Pus1 (pseudouridylate synthase 1) is an enzyme that converts uridine to pseudouridine in non-coding RNA substrates. Pus1 modifies uridines at specific positions in a subset of tRNAs. Pseudouridylation is known to affect the structure of tRNAs and stabilize base-stacking and base-pairing in the anticodon loop. Pus1 activity is important for proper folding and function of tRNAs. Pus1 has also been shown to pseudouridylate SRA (steroid receptor RNA activator) and function as a coactivator. Defects in pseudouridylation are associated with the human disease, MLASA (myopathy with lactic acidosis and sideroblastic anemia), an autosomal recessive oxidative phosphorylataion disorder specific to skeletal muscle and bone marrow.