The T-box riboswitch is essential in the control of amino acid synthesis enzyme expression. Using these riboswitches to detect the levels of uncharged tRNAs present in the cell, it is possible to suppress the expression of these enzymes when high levels of an amino acid are present, and upregulate expression with low concentrations. This allows constitutive expression of mRNA, allowing amino acid synthesis to be induced rapidly on a drop in concentration.
For example, when high levels of tryptophan are present in a cell, charged tRNA-Trp complexes bind to the attenuator loop of the T-box riboswitch. This binding, through the anticodon, prevents the biosynthetic enzymes from being expressed, preventing the cell from synthesising excess tryptophan.
At low tryptophan concentrations, there are more uncharged tRNA-Trp molecules present in the cell. The binding of the uncharged tRNA stabilises the anti-terminator conformation, preventing premature termination of transcription, allowing successful expression of the necessary biosynthetic enzymes.