At least 18 aminoacyl-tRNA synthetase and amino acid biosynthesis genes in several Gram-positive genera appear to be regulated by a common transcription anti-termination mechanism. Each gene is induced by limitation for the appropriate amino acid, and not by general amino acid limitation. The mRNA leader regions of these genes exhibit extensive structural conservation. haracterization of the Bacillus subtilis tyrS gene revealed that uncharged tyrosyl-tRNA promotes readthrough of a leader-region terminator; a conformational switch in the leader mRNA between a terminator structure and an antiterminator structure is postulated to mediate antitermination. Two sites of interaction between the tRNA and the leader have been identified by genetic analysis: the tRNA anti-codon interacts with a single codon displayed at a precise position in the leader-region structure, and the acceptor end of the tRNA interacts with a side-bulge on the antiterminator.