Project Summary The proposed project is a renewal of a previously funded project that was focused on the discovery and characterization of assembly factors for the mitochondrial respiratory complexes. During these studies, we made the unexpected observation that the assembly and activation of those complexes is strictly dependent upon the rather poorly understood mitochondrial fatty acid synthesis (mtFAS) pathway. When this pathway is active, it synthesizes acyl chains that are covalently attached to the Acp1 scaffold protein, which when acylated binds and activates the LYR family of regulators that are required for respiratory complex assembly and iron-sulfur cluster biogenesis. We further demonstrated that the activity of mtFAS and the acylation state of Acp1 is sensitive to the availability of mitochondrial acetyl-coA, which is also the principal fuel for mitochondrial oxidative metabolism. Therefore, we hypothesize that this system provides an elegant mechanism to enable the coupling of assembly and activation of the mitochondrial respiratory system with the availability of substrates for that system. To test this hypothesis and better define the underlying mechanisms, we propose the follow aims. Aim 1) Define the role of mtFAS in the regulation of iron-sulfur cluster synthesis. Aim 2) Define the mtFAS and LYR-dependent regulation of OXPHOS biogenesis. Aim 3) Discover and characterize novel targets of mtFAS and ACP regulation. We propose that a better understanding of mtFAS and how it regulates activation of Acp1 targets could lead to therapeutic approaches to boost and optimize mitochondrial respiration.