Final answer:
The Capping enzyme is recruited by the CTD of RNA polymerase II through interactions with specific amino acid residues in the YSPTSPS repeats. It adds a 7-methylguanosine cap to the 5' end of the RNA transcript, protecting it from degradation and aiding in translation initiation.
Step-by-step explanation:
The capping enzyme is recruited by the C-terminal domain (CTD) of RNA polymerase II during the process of transcription. The CTD of RNA polymerase II consists of a repeating amino acid sequence, often referred to as the YSPTSPS repeats. These repeats provide binding sites for various proteins involved in transcription. One of these proteins is the capping enzyme, which binds to the CTD through interactions with specific amino acid residues within the YSPTSPS repeats.
Once recruited, the capping enzyme adds a 7-methylguanosine (m7G) cap to the 5' end of the newly synthesized RNA transcript. This cap protects the RNA from degradation and helps in the initiation of translation by ribosomes. The capping enzyme catalyzes the addition of the m7G cap by forming a 5'-5' triphosphate linkage between the first nucleotide of the RNA transcript and the m7G cap.