(A) S-adenosyl methionine (SAM) riboswitch regulates several operons encoding enzymes and transporter proteins. SAM binds to an aptamer domain and stabilizes the formation of a terminator hairpin (the alternate pairings are in red) to arrest transcription . (B) Schematic representation of a T-Box involved in the regulation of aminoacyl-tRNA synthetases (aaRS). Non aminoacylated tRNA binds to the leader region at two different sites: the anticodon sequence of the tRNA base paired with a codon-like triplet present in the “specifier loop”, and the ACCA end of the tRNA binds to a complementary sequence located in the T-Box motif . This interaction stabilizes an anti-terminator structure allowing transcription of the downstream genes. *The aminoacyl-tRNA synthetases regulated by this mechanism are the ValRS, MetRS, IleRS, PheRS, GlyRS, SerRS, HisRS, and the AspRS. (C) The SprD pathogenicity island RNA (in red) binds to the ribosome binding site (The SD is green) of sbi mRNA to repress translation initiation . (D) The RsaOX (in red) (or Teg14as) cis-acting asRNA , is complementary to the coding sequence of tnp mRNA and is predicted to induce rapid degradation of tnp mRNA. Both the asRNA and the mRNA target site are highly folded, suggesting that the pairing is initiated by a loop–loop interaction.