The Riboswitch Regulates a Thiamine Pyrophosphate ABC Transporter of the Oral Spirochete Treponema denticola

Thiamine pyrophosphate (TPP), a biologically active form of thiamine (vitamin B-1), is an essential cofactor in all living systems. Microorganisms either synthesize TPP via de novo biosynthesis pathways or uptake exogenous thiamine from the environment via specific transporters. The oral spirochete Treponema denticola is an important pathogen that is associated with human periodontal diseases. It lacks a de novo TPP biosynthesis pathway and needs exogenous TPP for growth, suggesting that it may obtain exogenous TPP via a thiamine transporter. In this study, we identified a gene cluster that encodes a TPP ABC transporter which consists of a TPP-binding protein (TDE0143), a transmembrane permease (TDE0144), and a cytosolic ATPase (TDE0145). Transcriptional and translational analyses showed that the genes encoding these three proteins are cotranscribed and form an operon (tbpABC(Td)) that is initiated by a sigma(70)-like promoter. The expression level of this operon is negatively regulated by exogenous TPP and is mediated by a TPP-sensing riboswitch (Td(thi-box)). Genetic and biochemical studies revealed that the TDE0143 deletion mutant (T. denticola Delta tbpA) had a decreased ability to transport exogenous TPP, and the mutant failed to grow when exogenous TPP was insufficient. These results taken together indicate that the tbpABC(Td) operon encodes an ABC transporter that is required for the uptake of exogenous TPP and that the expression of this operon is regulated by a TPP-binding riboswitch via a feedback inhibition mechanism.