Chapter 9

Genetics of Bacteria and Archaea

Introduction

The genetics of bacteria and archaea is a foundational topic in microbiology, focusing on how these organisms exchange genetic material and adapt to their environments. Horizontal gene transfer (HGT) is a key process that allows prokaryotic cells to rapidly acquire new characteristics, enhancing their fitness and evolutionary potential.

Horizontal Gene Transfer in Bacteria and Archaea

Overview of Horizontal Gene Transfer

Horizontal gene transfer refers to the movement of genetic material between organisms, bypassing the traditional parent-to-offspring inheritance. This process is crucial for genetic diversity and adaptation in prokaryotes.

• Definition: The transfer of genes between cells, not involving reproduction.

• Importance: Enables rapid acquisition of traits such as antibiotic resistance and metabolic capabilities.

• Main Mechanisms:

  1. Transformation

  2. Transduction

  3. Conjugation

Mechanisms of Horizontal Gene Transfer

Transformation

Transformation is the process by which a cell takes up free DNA from its environment. This DNA is often released from other cells and can be incorporated into the recipient's genome if the cell is competent.

• Competent Cells: Cells that are genetically able to take up DNA from their surroundings.

• Selective Barriers: Cell membranes restrict the entry of DNA; only competent cells can overcome this barrier.

• Genetic Regulation: Competence is genetically determined and often regulated by quorum sensing and chemical signals.

• Example: Streptococcus pneumoniae can naturally take up DNA from its environment.

Key Steps in Transformation:

1. DNA is released from a donor cell.

2. A competent recipient cell takes up the DNA.

3. Incorporation of the DNA into the recipient's genome by recombination.

Transduction

Transduction is the transfer of genetic material mediated by bacterial viruses (bacteriophages). This process can move DNA from one cell to another via viral infection.

• Generalized Transduction: Any portion of the host genome can be packaged into a phage particle and transferred.

• Specialized Transduction: Only specific regions of the host genome adjacent to the phage integration site are transferred.

• Lysogeny: The viral genome integrates into the host chromosome, allowing for specialized transduction.

• Defective Phages: Sometimes, defective viruses are used by prokaryotic cells to exchange DNA.

• Example: Escherichia coli infected by lambda phage can undergo specialized transduction.

Conjugation

Conjugation is the direct transfer of DNA between two cells via cell-to-cell contact, typically mediated by a plasmid.

• Plasmids: Small, circular DNA molecules that can replicate independently of the chromosome.

• Mechanism: Requires a conjugative plasmid and a pilus for cell contact.

• Transfer: Can occur between closely or distantly related cells.

• Example: The F plasmid in E. coli enables conjugation and transfer of genetic material.

Fates of Transferred DNA

Possible Outcomes

• Replication: The transferred DNA replicates independently (e.g., plasmids).

• Recombination: The DNA integrates into the recipient's chromosome via homologous recombination.

Genetic Recombination

Homologous Recombination

Homologous recombination is the physical exchange of DNA between similar or identical sequences. It is essential for the integration of transferred DNA and for maintaining genetic integrity.

• Definition: Exchange between homologous DNA sequences from different sources.

• Homologous DNA: Sequences with nearly identical nucleotide composition.

• Key Proteins: RecA protein in bacteria and similar proteins in archaea and eukaryotes.

Steps in Homologous Recombination:

1. Endonuclease nicks the DNA.

2. Single-strand binding (SSB) proteins bind to the DNA.

3. Strand invasion occurs, mediated by RecA protein.

4. Development of cross-strand exchange (formation of heteroduplex DNA).

5. Resolution of the recombination intermediate, resulting in patches or splices.

Equation:

$\text{Homologous Recombination:} \, \text{DNA}_{\text{donor}} + \text{DNA}_{\text{recipient}} \rightarrow \text{Recombined DNA}$

Horizontal Gene Transfer in Archaea

Mechanisms in Archaea

Archaea also utilize transformation, transduction, and conjugation, though the prevalence and mechanisms may differ from bacteria. Restriction systems are less common, and more research is needed to fully understand these processes.

• Transformation, Transduction, Conjugation: All observed in some archaeal species.

• Restriction: Rare in archaea compared to bacteria.

Maintaining Genomic Integrity

Immune Systems in Prokaryotes

Bacteria and archaea have evolved immune systems to protect against foreign DNA and maintain genomic integrity.

• Innate Immunity: Nonspecific defense mechanisms.

• Adaptive Immunity: Target specific nucleic acids, such as the CRISPR/Cas system.

CRISPR/Cas9 System

The CRISPR/Cas9 system is a form of adaptive immunity in prokaryotes, originally discovered in bacteria and archaea. It is now widely used in genome editing.

• Mechanism: Viral DNA sequences are integrated into the host genome as a memory bank.

• Process:

  1. Viral DNA is integrated into the host genome.

  2. Sequences are transcribed into RNA.

  3. RNAs bind to Cas9 proteins.

  4. Cas9-RNA complexes recognize and cleave matching viral DNA upon reinfection.

• Application: Used for targeted genome editing in research and biotechnology.

Summary Table: Mechanisms of Horizontal Gene Transfer

Mechanism	Description	Key Features	Example Organism
Transformation	Uptake of free DNA from environment	Requires competent cells; regulated by genetics and environment	Streptococcus pneumoniae
Transduction	DNA transfer via bacteriophage	Generalized and specialized types; involves viral infection	Escherichia coli (lambda phage)
Conjugation	Direct cell-to-cell DNA transfer	Requires plasmid and pilus; can occur between different species	Escherichia coli (F plasmid)

Additional info: Some details about the molecular steps of recombination and CRISPR/Cas9 were inferred and expanded for academic completeness.