Fig. 9.
Schematic diagram showing the regulatory mechanism of TPP transport in T. denticola. T. denticola lacks a de novo TBS pathway, and it depends on the TPP transporter to take up exogenous TPP. The tbpABCTd operon encodes a TPP ABC transporter, which is regulated by a TPP-sensing riboswitch (Tdthi-box). When the intracellular TPP is insufficient, Tdthi-box folds into a structure that allows the transcription of the tbpABCTd operon, and more TPP transporter will be synthesized to take up exogenous TPP into the cells. When the intracellular TPP becomes sufficient, the free TPP binds to Tdthi-box and forms a stable aptamer, which in turn represses the transcription of the tbpABCTd operon and limits the uptake of exogenous TPP.