Genetic Transfer and Mapping in Bacteria

Introduction to Bacterial Genetics

Bacteria, like eukaryotes, exhibit allelic differences that affect their cellular traits. However, because bacteria are haploid and reproduce asexually via binary fission, traditional genetic crosses are not used. Instead, geneticists study genetic transfer, the movement of DNA from one bacterium to another, which increases genetic diversity. Bacteriophages, viruses that infect bacteria, also play a role in genetic transfer.

Mechanisms of Genetic Transfer in Bacteria

Overview of Genetic Transfer

• Conjugation: Direct transfer of DNA between two bacterial cells via physical contact.

• Transformation: Uptake of free DNA from the environment by a bacterial cell.

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

Conjugation

Conjugation is a process requiring direct contact between a donor and a recipient cell. The donor transfers a strand of DNA, often a plasmid, to the recipient. This process was first discovered by Lederberg and Tatum in 1946 using Escherichia coli strains with different nutritional requirements.

• Prototrophs: Wild-type bacteria that can synthesize all required nutrients.

• Auxotrophs: Mutant bacteria that cannot synthesize a specific nutrient and require it in the medium.

The replica plating technique is used to isolate auxotrophic mutants by transferring colonies to selective media and identifying those that fail to grow.

Lederberg and Tatum Experiment

• Mixed two auxotrophic strains of E. coli and plated them on minimal medium.

• Only the mixture produced prototrophic colonies, indicating genetic recombination.

Physical Contact Requirement

• Davis's U-tube experiment showed that direct cell-to-cell contact is necessary for conjugation; DNA alone cannot pass through a filter to mediate transfer.

F Factor and Conjugation Mechanism

• F factor (Fertility factor): A small circular DNA molecule (plasmid) in donor cells (F+).

• F+ cells produce sex pili (F pili) that attach to F− cells, forming a conjugation bridge.

• Transfer begins at the origin of transfer (oriT) on the F factor.

• The relaxosome complex nicks the DNA, and the relaxase protein remains attached to the single-stranded DNA (T-DNA) as it is transferred.

• After transfer, the recipient synthesizes the complementary strand, becoming F+.

Plasmids

• Plasmids: Extra-chromosomal, usually circular DNA molecules with their own origin of replication.

• Not essential for survival but can confer advantages (e.g., antibiotic resistance).

Hfr Strains and Chromosome Mapping

• Hfr (High frequency of recombination) strains: F factor integrates into the bacterial chromosome, enabling efficient transfer of chromosomal genes.

• F' (F-prime) factors: Formed when the F factor excises imprecisely, carrying some chromosomal genes with it.

• During Hfr × F− conjugation, chromosomal genes are transferred in a linear sequence starting at oriT; the entire chromosome rarely transfers.

Interrupted Conjugation and Gene Mapping

• Wollman and Jacob developed the interrupted mating technique to map gene order and distance.

• By interrupting conjugation at various times, the order and timing of gene transfer can be determined.

Key Result: Genes enter the recipient in a specific order, and the time of entry reflects their position on the chromosome.

Genetic Mapping

• The E. coli chromosome is circular and about 100 minutes in length (the time to transfer the entire chromosome).

• Gene distances are measured in minutes based on entry times during conjugation.

Example: If lacZ enters at 16 minutes and galE at 25 minutes, they are 9 minutes apart.

Transformation

Transformation is the uptake of free DNA from the environment by a competent bacterial cell. Discovered by Frederick Griffith in 1928, transformation can occur naturally or be induced artificially in the lab.

• Natural transformation: Occurs without experimental manipulation; requires competence factors (proteins that mediate DNA uptake).

• Artificial transformation: Induced by laboratory techniques such as calcium chloride/heat shock or electroporation.

Steps of Transformation

1. DNA fragment from a dead donor cell binds to a receptor on a competent recipient cell.

2. An extracellular endonuclease cuts the DNA; one strand is degraded, and the other enters the cell.

3. The single-stranded DNA aligns with a homologous region on the chromosome and recombines, forming a heteroduplex.

4. DNA repair enzymes resolve the heteroduplex, potentially converting the recipient's allele to that of the donor.

Homologous recombination occurs if the DNA is similar to the recipient's chromosome; non-homologous recombination can occur at random sites if not.

Species-Specific DNA Uptake

• Some bacteria preferentially take up DNA from the same species using DNA uptake signal sequences or quorum sensing (e.g., Streptococcus pneumoniae).

Transduction

Transduction is the transfer of bacterial DNA from one cell to another via a bacteriophage. It can occur via two main types of phage life cycles:

• Lytic cycle: Phage replicates and lyses the host cell.

• Lysogenic cycle: Phage DNA integrates into the host genome and replicates with it.

During the lytic cycle, fragments of host DNA may be mistakenly packaged into phage particles (transducing phages), which then inject this DNA into new recipient cells, leading to recombination.

Example: Phage P22 (infects Salmonella typhimurium) and P1 (infects E. coli) can mediate transduction. If a his+ gene is packaged and transferred to a his− recipient, the recipient can become his+ after recombination.

Medical Relevance of Bacterial Genetic Transfer

Horizontal gene transfer (HGT) is the movement of genetic material between organisms other than by descent. Conjugation, transformation, and transduction are all forms of HGT and are major contributors to the spread of antibiotic resistance among bacteria.

• Antibiotic resistance: Genes conferring resistance can be acquired via HGT, leading to multidrug-resistant strains such as MRSA (Methicillin-resistant Staphylococcus aureus).

• Resistance mechanisms include antibiotic degradation, efflux pumps, and target modification.

Example: The percentage of S. aureus isolates resistant to methicillin increased from ~2% in 1981 to ~60% by 2001, illustrating the rapid spread of resistance genes.

Summary Table: Three Mechanisms of Genetic Transfer

Key Terms and Concepts

• Prototroph: Bacterium able to synthesize all required nutrients.

• Auxotroph: Bacterium unable to synthesize a specific nutrient.

• F factor: Fertility plasmid enabling conjugation.

• Hfr strain: Bacterial strain with F factor integrated into the chromosome.

• Competence: Ability of a cell to take up DNA from the environment.

• Horizontal gene transfer: Non-vertical transfer of genetic material between organisms.

Formulas and Equations

• Gene distance (in minutes):

Examples and Applications

• Mapping bacterial genes: Interrupted conjugation experiments allow construction of genetic maps based on the timing of gene transfer.

• Antibiotic resistance: Spread of resistance genes via plasmids and HGT is a major clinical concern.

Additional info: The above notes synthesize and expand upon the accessible reading version of Chapter 7, providing definitions, context, and examples for key concepts in bacterial genetic transfer and mapping.