Fig. 5.
Two metabolically related, albeit distinct, pathways regulated by a dual-specificity T-box riboswitch. (A) When Asn (marked as “N”) is limited but in the presence of Glu (marked as “E”) (−N/+E), the binding of tRNAAsn to the NT-box triggers the expression of the transamidation pathway (ND-DRS2 and GatCAB) for the tRNA-dependent synthesis of Asn. (B) When both Asn and Glu are limited (−N/−E), the binding of tRNAAsn and tRNAGlu to the NT-box triggers the expression of the transamidation pathway and fosters the glutaminase activity of the tRNA-dependent amidotransferase (GatCAB) for the tRNA-independent synthesis of Glu. Consequently, Glu becomes available for Glu-tRNAGlu synthesis through the activity of the nondiscriminating glutamyl-tRNA synthetase (ND-ERS). (C) In the case of limited Glu and in the presence of Asn (+N/−E), most of the tRNAAsn is charged with Asn, and therefore the expressed GatCAB would be used for glutaminase activity. The produced Glu is then used for Glu-tRNAGlu synthesis. The switch from the terminator (Ter) to the antiterminator (Anti) conformation is triggered when Asn and/or Glu are not available. Finally, the activation of the transamidation and glutaminase activity increases the charging of tRNAAsn and/or tRNAGlu, which triggers the switch to the terminator conformation. Q, glutamine or Gln.