Final answer:
The C-terminal domain of RNA polymerase II must be phosphorylated to allow the 5'-Capping Enzyme to bind, which facilitates the addition of the 7-methylguanosine cap to the mRNA transcript for protection and recognition by translation factors.
Step-by-step explanation:
Before the 5'-Capping Enzyme can bind to the C-terminal domain (CTD) of the largest subunit of RNA polymerase II, the CTD must be phosphorylated. The phosphorylation of the CTD changes its conformation and allows the binding of various factors necessary for mRNA processing, including the capping enzyme.
This phosphorylation occurs as part of the transition from transcription initiation to elongation, with the RNA polymerase II being modified by the addition of phosphate groups by kinases, particularly TFIIH which is part of the transcription initiation complex.