What role does the capping enzyme play in eukaryotic RNA processing?

The capping enzyme plays a crucial role in eukaryotic RNA processing by adding a 5' cap to the nascent RNA transcript. This modification is essential for several reasons, primarily influencing RNA stability, export from the nucleus, and translation efficiency.

By attaching the 5' cap, the capping enzyme helps protect the RNA from degradation by exonucleases. This protective feature ensures that the mRNA can be transported successfully to the cytoplasm where it is translated into protein. The cap structure also serves as a recognition signal for the ribosome during the initiation of translation, facilitating the binding of the ribosome to the mRNA.

In terms of splicing, while the capping process is necessary for proper RNA processing, it does not directly recruit the splicing machinery. Instead, the splicing of introns out of the pre-mRNA occurs as part of the broader RNA processing events, which also include the addition of the cap.

Adding a poly-A tail to mRNA is a separate but related process that occurs at the 3' end of the RNA molecule, and it is performed by a different enzyme complex. The initiation of transcription is the function of RNA polymerase, not the capping enzyme, and the capping enzyme is