Peroxisome proliferator activated receptor-␥ co-activator-1␣ (PGC-1␣) is a transcriptional co-activator that coordinately regulates the expression of distinct sets of metabolism-related genes in different tissues. Here we show that PGC-1␣ expression is reduced in skeletal muscles from mice lacking the sirtuin family deacetylase SIRT1. Conversely, SIRT1 activation or overexpression in differentiated C2C12 myotubes increased PGC-1␣ mRNA expression. The transcription-promoting effects of SIRT1 occurred through stimulation of PGC-1␣ promoter activity and were enhanced by co-transfection of myogenic factors, such as myocyte enhancer factor 2 (MEF2) and, especially, myogenic determining factor (MyoD). SIRT1 bound to the proximal promoter region of the PGC-1␣ gene, an interaction potentiated by MEF2C or MyoD, which also interact with this region. In the presence of MyoD, SIRT1 promoted a positive autoregulatory PGC-1␣ expression loop, such that overexpression of PGC-1␣ increased PGC-1␣ promoter activity in the presence of co-expressed MyoD and SIRT1. Chromatin immunoprecipitation showed that SIRT1 interacts with PGC-1␣ promoter and increases PGC-1␣ recruitment to its own promoter region. Immunoprecipitation assays further showed that SIRT1-PGC-1␣ interactions are enhanced by MyoD. Collectively, these data indicate that SIRT1 controls PGC-1␣ gene expression in skeletal muscle and that MyoD is a key mediator of this action. The involvement of MyoD in SIRT1-dependent PGC-1␣ expression may help to explain the ability of SIRT1 to drive musclespecific gene expression and metabolism. Autoregulatory control of PGC-1␣ gene transcription seems to be a pivotal mechanism for conferring a transcription-activating response to SIRT1 in skeletal muscle.Peroxisome proliferator activated receptor-␥ co-activator-1␣ (PGC-1␣) 2 is a transcriptional co-activator that is recognized as a master controller of the expression of genes involved in metabolic regulation. PGC-1␣ exerts differential effects on metabolism in different tissues. In brown adipose tissue, PGC-1␣ increases the expression of uncoupling protein 1 and genes involved in mitochondrial oxidative pathways. In the liver, PGC-1␣ induces the expression of genes involved in gluconeogenesis and the metabolic response to starvation. In skeletal muscle, PGC-1␣ expression is rapidly induced by exercise in vivo (1), a response considered to be a mechanism for modulating metabolic flux in response to decreased ATP levels (2). Chronic exercise also increases PGC-1␣ expression in association with fiber-type switching toward the more oxidative and high endurance type IIa and type I fibers. The fiber-type switch promoted by PGC-1␣ is characterized by increased mitochondrial density and function, increased oxidative metabolism, increased expression of myofibrillar proteins characteristic of type I and type IIa muscle fibers and a switch in substrate fuel usage (3). Furthermore, greater levels of PGC-1␣ are found in oxidative fibers compared with glycolytic fibers, even in a rested state (3). Mo...