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Chapter 36: Transcription: RNA Synthesis, Processing, and Regulation

You are studying lipoprotein processing in a strain of mutant mice. You find that these mice synthesize the same apolipoprotein composition contained in lipoproteins made in the liver and the intestines, whereas, in wild-type mice there are 2 different sized apolipoproteins in these same tissues. Examination of the mRNA processing in the mutant mice would most likely show a defect in which of the following processes?

A. alternative polyadenylation

B. alternative splicing

C. differential capping

D. posttranscriptional modification converting a U to a T residue

E. RNA editing

Answer E: RNA editing is a molecular process through which some cells can make discrete changes to specific nucleotide sequences within an mRNA molecule after it has been generated by RNA polymerase. There are 2 major types of RNA editing with 1 being a C-to-U change catalyzed by cytidine deaminase that deaminates a cytidine base into a uridine base. An example of C-to-U editing is with the apolipoprotein B gene in humans. ApoB-100 is expressed in the liver and apoB-48 is expressed in the intestines. The B-100 form has a CAA sequence that is edited to UAA, a stop codon, in the intestines.

You are studying the processes of RNA transcription in a cell-free system. You find that although the RNA polymerase II in your system initiates transcription as expected, it does not complete the process. Which of the following modifications to the enzyme is most likely not taking place, thereby, explaining the lack of complete transcription?

A. 5′ capping

B. acetylation

C. methylation

D. phosphorylation

E. ubiquitination

Answer D: In order for RNA pol II to initiate, as well as to continue, transcription it must be phosphorylated. The transcription factor complex TFIIH contains a kinase that phosphorylates serine residues in the C-terminal domain (CTD) of the large subunit of RNA pol II. This phosphorylation event allows transcription initiation to commence. The 5′ end of all eukaryotic mRNAs are capped and this process occurs after the newly synthesizing mRNAs are around 20-30 bases long. At this point, RNA pol II pauses and a different kinase phosphorylates additional residues in the CTD. This pausing and regulatory phosphorylation event allows for the potential of attenuation in the rate of transcription. Lack of phosphorylation would, therefore, prevent continued transcription.

Which of the following represents a characteristic feature of the ...

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