Identity elements in tRNA-mediated transcription antitermination: implication of tRNA D- and T-arms in mRNA recognition

tRNA-mediated transcription antitermination has been shown to control the e xpression of several amino acid biosynthesis operons and aminoacyl-tRNA-syn thetase-encoding genes in Gram-positive bacteria. A model originally put fo rward by Grundy & Henkin describes the conserved structural features of the leader sequences of these operons and genes. Two sequences of 3 and 4 nt, respectively, take a central position in this model and are thought to be r esponsible for the binding of the system-specific uncharged tRNA, an intera ction which would stabilize the antiterminator conformation of the leader. Here a further evolution of this model is presented based on an analysis of trp regulation in Lactococcus lactis in which a function is assigned to hi therto unexplained conserved structures in the leader sequence. It is postu lated that the mRNA-tRNA interaction involves various parts of the tRNA in addition to the anticodon and the acceptor in the original model and that t hese additional interactions contribute to the recognition of a specific tR NA, and hence to the specificity and efficacy of the regulatory response.