Final answer:
The pTEFb factor is critical for transcription initiation as it is recruited to the promoter by basal transcription factors and phosphorylates the RNA polymerase II to allow mRNA synthesis to commence.
Step-by-step explanation:
The pTEFb factor is recruited to the promoter by various transcription factors that form a complex around RNA polymerase II. This complex includes basal transcription factors denoted as TFIIA, TFIIB, TFIIE, TFIIF, and TFIIH, along with the TATA-binding protein which is a part of TFIID. The pTEFb factor is crucial for transitioning RNA polymerase II from a pre-initiation phase to elongation, as it phosphorylates the RNA polymerase II at specific sites on its C-terminal domain (CTD), namely the serine residues 2 and 5. This phosphorylation event is necessary to free the RNA polymerase II from its stalled state, allowing it to progress along the DNA template and synthesize mRNA.