Many genes in both bacteria and eukaryotes contain numerous sequences that can c

Question

Many genes in both bacteria and eukaryotes contain numerous sequences that can cause pauses in or premature termination of transcription. Nevertheless, the transcription of these genes within a cell normally produces multiple RNA molecules thousands of nucleotides long without pausing or premature termination. However, when a single round of transcription of these genes takes place in a test tube, RNA synthesis is frequently interrupted by pauses and premature terminations, which reduce the rate at which transcription takes place and frequently shorten the lengths of the mRNA molecules produced. Most pauses and premature terminations occur when RNA polymerase temporarily backtracks (i.e., backs up) for one or two nucleotides along the DNA. Experimental findings have demonstrated that most pauses and premature terminations disappear if several RNA polymerases are simultaneously transcribing the DNA molecule. Propose an explanation for this observation of faster transcription and longer mRNAs when the template DNA is being transcribed by multiple RNA polymerases.

Explanation / Answer

Transcription is the process of copying genetic information stored in a DNA strand into a transportable complementary strand of RNA. Transcription elongation is not a smooth ride along the DNA. For proofreading, the polymerase is made to back-up, erase some of the RNA it has already made and have another go at transcription. RNA polymerase does not transcribe through a gene at a constant pace. Rather it pauses periodically at certain sequences, sometimes for long periods of time before resuming transcription. In extreme cases, for example, when the polymerase encounters a damaged nucleotide, it comes to a complete halt. More often, an elongating polymerase is stalled near the promoter. Promoter-proximal pausing during early elongation is a commonly used mechanism for regulating genes poised to be expressed rapidly or in a coordinated fashion. Pausing is mediated by a complex called NELF (negative elongation factor). The blockage is released once the polymerase receives an activation signal. Other elongation factors stimulate the rate of elongation by limiting the length of time that polymerase pauses. When a single RNA polymerase is transcribing a template in a test tube, the backtracking often may lead to the RNA polymerase and the 3 ? end of the RNA transcript losing contact with each other resulting in premature termination. If other RNA polymerases are present on the template, they may act cooperatively with the leading RNA polymerase that has backtracked by pushing the leading RNA polymerase foward, thus continuing transcription before termination can occur.

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