Final answer:
TFIIIB is the 'commitment factor' that specifies the start-point for RNA polymerase III transcription, playing a crucial role in the initiation complex formation and stability.
Step-by-step explanation:
The transcription initiation complex for RNA polymerase III is an assembly of different factors, crucial for the accurate initiation of transcription. Within this context, the TFIIIB complex acts as the 'commitment factor' that specifies the start-point for RNA polymerase III transcription. This complex includes multiple subunits that together with other transcription factors, such as TFIIIA and TFIIIC, help in the proper positioning of RNA polymerase III at the target promoters. TFIIIB is critical for the functioning of RNA polymerase III because it stabilizes the binding of RNA polymerase to DNA and promotes the transcription of small structural RNAs like tRNA and 5S rRNA.