Protocol to identify amino acids bound to tRNA by mass

tRNA-bound amino acids often need to be identified, for instance, in cases where different amino acids compete for binding to the same tRNA. Here, we present a mass-spectrometry-based protocol to determine the amino acids bound to tRNA by aminoacylation. We detail how to perform the aminoacylation reaction, the preparation of the aminoacyl-tRNA for measurement, and the mass spectro-metric analysis. We use arginine acylation as an example; however, this protocol can be applied to any other amino acid.