CBG.21 Bacterial Transformation and Genetic Mapping

Bacteria operate within the framework of Darwinian Evolution, characterized by specific adaptive behaviors and genetic mechanisms.
Adaptation: Bacteria continuously adapt to their specific micro-environment to ensure survival.
DNA Mutation: Bacterial DNA undergoes mutations at significantly higher rates than eukaryotic cells.
Trait Development: New traits appear through two primary paths: mutation or the acquisition of genetic material.
Housekeeping and Energy Conservation:
- Energy is described as the "key to good housekeeping."
- Bacteria invest energy into DNA replication.
- The energy cost is proportional to the size of the bacterial chromosome; the larger the chromosome, the more energy is required for replication.
- The principle of "use it or lose it" applies to genetic information to maintain metabolic efficiency.
Heritability: Any traits acquired through these mechanisms are heritable and passed on to subsequent generations.

Horizontal Gene Transfer (HGT), also referred to as Lateral Gene Transfer (LGT), is the process of incorporating DNA from another organism into the bacterial chromosome.
Consequences of HGT:
- Results in "regions of difference" appearing on the bacterial chromosome.
- These regions are identifiable because they differ from the rest of the host chromosome in terms of:
- $G+C$ content
- $GC$ skew
- Codon usage

Evolutionary Maturation: Over time, mutations accumulate within these acquired regions. These mutations eventually cause the foreign DNA to resemble the host bacterial chromosome more closely.
Evolutionary Tool: Scientists can use these distinct regions as tools to hypothesize exactly when and under what environmental circumstances a DNA uptake event occurred.

There are three standard (canonical) ways bacteria exchange genetic information: - Transformation - Conjugation - Transduction
Additionally, there are non-canonical methods of transfer involving specialized structures:
- Gene transfer agents - Vesicles containing DNA - Nanotubes

Definition: Some bacteria possess the ability to take up DNA directly from their surrounding environment. This DNA is integrated into the host chromosome via recombination.
Genotypic Change: If the introduced DNA has a different genotype than the recipient bacterium, a permanent change to the bacterial genotype occurs. This specific process is defined as transformation.
Sources of Uptaken DNA: - Dead bacterial cells. - Free DNA in the environment. - DNA actively secreted by other living cells.
Molecular Mechanism in Gram-Positive Bacteria:
- Model organism: Streptococcus pneumoniae. - Periplasm thickness: $30-40\,nm$ .
- DNA-uptake pilus: Interacts with transforming DNA (lengths of $10-12\,bp$ modeled). - Key components: ATPase motors, specific channels, and the Endonuclease ComGB.
- Polarity: DNA is processed with a $3' \to 5'$ polarity.
Molecular Mechanism in Gram-Negative Bacteria:
- Model organism: Neisseria gonorrhoeae.
- Structure involves a DNA-uptake pilus and a periplasm.
- ATPase involvement: Uses the extension ATPase (pilB) or the retraction ATPase (pilT) to facilitate the movement of DNA into the cell.

Natural Competence:
- Defined as the inherent ability of certain bacterial species to perform transformation.
- Examples of naturally competent species:
- Bacillus subtilis
- Haemophilus influenzae
- Helicobacter pylori
- Neisseria gonorrhoeae
- Streptococcus pneumoniae
- Specific limitations and conditions:
- N. gonorrhoeae: Takes up only linear DNA.
- S. pneumoniae and B. subtilis: Competent only at the onset of the stationary phase and in high cell density environments.
- Timing: S. pneumoniae remains competent for only a few minutes; B. subtilis remains competent for several hours.
- Efficiency: Competence can reach $100\%$ in S. pneumoniae, but only up to $20\%$ in B. subtilis.
Artificial Competence:
- Necessary for biotechnological applications involving species that are not naturally competent, such as Escherichia coli, particularly when plasmids must be introduced.
- Techniques for induction:
- Chemical Transformation: Often utilizes Calcium Chloride ( $CaCl_2$ ) followed by a heat-shock. - Electroporation: Application of an electric field of $10-20\,kV/cm$ . - Outcomes: Membrane integrity is restored naturally after the process, though overall efficiency remains low.

Principle: Gene mapping involves determining the physical proximity of genes on a chromosome. Two markers (a and b) are taken up together if they are close enough to reside on the same DNA fragment.
Mathematical Probabilities in Mapping:
- In highly competent cells, transformation occurs in approximately $1$ cell per $10^3$ .
- Unlinked Genes: If two genes are widely separated and always carried on separate fragments, the probability of simultaneous transformation (co-transformation) is the product of individual probabilities: $(10^{-3})^2 = 10^{-6}$ .
- Linked Genes: If genes are near enough to stay on the same fragment, the probability of co-transformation is roughly equal to a single gene transformation event: $10^{-3}$ .
Mapping Logic:
- Higher frequency of co-transformation implies physical proximity between genes.
- Caution: In populations where most cells are not competent, a high transformation frequency alone is not sufficient to conclude genetic linkage.
Mapping Case Study Example:
- Donor genotype: $a^+ b^+ c^+ d^+$
- Recipient genotype: $a^- b^- c^- d^-$
- Co-transformation data provided:
- $a^+$ and $b^+$ : $200$
- $a^+$ and $c^+$ : $0$
- $a^+$ and $d^+$ : $500$
- $b^+$ and $c^+$ : $500$
- $b^+$ and $d^+$ : $0$
- $c^+$ and $d^+$ : $0$
- Interpretation: The high values between $(a, d)$ and $(b, c)$ and $(a, b)$ suggest these pairs are linked. Zero values indicate great distance. The inferred gene order is either $c-b-a-d$ or $d-a-b-c$ .