I'm having trouble understanding how eukaryotes terminate transcription. Studying Campbell Biology (pg. 342, 10th ed.), I read:

In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence on DNA, which specifies a polyadenylation signal (AAUAAA) in the pre-mRNA. This is is called a "signal" because once this stretch of six RNA nucleotides appears, it is immediately bound by certain proteins in the nucleus.

The text continues with:

Then, at a point about 10-35 nucleotides downstream from the AAUAA, these proteins cut it free from the polymerase, releasing the pre-mRNA.

Does this mean pre-mRNA continues to be synthesized for 10-35 nucleotides after AAUAA is transcribed before finally being cut off?

Thanks so much.


1 Answer 1


According to Lewin's Genes XI - Krebs et. al on Eukaryotic Transcription, RNA Splicing, and Processing

It is not clear whether RNA polymerase II actually engages in a termination event at a specific site. It is possible that its termination is only loosely specified. In some transcription units, termination occurs more than 1000 bp downstream of the site, corresponding to the mature 3' end of the mRNA (which is generated by cleavage at a specific sequence) Instead of using specific terminator sequences, the enzyme ceases RNA synthesis within multiple sites located in rather long "terminator regions." The nature of the individual termination sites is largely unknown.

It does go on to say,

The site of cleavage/polyadenylation in most pre-mRNAs is flanked by two cis-acting signals: an upstream AAUAAA motif, which is located 11 to 30 nucleotides from the site, and a downstream U-rich or GU-rich element. The AAUAAA is needed for cleavage and polyadenylation because deletion or mutation of the AAUAAA hexamer prevents generation of the polyadenylated 3' end (though in plants and fungi there can be considerable variation from the AAUAAA motif).

I should add that the significance of 11 to 30, or as you said 10 to 35 nucleotides is that it is telling you the approximate size of the protein complex that is sitting on the RNA and where the active site of the protein is. The domain that recognizes the termination sequence is about 10 to 35 bases wide. It is likely that the variation is due to the RNA sequence following the termination sequence ability to form a stem loop structure.

Campbell takes a necessarily basic and less nuanced approach to the information that they provide on molecular biology. The approach is to provide some details and show the evolutionary commonalities between a diverse range of biological systems. The approach is okay as it gets you into the frame of mind of thinking about the interconnectedness of biological systems.

As you progress in your studies you will see that the reality and specifics are far more complex and exception based than was presented in introductory courses. But the more advanced courses spend an entire semester focused on what Campbell covers in one chapter. Campbell is accurate but incomplete out of necessity.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .