3.4 RNA polymerase and ribosomes (b) Rifampin is an antibiotic used to treat Myc

Question

3.4 RNA polymerase and ribosomes

(b) Rifampin is an antibiotic used to treat Mycobacterium infections such as tuberulosis. It inhibits the initiation of transcription, but not the elongation of RNA transcripts. The time evolution of an E. coli  ribosomal RNA (rRNA) operon after addition of rifampin is shown in figure 3.36. Use the figure to estimate the rate of transcript elongation. Use the beginning of the "Christmas-tree" morphology on the left of figure 3.36A as the start point of transcription.

(c) Using the calculated elongation rate, estimate the frequency of initiation off of the rRNA operon. These genes are among the most transcribed in E. coli.

(d) As we saw in the chapter, a typical E. coli cell with a division time of 3000 s contains roughly 20,000 ribosomes. Assuming there is no ribosome degradation, how many RNA polymerase molecules must be synthesizing rRNA at any instant? What percentage of the RNA polymerase molecules in E. coli are involved in transcribing rRNA genes?

Explanation / Answer

RNA polymerase (ribonucleic acid polymerase), both abbreviated RNAP or RNApol, official name DNA-directed RNA polymerase, is a member of a family of enzymes that are essential to life: they are found in all organisms(species) and many viruses. RNAP locally opens the double-stranded DNA (usually about four turns of the double helix) so that one strand of the exposed nucleotides can be used as a template for the synthesis of RNA, a process called transcription. A transcription factor and its associated transcription mediator complex must be attached to a DNA binding site called a promoter region before RNAP can initiate the DNA unwinding at that position. RNAP has intrinsic helicase activity, therefore no separate enzyme is needed to unwind the DNA (in contrast to DNA polymerase). RNAP not only initiates RNA transcription, it also guides the nucleotides into position, facilitates attachment and elongation, has intrinsic proofreading and replacement capabilities, and termination recognition capability. In eukaryotes, RNAP can build chains as long as 2.4 million nucleotides.

RNAP was discovered independently by Charles Loe, Audrey Stevens, and Jerard Hurwitz in 1960.[18] By this time, one half of the 1959 Nobel Prize in Medicine had been awarded to Severo Ochoa for the discovery of what was believed to be RNAP,[19] but instead turned out to be polynucleotide phosphorylase.

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