Final answer:
False. The conformation of the TPP riboswitch when bound to thiamine pyrophosphate (TPP) does not allow the ribosome to access the Shine-Dalgarno sequence; instead, it hides the sequence and blocks translation, making the statement false.
Step-by-step explanation:
The statement that the conformation of the TPP riboswitch allows the ribosome to access the Shine-Dalgarno sequence only when bound to thiamine pyrophosphate is false. Riboswitches regulate gene expression by changing the structure of mRNA in response to binding of a small molecule. In the case of the TPP riboswitch, the presence of thiamine pyrophosphate (TPP) indeed influences the mRNA structure; however, the binding leads to a sequestration of the Shine-Dalgarno sequence.
When TPP is bound to the TPP riboswitch, it promotes the formation of a specific stem-loop structure that hides the Shine-Dalgarno sequence, effectively preventing the recruitment of the ribosome and thus translation of the mRNA. Therefore, the conformation of the TPP riboswitch when TPP is bound does not allow the ribosome to access the Shine-Dalgarno sequence and, as a result, blocks translation.