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Chapter 12: Problem 18

Describe the structure of RNA polymerase in bacteria. What is the core enzyme? What is the role of the \(\sigma\) factor?

Short Answer

Expert verified
Answer: The sigma factor plays a crucial role in bacterial RNA polymerase by helping the core enzyme recognize specific DNA sequences known as promoters, where transcription should begin. It guides the RNA polymerase to the correct initiation site, facilitating the accurate start of the transcription process. After transcription has initiated, the sigma factor dissociates from the core enzyme.

Step by step solution

01

Introduction to RNA Polymerase

RNA polymerase is an essential enzyme in bacteria responsible for the synthesis of RNA from a DNA template through a process called transcription. It consists of multiple subunits, which combine to form functional complexes. Understanding the structure of RNA polymerase and the function of its components is crucial in explaining its role in bacterial protein synthesis.
02

The Structure of Bacterial RNA Polymerase

In bacteria, RNA polymerase is composed of multiple subunits, typically five: two alpha (\(\alpha\)) subunits, a beta (\(\beta\)) subunit, a beta prime (\(\beta'\)) subunit, and an omega (\(\omega\)) subunit. These subunits come together to form a complex structure responsible for binding to the DNA template and catalyzing the formation of RNA molecules.
03

The Core Enzyme

The core enzyme of bacterial RNA polymerase consists of the aforementioned subunits: two \(\alpha\) subunits, one \(\beta\) subunit, one \(\beta'\) subunit, and one \(\omega\) subunit. The core enzyme alone has the ability to synthesize RNA, but it cannot specifically recognize promoter sequences in the DNA template, which is required to initiate transcription accurately.
04

The Sigma Factor

The sigma (\(\sigma\)) factor is a protein that binds to the core enzyme and helps it recognize the specific DNA sequences called promoters, where transcription should begin. The sigma factor's primary role is to guide the RNA polymerase to the correct initiation site and facilitate the precise start of transcription. The combined structure of the core enzyme and the sigma factor is known as the RNA polymerase holoenzyme. Once the transcription process starts, the sigma factor dissociates from the core enzyme, allowing the latter to continue synthesizing RNA without the help of the sigma factor.

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Most popular questions from this chapter

When the amino acid sequences of insulin isolated from different organisms were determined, some differences were noted. For example, alanine was substituted for threonine, serine was substituted for glycine, and valine was substituted for isoleucine at corresponding positions in the protein. List the single-base changes that could occur in triplets to produce these amino acid changes.

The mRNA formed from the repeating tetranucleotide UUAC incorporates only three amino acids, but the use of UAUC incorporates four amino acids. Why?

One form of posttranscriptional modification of most eukaryotic RNA transcripts is the addition of a poly-A tail at the \(3^{\prime}\) -end. The absence of a poly-A tail leads to rapid degradation of the transcript. Poly-A tails 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 tails. Considering the activities of RNAs, what might be the general functions of \(3^{\prime}\) -polyadenylation??

A glycine residue exists at position 210 of the tryptophan synthetase enzyme of wild-type \(E .\) coli. If the codon specifying glycine is GGA, how many single-base substitutions will result in an amino acid substitution at position 210 , and what are they? How many will result if the wild-type codon is GGU?

Alternative splicing is a common mechanism for eukaryotes to expand their repertoire of gene functions. At least one estimate indicates that approximately 50 percent of human genes use alternative splicing, and approximately 15 percent of diseasecausing mutations involve aberrant alternative splicing. Different tissues show remarkably different frequencies of alternative splicing, with the brain accounting for approximately 18 percent of such events. (a) Define alternative splicing and speculate on the evolutionary strategy alternative splicing offers to organisms. (b) Why might some tissues engage in more alternative splicing than others?
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