RNA Processing II: Capping and Polyadenylation (CMB part 13)

Capping occurs in several steps: initially, RNA triphosphatase removes the terminal phosphate from pre-mRNA. Subsequently, guanylyl transferase adds the capping GMP derived from GTP, followed by two methyl transferases that methylate the N7 position of the capping guanosine and the 2'-O-methyl group of the penultimate nucleotide. These processes take place early in transcription, before the RNA chain exceeds 30 nucleotides in length. The cap plays a crucial role in ensuring proper splicing of some pre-mRNAs, facilitating the transport of mature mRNAs out of the nucleus, protecting mRNA from degradation, and enhancing its translatability. Most eukaryotic mRNAs and their precursors possess a poly(A) tail approximately 250 nucleotides long at their 3'-ends, added post-transcriptionally by poly(A) polymerase. The poly(A) tail increases both the stability and translatability of the mRNA, with the relative importance of these effects differing across systems. Transcription of eukaryotic genes beyond the polyadenylation site, after which the transcript is cleaved and polyadenylated at the newly formed 3'-end. An efficient mammalian polyadenylation signal includes an AAUAAA motif about 20 nucleotides upstream of the polyadenylation site, followed 23–24 base pairs later by a GU-rich sequence and then a U-rich motif. Variations in these sequences influence polyadenylation efficiency, with plant signals allowing more flexibility around the AAUAAA motif than animal signals, and yeast signals rarely containing the AAUAAA motif. Polyadenylation involves both cleavage of the pre-mRNA and the addition of the poly(A) tail at the cleavage site. The cleavage process requires multiple proteins, including CPSF, CstF, CF I, CF II, poly(A) polymerase, and the CTD of the largest subunit of RNA polymerase II. Among these, CPSF-73 is responsible for cleaving the pre-mRNA.