Final answer:
The carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II must be phosphorylated to initiate transcription, which activates the transcription initiation complex and enables RNA polymerase II to synthesize mRNA.
Step-by-step explanation:
In the process of transcription initiation by RNA polymerase II in eukaryotes, it is essential that the carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II be phosphorylated. This phosphorylation event is critical for the transition from transcription initiation to elongation. Initially, RNA polymerase II, along with multiple transcription factors, binds to the promoter DNA sequence, forming the transcription initiation complex.
Transcription factors such as TFIIA, TFIIB, TFGIIE, TFIIF, and TFIIH are involved in this process. Once the RNA polymerase II is bound to its upstream sequence, the CTD is phosphorylated, which activates the transcription initiation complex and allows RNA polymerase II to start synthesizing mRNA. The phosphorylation of the RNA polymerase's CTD thus facilitates the shift from the transcription initiation stage to the elongation of the nascent RNA transcript.