MCB 150 lecture 16

Course Information

Course Title: MCB 150: The Molecular and Cellular Basis of Life

Lecture: Lecture 16: Transcription in Bacteria

Transcription in Bacteria

Purpose

The primary purpose of transcription in bacteria is to amplify gene expression. Since many genes exist in only 1 or 2 copies within the bacterial genome, the availability of templates for protein synthesis is limited. Transcription acts as a critical regulatory step, enabling bacteria to produce proteins necessary for survival and adaptation in diverse environments.

Process Overview

The transcription process encompasses the synthesis of RNA from a DNA template, and it follows the general pathway of DNA ➞ RNA ➞ Protein, illustrating the central dogma of molecular biology. This pathway highlights how genetic information is conveyed from the genetic blueprint (DNA) to functional products (proteins).

RNA Polymerase

• Role: RNA Polymerase is the essential enzyme that synthesizes RNA by reading the DNA template. Its activity is pivotal to initiating transcription and ensuring the accurate assembly of RNA molecules.

• Structure: The enzyme in E. coli, known as RNA Polymerase, comprises 6 subunits that form a complex called the Holoenzyme. This includes two alpha (α) subunits, one beta (β) subunit, one beta prime (β′), one omega (ω) subunit, and one sigma (σ) subunit. In the absence of the σ subunit, the remaining components form the Core Enzyme.

• Function: RNA Polymerase does not simply transcribe the entire bacterial genome; rather, it specifically locates and synthesizes individual genes, ensuring that only relevant genetic information is utilized to produce necessary proteins.

Promoter Recognition

• Promoters: These are specialized DNA sequences that function as critical start signals for the transcription process, marking the beginning of a gene.

• Promoter Sequences: In bacterial promoters, two major regions are recognized, situated at positions -10 and -35 relative to the transcription start site.

• Consensus Sequences: Promoter efficiency depends on its proximity to these consensus sequences; the closer the sequence aligns with the ideal consensus, the more effectively RNA Polymerase binds to it, thereby enhancing RNA production rates.

Sequence Logos

• Analysis Tool: Sequence logos are a useful analytical tool for assessing conserved promoter regions. They provide a graphical representation of nucleotide frequency at specific positions, allowing researchers to visualize how certain sequences are favored across various organisms.

Transcription Steps in Bacteria

1. RNA Polymerase Scanning: The σ subunit actively scans the DNA to locate promoter sites, initiating the transcription process.

2. Transcription Phases: The transcription process can be broken down into several distinct phases:

   • Promoter Recognition: RNA Polymerase identifies and binds to the promoter region.

   • Initiation: The synthesis of RNA begins as RNA Polymerase unwinds the DNA strands.

   • Elongation: During this phase, mRNA is synthesized in the 5' to 3' direction, while the DNA strands are rewound behind the polymerase. The newly synthesized mRNA strand separates from the DNA template.

   • Termination: The transcription process concludes when RNA Polymerase encounters specific signals in the DNA sequence, leading to the release of the newly formed mRNA.

Transcription Bubble

• Inside the Bubble: The transcription bubble is a transient structure that forms around the RNA Polymerase as it synthesizes RNA. It includes features such as the movement of the polymerase along the DNA strand, the dynamic interaction of upstream and downstream DNA regions, and the unwinding and rewinding of double-stranded DNA.

• Strands:

  • Coding Strand: This strand of DNA has a sequence that is similar to the mRNA transcript and serves as a reference for understanding RNA sequence functionality.

  • Template Strand: Known as the non-coding strand, this strand is the actual template used during the transcription process to guide RNA synthesis.

Template Strand Selection

• Choosing the Template: In the bacterial genome, either DNA strand can function as a template for transcription. The selection of the template strand is influenced by the specific location and orientation of the promoter associated with the gene, determining the directionality of RNA synthesis.