotic cell.

Mitochondrial fusion and fission play important roles for mitochondrial morphology and function. We identified Mdm30 as a novel component required for maintenance of fusion-competent mitochondria in yeast. The Mdm30 sequence contains an F-box motif that is commonly found in subunits of Skp1-Cdc53-F-box protein ubiquitin ligases. A fraction of Mdm30 is associated with mitochondria. Cells lacking Mdm30 contain highly aggregated or fragmented mitochondria instead of the branched tubular network seen in wild-type cells. ⌬mdm30 cells lose mitochondrial DNA at elevated temperature and fail to fuse mitochondria in zygotes at all temperatures. These defects are rescued by deletion of DNM1, a gene encoding a component of the mitochondrial division machinery. The protein level of Fzo1, a key component of the mitochondrial fusion machinery, is regulated by Mdm30. Elevated Fzo1 levels in cells lacking Mdm30 or in cells overexpressing Fzo1 from a heterologous promoter induce mitochondrial aggregation in a similar manner. Our results suggest that Mdm30 controls mitochondrial shape by regulating the steadystate level of Fzo1 and point to a connection of the ubiquitin/26S proteasome system and mitochondria. INTRODUCTIONMitochondria are highly dynamic organelles. In many eukaryotic cell types they continuously move along cytoskeletal tracks and frequently fuse and divide. Their shape varies depending on the cell type, physiological status, and nutritional conditions (Bereiter-Hahn and Vö th, 1994;Yaffe, 1999;Griparic and van der Bliek, 2001;Westermann, 2002). Recent studies have shown that mitochondrial dynamics is crucial for a variety of cellular functions. For example, fusion of mitochondria is important for sperm development in Drosophila (Hales and Fuller, 1997) and for maintenance of mitochondrial DNA (mtDNA) in yeast (Berger and Yaffe, 2000). Furthermore, fusion is required for the formation of intracellular mitochondrial networks that allow the dissipation of energy in the cell (Skulachev, 2001) and for complementation of mitochondrial gene products, a mechanism thought to constitute a defense against cellular aging (Ono et al., 2001). Mitochondrial fission is an important step in apoptosis (Frank et al., 2001) and is required for embryonic development in nematodes (Labrousse et al., 1999). Several of the proteins determining mitochondrial behavior have been identified in recent years Yaffe, 1999;Jensen et al., 2000;Boldogh et al., 2001;Shaw and Nunnari, 2002;Westermann and Prokisch, 2002). Yet, many of the molecular processes regulating mitochondrial dynamics remain to be described.The molecular components of mitochondrial fusion and fission have been most extensively studied in budding yeast. Yeast mitochondria form an extended tubular network located below the cell cortex (Hoffmann and Avers, 1973). Mitochondria undergo gross structural changes during adaptation to nutritional conditions and growth phase (Stevens, 1981;Pon and Schatz, 1991;Egner et al., 2002). During vegetative growth, the continuity of the mitochondr...