N et al., 2002). Uridine modified tRNAs have an enhanced capability to “wobble” and study G-ending codons, forming a functionally redundant decoding technique (Johansson et al., 2008). Nonetheless, only a handful of biological roles for these modifications are known. Uridine mcm5 modifications permit the translation of AGA and AGG codons during DNA damage (Begley et al., 2007), influence distinct telomeric gene silencing or DNA damage responses (Chen et al., 2011b), and function in exocytosis (Esberg et al., 2006). These roles can’t totally clarify why these modifications are ubiquitous, or how they’re advantageous to cells. Interestingly, studies in yeast hyperlink these tRNA modifications to nutrient-dependent responses. Each modifications consume metabolites Potassium Channel Storage & Stability derived from sulfur metabolism, mostly S-adenosylmethionine (SAM) (Kalhor and Clarke, 2003; Nau, 1976), and cysteine (Leidel et al., 2009; Noma et al., 2009). These modifications seem to become downstream from the TORC1 pathway, as yeast lacking these modifications are hypersensitive to rapamycin (Fichtner et al., 2003; Goehring et al., 2003b; Leidel et al., 2009; Nakai et al., 2008), and interactions might be detected between Uba4p and Kog1/TORC1 (Laxman and Tu, 2011). These modification pathways also play essential roles in nutrient stress-dependent dimorphic foraging yeast behavior (Abdullah and Cullen, 2009; Goehring et al., 2003b; Laxman and Tu, 2011). We reasoned that deciphering the interplay involving these modifications, nutrient availability and cellular metabolism would reveal a functional logic to their biological significance. Herein, we show that tRNA uridine thiolation abundance reflects sulfur-containing amino acid availability, and functions to regulate translational capacity and amino acid homeostasis. Uridine thiolation represents a important mechanism by which translation and development are regulated synchronously with metabolism. These findings have important implications for our understanding of cellular amino acid-sensing mechanisms, and with all the accompanying manuscript (Sutter et al., 2013), show how sulfur-containing amino acids serve as sentinel metabolites for cell development control.NIH-PA CRAC Channel custom synthesis Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptCell. Author manuscript; accessible in PMC 2014 July 18.Laxman et al.PageRESULTStRNA uridine thiolation amounts reflect intracellular sulfur amino acid availabilityNIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptWe have been intrigued by connections among tRNA uridine modification pathways and nutrients, specially due to the fact mutants of tRNA uridine-modifying enzymes have been hypersensitive to rapamycin (Figure S1A). We initially tested whether or not tRNA uridine modification amounts changed in response to distinct nutrient environments. To qualitatively assay tRNA uridine thiolation, tRNAs had been resolved on urea-PAGE gels containing the sulfur-coordinating mercury agent APM (Nakai et al., 2008) (Supplemental Information). We confirmed that the enzyme Uba4p is needed for all tRNA thiolation (Figure S1B). Although the majority of tRNALys (UUU), tRNAGlu (UUC) and tRNAGln (UUG) have been thiolated in cells increasing either in YPD (wealthy medium) or below continuous glucose-limitation, a fraction of these tRNAs remained unthiolated (Figure S1B), suggesting that this modification was not constitutive, and may well alter in abundance beneath distinct situations. We then developed targeted LC-MS/MS approaches to quantitatively measure amounts of thiolated, m.