Termination of RNA synthesis in eukaryotes AAUAAA sequence and 'snurp' in post-transcriptional cleavage. In eukaryotes, the actual termination of RNA polymerase II activity during transcription may take place through termination sites similar to those found in prokaryotes (the nature of individual termination sites in not known). But these termination sites are believed to be present away (sometimes upto one kilobase away) from the site of the 3' end of mRNA. Obviously 3' end of mRNA will be generated due to post-transcriptional cleavage. This cleavage, at the end, is believed to be achieved by what is popularly called 'snurp' (small nuclear RNA-protein complex). 'Snurp' used for post-transcription cleavage has not been identified so far but is believed to be certainly different than the U1 snurp, which is believed to be involved in intron splicing in split genes (see Expression of Gene : Protein Synthesis 3. RNA Processing (RNA Splicing, RNA Editing and Ribozymes)). Moreover, a sequence 5'AAUAAA 3' has been found just on the 5' side of poly(A) addition site in several eukaryotic mRNAs (e.g. αand βglobins in mouse and ovalbumin in chicken). We will notice in the next topic that poly(A) tail is added to 3' end of eukaryotic mRNA after processing of precursor mRNA. The sequence 5'AAUAAA3' in mRNA 3' end seems to be common in eukaryotic mRNA and mutations in this sequence cause elongation of mRNA. This will suggest that this sequence contains the signal for endonucleolytic post-transcriptional cleavage. This sequence, therefore, is not involved in the termination of the synthesis of mRNA, but helps in generating 3' end later through endonuclease cleavage, in which snurp helps in an unknown manner.
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