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Ch. 12 - The Genetic Code and Transcription
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook
All textbooksKlug 10th EditionCh. 12 - The Genetic Code and TranscriptionProblem 18
Chapter 12, Problem 18

Describe the structure of RNA polymerase in bacteria. What is the core enzyme? What is the role of the σ subunit?

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Begin by explaining that bacterial RNA polymerase is a multi-subunit enzyme responsible for synthesizing RNA from a DNA template during transcription.
Describe the overall structure of bacterial RNA polymerase, which consists of a core enzyme and an additional subunit called sigma (σ). The core enzyme is composed of five subunits: two alpha (\alpha) subunits, one beta (\beta), one beta prime (\beta\'), and one omega (\omega) subunit.
Clarify that the core enzyme (\alpha_2\beta\beta\'\omega) is capable of elongating the RNA chain but cannot initiate transcription on its own because it lacks specificity for promoter regions on DNA.
Explain that the sigma (σ) subunit associates with the core enzyme to form the holoenzyme, which is necessary for the initiation phase of transcription. The σ subunit recognizes and binds to specific promoter sequences on the DNA, guiding the RNA polymerase to the correct start site.
Summarize that once transcription is initiated, the σ subunit often dissociates from the core enzyme, allowing the core enzyme to continue RNA synthesis during elongation.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Structure of Bacterial RNA Polymerase

Bacterial RNA polymerase is a multi-subunit enzyme responsible for synthesizing RNA from a DNA template. It typically consists of a core enzyme made up of five subunits (2 α, β, β', and ω) and a sigma (σ) factor that associates transiently. This complex structure allows the enzyme to perform transcription efficiently.
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Core Enzyme of RNA Polymerase

The core enzyme of bacterial RNA polymerase includes the subunits α2, β, β', and ω. It carries out the catalytic activity of RNA synthesis but cannot initiate transcription on its own. The core enzyme binds to DNA and elongates the RNA chain once transcription has started.
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Role of the Sigma (σ) Subunit

The sigma (σ) subunit is a detachable factor that directs the RNA polymerase core enzyme to specific promoter regions on the DNA. It enables the enzyme to recognize and bind to the correct initiation sites, facilitating the start of transcription. After initiation, the σ subunit often dissociates from the core enzyme.
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Related Practice
Textbook Question

Refer to the genetic coding dictionary to respond to the following:

A base-substitution mutation that altered the sequence shown in part (a) eliminated the synthesis of all but one polypeptide. The altered sequence is shown here:

5'-AUGCAUACCUAUGUGACCCUUGGA-3'

Determine why.

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Textbook Question

Most proteins have more leucine than histidine residues, but more histidine than tryptophan residues. Correlate the number of codons for these three amino acids with this information.

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Textbook Question

Define the process of transcription. Where does this process fit into the central dogma of molecular biology (DNA makes RNA makes protein)?

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Textbook Question

Write a paragraph describing the abbreviated chemical reactions that summarize RNA polymerase-directed transcription.

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Textbook Question

Messenger RNA molecules are very difficult to isolate in bacteria because they are rather quickly degraded in the cell. Can you suggest a reason why this occurs? Eukaryotic mRNAs are more stable and exist longer in the cell than do bacterial mRNAs. Is this an advantage or a disadvantage for a pancreatic cell making large quantities of insulin?

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Textbook Question

One form of posttranscriptional modification of most eukaryotic pre-mRNAs is the addition of a poly-A sequence at the 3' end. The absence of a poly-A sequence leads to rapid degradation of the transcript. Poly-A sequences of various lengths are also added to many bacterial RNA transcripts where, instead of promoting stability, they enhance degradation. In both cases, RNA secondary structures, stabilizing proteins, or degrading enzymes interact with poly-A sequences. Considering the activities of RNAs, what might be general functions of 3'-polyadenylation?

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