Bacterial Genetics: Horizontal Gene Transfer, Transformation, and Conjugation

Horizontal Gene Transfer and Modes of Genetic Exchange

Horizontal Gene Transfer (HGT) allows individuals of the same generation to share genes with each other, facilitating rapid evolution and the spread of traits such as antibiotic resistance. This stands in contrast to vertical gene transfer, which is the transmission of genes from parent to offspring. There are three primary modes of HGT commonly found in bacteria: Transduction, Transformation, and Conjugation. Transduction is a virally-mediated gene transfer carried out by bacteriophages. Transformation involves the uptake of naked, extracellular DNA from the environment. Conjugation, often referred to as bacterial mating, is the direct transfer of DNA between two living bacterial cells through physical contact.

Transduction: Viral-Mediated Material Transfer

Transduction is the process by which a bacteriophage (or phage) transfers DNA from one bacterium to another. This occurs in two forms: Generalized and Specialized transduction. Generalized transduction occurs when phages are "sloppy" during the packaging of viral particles. Instead of packaging viral DNA, they accidentally package fragments of the host cell's DNA into the viral capsid (virion). These virions can then bind to receptors on a new host cell and inject the bacterial DNA. Once inside, the injected DNA can engage in homologous recombination, mediated by the host cell's recombination system, potentially replacing a homologous region of the recipient's chromosome. A common example involves the transfer of a metabolic gene, such as moving a $Pro^+$ gene into a cell that is $Pro^-$. Specialized transduction is specific to genes located near the integration sites of lysogenic phages (prophages) in the host genome. When the prophage excises from the chromosome to enter the lytic cycle, it may take adjacent bacterial genes with it.

Transformation: Uptake of Naked DNA and the Discovery of Genetic Material

Transformation is the process by which competent bacterial cells take up small amounts of DNA from their environment. This mechanism led to the monumental discovery that DNA is the "transforming principle" and the carrier of genetic information. The discovery began with Frederick Griffith in 1928, who worked with two strains of Streptococcus pneumoniae: smooth (S) colonies and rough (R) colonies. The smooth strain is pathogenic because it possesses a polysaccharide capsule that protects the bacteria from host white blood cells (phagocytosis). The rough strain lacks this capsule and is nonpathogenic. Griffith observed that while heat-killed S cells and living R cells were separately non-lethal to mice, a mixture of the two caused death. Living S cells were recovered from the dead mice, demonstrating that a "transforming principle" from the dead S cells had converted the harmless R cells into virulent S cells.

In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty sought to identify the chemical identity of this transforming principle. They extracted components from heat-killed virulent cells and treated them with specific enzymes: RNase (to destroy RNA), Protease (to destroy proteins), and DNase (to destroy DNA). They found that only the extract treated with DNase lost the ability to transform nonvirulent cells into virulent cells, proving that DNA was the factor responsible. Despite this evidence, the prevailing view of the time was that genes were made of protein. The matter was definitively settled in 1952 by Alfred Hershey and Martha Chase using bacteriophage T2 (or T7). They specifically labeled phage protein with radioactive sulfur ($^{35}S$) and phage DNA with radioactive phosphorus ($^{32}P$). After infection and centrifugation, they found that the radioactive DNA ($^{32}P$) was located in the pellet with the bacterial cells and in the progeny phages, while the radioactive protein ($^{35}S$) remained in the liquid supernatant and was absent from progeny. This confirmed that DNA, not protein, is the genetic material injected into cells to reprogram them.

Plasmids: Characteristics and the pGLO System

Plasmids are extra-genomic, typically circular DNA molecules that are much smaller than the bacterial chromosome. They require host proteins for replication and can encode genes that provide a selective advantage, such as antibiotic resistance, metabolic capabilities (e.g., the ProA gene), resistance to toxic metals, or pathogenesis. Plasmids are vital tools in research and genetic engineering. An example is the plasmid $pBR322$, which is $4,362_{ }bp$ and contains genes for ampicillin resistance ($amp$) and tetracycline resistance ($tet$), an origin of replication ($ori$), and restriction sites like $PstI$, $HindIII$, and $BamHI$.

Another significant example is the $pGLO$ plasmid used in bacterial transformation experiments. It contains the $bla$ gene (encoding $\beta$-lactamase) for ampicillin resistance, the $GFP$ gene (Green Fluorescent Protein) from jellyfish, and the $araC$ gene, which encodes a regulatory protein. Transformation with $pGLO$ makes $Escherichia_{ }coli$ cells ampicillin-resistant and causes them to glow green under UV light when induced. The protocol involves adding $E. coli$ and $pGLO$ to a transformation buffer, cooling on ice, subjecting the cells to heat shock, adding LB broth for recovery, and finally plating on LB agar containing ampicillin and arabinose ($LB/amp/ara$).

Regulation of the araBAD Operon

Transformation with $pGLO$ demonstrates gene regulation via the $araBAD$ operon. The $AraC$ protein is a bifunctional regulator that forms a dimer. When arabinose is absent, the $AraC$ dimer exists in an elongated form that binds to the $araI_1$ and $araO_2$ sites, causing the DNA to form a loop. This loop prevents RNA polymerase from interacting with the $P_{BAD}$ promoter, so transcription of the downstream genes (such as $GFP$ in the $pGLO$ system) is blocked. When arabinose is present, it binds to the $N$-terminal dimerization domain of $AraC$, causing the protein to adopt a compact conformation. In this state, $AraC$ binds to the $araI_1$ and $araI_2$ sites and interacts with RNA polymerase to permit transcription. This mechanism ensures that the protein (or $GFP$) is produced only when the inducer (arabinose) is available. Transformation efficiency in these experiments is calculated as the number of transformants per microgram of plasmid using the formula: $\text{Efficiency} = \frac{\text{Number of fluorescent colonies}}{\mu\text{g of plasmid spread on the plate}}$. The amount of DNA is calculated by $\text{DNA } (\mu\text{g}) = \text{concentration of DNA } (\mu\text{g}/\mu\text{l}) \times \text{volume of DNA } (\mu\text{l})$.

DNA Cloning, Selection, and Analysis

DNA cloning involves inserting a gene of interest into a plasmid vector. In a hypothetical procedure to clone the complete wild-type $ProA$ gene (a $1.25_{ }kb$ fragment) from plasmid $pSF402$ ($5_{ }kbp$ total) into vector $pBR322$ ($4,362_{ }bp$), researchers would: 1. Digest both plasmids with restriction endonucleases $HindIII$ and $BamHI$. 2. Heat the mixture to $65^{\circ}C$ to inactivate the enzymes. 3. Incubate with DNA ligase to seal the "sticky ends." 4. Transform $E. coli$ with the ligation mixture. 5. Select and screen for the recombinant plasmid, designated $pBR322-proA$.

Restriction mapping helps confirm successful cloning. If the $1.25_{ }kb$ $ProA$ fragment is inserted between the $HindIII$ and $BamHI$ sites of $pBR322$, a double digest of the resulting $pBR322-proA$ plasmid with those same enzymes would yield two fragments: the $1.25_{ }kb$ insert and the vector backbone. Agarose gel electrophoresis is used to visualize these patterns, where a porous matrix separates negatively charged DNA fragments by size; smaller fragments migrate faster toward the positive electrode. If transforming a strain with a $proA$ mutation, one could select for the plasmid without antibiotics by growing the cells on minimal media lacking proline, as only those that took up the $ProA$ gene could synthesize the amino acid and survive.

Bacterial Conjugation

Conjugation is the transfer of DNA through cell-to-cell contact, typically initiated by a sex pilus protruding from a donor cell to a recipient cell. It requires specialized plasmids, such as the fertility factor ($F$ factor). The $F$ factor includes two origins of replication: $oriV$ for non-conjugating cells and $oriT$ for DNA transfer. During conjugation, the donor ($F^+$) cell transfers a copy of the plasmid to the recipient ($F^-$) cell. Occasionally, the $F$ factor integrates into the host chromosome, creating a High-Frequency Recombination ($Hfr$) strain. $Hfr$ cells can transfer parts of the chromosome to a recipient in a specific order; the entire $E. coli$ chromosome takes approximately $100\text{ minutes}$ to transfer, a property used to map gene locations. If an integrated $F$ factor excises incorrectly, it becomes an $F'$ factor, which is a plasmid that carries chromosomal genes.

Experimental Analysis of Conjugation and Selective Growth

In a laboratory setting, conjugation can be studied using specific donor and recipient strains. The donor strain $KS1000$ has the genotype $F' \text{ lac proA/}\Delta\text{lac}\Delta\text{proA arg}^-$, meaning it is $\text{Lac}^+$, $\text{Pro}^+$, sensitive to streptomycin ($\text{Str}^s$), and requires arginine for growth ($\text{Arg}^-$). The recipient strain $CSH50$ has the genotype $F^-, \Delta\text{lac}\Delta\text{proA, StrA}$, making it $\text{Lac}^-$, $\text{Pro}^-$, and resistant to streptomycin ($\text{Str}^r$).

To differentiate these strains and identify exconjugants (recipients that received the plasmid), selective media is used:

• To grow $KS1000$, minimal media must be supplemented with arginine ($GMA$).
• To grow $CSH50$, minimal media must be supplemented with proline and streptomycin ($GMPS$).
• Neither donor nor recipient alone can grow on Glucose-Minimal Salts-Streptomycin ($GMS$) plates because the donor is killed by streptomycin and the recipient cannot synthesize proline. However, exconjugants ($CSH50$ cells that received the $F'\text{ lac proA}$ plasmid) will grow on $GMS$ because they gain the ability to synthesize proline and possess the innate resistance to streptomycin.

In experimental setups involving broth or spot mating, cell density is important for visualization. To see one Colony Forming Unit ($CFU$) in a $10^{-2}$ dilution (assuming a $5_{ }\mu\text{l}$ loopful), the undiluted broth would need a concentration of at least $2 \times 10^{4}\text{ cells/ml}$.