A tRNA-directed transcription antitermination mechanism is common in Gram-positive bacteria, regulating the expression of aminoacyl-tRNA synthetase and amino acid biosynthesis genes. When the level of an amino acid decreases, the cognate uncharged-tRNA accumulates. Uncharged-tRNA stimulates the conformation of mRNA leader region to change from a terminator structure to an antiterminator structure by interaction of two sites: the anticodon and 3′ acceptor end of tRNA interact with the specifier sequence and the T-box in mRNA leader, respectively. This procedure promotes the readthrough and the expression of the relative genes.