It is thought to help with mRNA recognition by the ribosome during translation. Proteins are synthesized by ribosomes translating mRNA into polypeptide chains which may then undergo PTM to form the mature protein product. Post Transcriptional Modification Splicing Mechanisms Things are compartmentalized and do not occur all in the same area. . The three main post-transcriptional modifications are. Where do post-transcriptional modifications occur. Post-translational modifications can occur on the amino acid side chains or at the proteins C- or N- termini. To the 3 end of the RNA chain 30-500 adenines are added in what is called a poly A tail. Where does post translational modification occur. September 3 2018 by Sagar Aryal. Post-transcriptional modifications of pre-mRNA such as capping splicing and polyadenylation take place in the nucleus. Similarly which occur during post translational modification. Febr...