Microbial Genetics: Horizontal Gene Transfer, Plasmids, Transposons, and Recombination

Microbial Genetics: Horizontal Gene Transfer and Mobile Genetic Elements

Introduction

This section covers the mechanisms by which bacteria exchange genetic material, focusing on horizontal gene transfer, plasmids, transposons, and the process of recombination. Understanding these processes is essential for appreciating microbial evolution, antibiotic resistance, and genetic diversity.

Transfer Mechanisms in Microbial Genetics

Vertical vs. Horizontal Gene Transfer

• Vertical gene transfer: Transmission of genetic material from parent to offspring during reproduction.

• Horizontal gene transfer (HGT): Movement of genetic material between organisms other than by descent. This process is especially common in prokaryotes and contributes significantly to genetic diversity.

Example: Escherichia coli strains K12 and O157 differ by over 1,500 genes, many acquired via HGT. About 20% of the E. coli genome is derived from other microbes.

Mechanisms of Horizontal Gene Transfer

Overview

Bacteria can acquire new genetic material through several mechanisms:

• Transformation: Uptake of free ("naked") DNA from the environment.

• Conjugation: Direct transfer of DNA via cell-to-cell contact, often mediated by a pilus.

• Transduction: Transfer of DNA from one bacterium to another via a bacteriophage (virus).

• Transposition: Movement of mobile genetic elements (transposons) within and between DNA molecules.

Transformation

• Involves the uptake of naked DNA fragments by a competent bacterial cell from its environment.

• Competence: The physiological state that allows a cell to take up DNA. Some bacteria are naturally competent; others can be made competent artificially (e.g., by chemical treatment or electroporation).

• DNA may integrate into the chromosome by recombination, becoming a permanent part of the genome.

• Example: Griffith's experiment (1928) with Streptococcus pneumoniae demonstrated transformation, showing that non-virulent bacteria could acquire virulence by uptake of DNA from dead virulent cells.

Conjugation

• Requires direct contact between donor and recipient cells, usually via a sex pilus.

• Mediated by conjugative plasmids (e.g., the F (fertility) factor in E. coli).

• Donor cells (F+) transfer plasmid DNA to recipient cells (F-).

• If the F factor integrates into the chromosome, the cell becomes an Hfr (high frequency of recombination) cell, capable of transferring chromosomal genes during conjugation.

• Recipient cells usually remain F- unless the entire F factor is transferred.

Transduction

• DNA is transferred from one bacterium to another by a bacteriophage.

• Generalized transduction: Any bacterial gene can be transferred; occurs when phages mistakenly package host DNA.

• Specialized transduction: Only specific bacterial genes near the phage integration site are transferred; occurs with temperate phages.

• Example: Toxin genes (e.g., diphtheria, cholera) can be transferred by specialized transduction.

Transposition

• Transposons are mobile genetic elements that can move within and between DNA molecules.

• Contain a transposase gene, which recognizes inverted repeat sequences at the ends of the transposon.

• Insertion sequences (IS elements): Simplest transposons, carrying only the transposase gene.

• Complex transposons: Carry additional genes, such as antibiotic resistance genes.

• Transposons can move between chromosomes and plasmids, facilitating the spread of resistance genes.

Fates of Acquired DNA

• Degradation: The cell may degrade the foreign DNA and use it as a nutrient source.

• Recombination: The DNA may integrate into the host chromosome via homologous recombination.

• Co-existence: The DNA may persist as an independent genetic element (e.g., plasmid).

Recombination

Role and Mechanism

• Mediated by the RecA protein, which facilitates homologous recombination between donor and recipient DNA.

• Homologous recombination involves crossing over at similar sequences, allowing integration of new genetic material.

• Functions:

  • Acquisition of new functions (e.g., metabolic pathways, resistance genes).

  • Repair of defective or damaged genes.

  • Increases genetic diversity within microbial populations.

Plasmids

• Small, circular, double-stranded DNA molecules (2–25 kbp) that replicate independently of the chromosome.

• Carry their own origin of replication.

• Types:

  • Conjugative plasmids: Can transfer themselves to other cells (e.g., F factor).

  • Resistance (R) plasmids: Carry antibiotic resistance genes.

  • Virulence plasmids: Carry genes for toxins or other virulence factors.

• Plasmids can be transferred between cells via conjugation, contributing to the spread of traits such as antibiotic resistance.

Transposons

• Mobile genetic elements (700–40,000 bp) capable of moving within and between DNA molecules.

• Can carry genes for antibiotic resistance or other traits.

• Facilitate genetic rearrangements and the spread of resistance genes between plasmids and chromosomes.

Summary Table: Mechanisms of Horizontal Gene Transfer

Mechanism	Description	Key Features	Example
Transformation	Uptake of naked DNA from environment	Requires competence; DNA may recombine with chromosome	Griffith's experiment with S. pneumoniae
Conjugation	Direct transfer of DNA via cell-to-cell contact	Involves plasmids (e.g., F factor); sex pilus	F+ to F- transfer in E. coli
Transduction	DNA transfer via bacteriophage	Generalized or specialized; phage as vector	Transfer of toxin genes
Transposition	Movement of transposons within/between DNA	Transposase enzyme; can carry resistance genes	Spread of antibiotic resistance

Key Equations and Terms

• Homologous recombination: Exchange of genetic material between similar or identical DNA sequences.

• RecA protein: Essential for homologous recombination in bacteria.

• Transposase: Enzyme that catalyzes movement of transposons.

Additional info:

• Horizontal gene transfer is a major driver of microbial evolution and adaptation, especially in the spread of antibiotic resistance.

• Understanding these mechanisms is crucial for biotechnology, medicine, and epidemiology.