Question

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

Short answer

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

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.

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.

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.

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.

Key concepts

Bacterial Transcription

Bacterial transcription is a fundamental biological process in which the genetic information stored in DNA is copied into RNA. This is the first step in the expression of genes, as it sets the stage for protein production. Transcription in bacteria is initiated by RNA polymerase, which binds to specific DNA sequences known as promoters.

During transcription, the double-stranded DNA unwinds to allow one of the strands to serve as a template. RNA polymerase catalyzes the synthesis of a complementary RNA strand by sequentially adding ribonucleotides. Bacterial transcription is crucial because it controls gene expression and allows bacteria to respond to environmental changes by producing necessary proteins.

Unlike eukaryotes, bacteria do not require separate RNA polymerases for different types of RNA. A single bacterial RNA polymerase is responsible for synthesizing all RNA types, making the process more streamlined but also dependent on accessory factors like the sigma factor to initiate transcription accurately.

Core Enzyme

The core enzyme of bacterial RNA polymerase is essential to the transcription process but lacks the ability to start transcription on its own. It is a multi-subunit complex constituting:

• Two alpha (\(\alpha\)) subunits: Important for enzyme assembly and interaction with regulatory factors.
• One beta (\(\beta\)) subunit: Plays a role in catalysis and binding to ribonucleotides.
• One beta prime (\(\beta'\)) subunit: Critical for DNA binding.
• One omega (\(\omega\)) subunit: Stabilizes the RNA polymerase complex.

These components together form the core enzyme, which can synthesize RNA.

However, the core enzyme is not sufficient for recognizing the precise starting point of the transcription, which is where the promoter sequences reside. This limitation requires assistance from another factor known as the sigma factor. The core enzyme alone can elongate RNA but depends on a sigma factor for proper initiation.

Sigma Factor

The sigma factor is a protein component that temporarily associates with the core enzyme of RNA polymerase, creating what is known as the RNA polymerase holoenzyme. This composition is necessary to initiate transcription at the correct promoter sequences in DNA.

Several key roles of the sigma factor include:

• Recognition of promoter regions: It enables the holoenzyme to identify and bind specifically to promoter sequences, ensuring the accurate start of transcription at the right site.
• Facilitation of strand separation: Helps open the DNA helix around the promoter region to allow entry of ribonucleotides and initiation of RNA synthesis.
• Dissociation post-initiation: After transcription begins, the sigma factor detaches, leaving the core enzyme to continue elongating the RNA chain.

Without the sigma factor, the core enzyme would struggle to find and bind the precise starting points on the DNA, making it critical for proper regulation and efficiency in the transcription process.