Bacterial Gene Regulation, Mutation, and Genetic Recombination Notes

Flow of Genetic Information and Microbial Systems

• Genetic information within a cell is utilized through three primary processes as shown in Figure 8.2:

  • Expression: The use of genetic information within a cell to produce the proteins required for cellular function and metabolism. This involves the processes of transcription and translation.

  • Recombination: The horizontal transfer of genetic information between cells of the same generation. This leads to new combinations of genes and the creation of a recombinant cell.

  • Replication: The vertical transfer of genetic information to the next generation of cells. DNA is duplicated and passed from parent cells to offspring cells.

Understanding Genotype, Genome, and Phenotype

• Genotype: Refers to the specific variations of genes present within an organism.

• Genome: Represents the total genetic material of an organism.

• Phenotype: The observable physical characteristics of an organism, which are dependent on:

  • The presence of specific genetic information (genes).

  • The expression of that information into functional proteins.

Examples of Gene Expression Variable Outcomes

• Polydactyly: A condition characterized by extra digits. Possession of the gene does not guarantee the trait:

  • $90\%$ of individuals with the gene express the trait.

  • $10\%$ of individuals remain normal with $5$ digits because the gene is not expressed.

• Antigenic Variation in $N. \text{gonorrhoeae}$: The bacterium alters the expression of its Opa genes. By switching expression between $10$ different genes, it changes its surface proteins to evade the host immune system via antigenic variation.

Gene Regulation in Bacteria

• Rationale for Regulation:

  • A genome encodes for significantly more proteins than are required at any single moment.

  • Regulation allows the cell to conserve energy and resources by only producing what is necessary.

• Points of Regulation: Although regulation can occur at any stage of gene expression, the most common methods include:

  • Initiation of Transcription: This is the most energy-efficient method as it stops production at the earliest stage. However, it results in a lag time when the protein is eventually needed.

  • Control of Protein Activity via Ligands: This is the most energetically expensive method (since the protein is already made) but allows for a very rapid response to environmental shifts.

Ligands and Their Functional Roles

• Ligand Definition: A small molecule that binds to a protein to alter its three-dimensional shape and, consequently, its function.

• Activating Protein Function: Accomplished by "co-worker" ligands.

  • Examples: Corempressor, coactivator, coenzyme.

• Inactivating Protein Function:

  • Examples: Inducer, enzyme inhibitor.

Classification of Bacterial Genes

• Constitutively Active Genes:

  • These genes are not part of an operon.

  • They are always expressed because their products are constantly needed by the cell.

  • The activity of their protein products is controlled by ligands rather than transcriptional blocks.

  • Examples: Genes for cell respiration, ribosomes, RNA polymerase, DNA polymerase, and repressors on regulatory genes.

• Structural Genes:

  • These are located on an operon.

  • They are regulated at the level of transcription initiation to save energy, expressed only when specific environmental conditions require them.

The Operon System in Bacteria

• An operon is the primary bacterial system for gene regulation, where multiple genes for a specific metabolic pathway are clustered under a single control region.

• Coordinated Regulation: All structural genes within the operon are activated or repressed as a single unit.

• Components of an Operon (Figure 8.12):

  • Regulatory Gene ($R$): A constitutively expressed gene that codes for the Repressor protein.

  • Promoter ($P$): The DNA sequence where RNA polymerase binds to initiate transcription.

  • Operator ($O$): The DNA sequence that acts as a "traffic light," where the repressor protein binds to block RNA polymerase.

  • Structural Genes (e.g., $Z, Y, A$): The portion of the DNA that encodes the actual functional enzymes.

Mechanisms of Negative Regulation

• Repressor Protein: A regulatory protein used to control expression at all operons.

  • Every repressor is encoded by a constitutively active regulatory gene (e.g., the LacI gene encodes the Lac repressor).

  • Form/activity is controlled by specific environmental signals (ligands).

  • Active Form Function: In its active state, it is a DNA-binding protein that binds to the Operator sequence to physically block transcription.

Inducible Operons (The Lac Operon)

• Normal State: Gene expression is typically OFF.

• Function: Usually encodes catabolic enzymes used to metabolize specific (non-glucose) nutrients.

• Environmental Signal: The ligand is called an Inducer.

  • Inducer binds to the repressor, inactivating it.

  • The inactivated repressor is released from the Operator DNA sequence.

  • Transcription proceeds, and gene expression is turned ON (induced).

• Lac Operon Specifics:

  • Presence of lactose (converted to allolactose, the inducer) triggers expression of enzymes for lactose catabolism: $\beta$-galactosidase, permease, and transacetylase.

Repressible Operons (The Trp Operon)

• Normal State: Gene expression is typically ON.

• Function: Usually encodes anabolic enzymes used to produce essential building blocks like amino acids and nucleotides.

• Environmental Signal: The ligand is called a Corepressor.

  • Corepressor binds to the repressor, converting it into its active form.

  • The active repressor binds to the Operator DNA sequence.

  • Transcription is blocked, and gene expression is turned OFF.

• Trp Operon Specifics:

  • The presence of the amino acid tryptophan acts as the corepressor to prevent the overproduction of tryptophan when internal concentrations are sufficient.

Genetic Changes in Bacteria

• Vertical Gene Transfer: Transfer from parent to offspring.

  • Example: Mutations occurring during DNA replication that are passed on during cell division.

• Horizontal (Lateral) Gene Transfer: Transfer between two bacteria, potentially of different species.

  • Transformation: Bacterial DNA moves from a donor (dead cell) to the environment to a recipient cell.

  • Transduction: Bacterial DNA is moved from a donor to a recipient via a bacteriophage virus.

  • Conjugation: DNA transfer via direct cell-to-cell contact.

Mutations: Random DNA Sequence Changes

1. Base Substitution (Point Mutation)

• Exchange of a single base due to a replication error, altering a single codon.

  • Silent Mutation: The new codon codes for the same amino acid; the protein remains functional.

  • Nonsense Mutation: The new codon is a STOP codon ($UAG$); causes early termination and a nonfunctional protein.

  • Missense Mutation: The new codon codes for a different amino acid; effect varies based on the chemical properties of the new amino acid.

2. Frameshift Mutation

• The insertion or deletion of one or more nucleotide pairs (not in multiples of three).

• This shifts the translational reading frame, altering every amino acid following the mutation.

• Result: Usually leads to a complete loss of protein function.

3. Large Chromosomal-Level Mutations

• Usually result in inactive protein products.

  • Deletions: The loss of large segments of DNA.

  • Insertions: The addition of large, unwanted DNA pieces, such as the integration of viral DNA into a host gene, disrupting protein production.

Sources of Mutations

• Spontaneous Mutations: Natural errors made by DNA polymerase during replication.

  • Kept at a low rate by proofreading and DNA repair mechanisms.

  • Provides the necessary variation for evolution.

• Induced Mutations: Caused by environmental mutagens.

  • Can increase mutation rates by $10$ to $1000 \times$.

Specific Mutagens

• Nucleotide Base Analogs: Chemicals structurally similar to bases that cause base substitutions and block replication. Example: AZT, used in HIV treatment.

• Intercalating Chemicals: Molecules that wedge themselves between base pairs, causing frameshift mutations.

  • Examples: Acridine dyes (ethidium bromide), benzopyrene (found in smoke), aflatoxin (produced by mold on grains).

• Ionizing Radiation: Creates free radicals that cause double-stranded DNA ($dsDNA$) breaks.

• Non-ionizing Radiation (UV): Causes the formation of thymine dimers, where adjacent thymines on a DNA strand become cross-linked, disrupting base pairing.

DNA Repair Mechanisms

• Nucleotide Excision Repair: A general repair mechanism used to fix DNA errors after replication.

• UV Repair Systems: Specifically uses Photolyases, which are enzymes activated by visible light that break the bonds of thymine dimers.

Mechanisms of Genetic Recombination

• Maintenance: Once new DNA enters a cell, it must be successfully integrated and copied to be passed on.

• Homologous Recombination: A process of "crossing over" between similar DNA sequences to integrate donor DNA into the recipient's chromosome.

• Degradation: DNA that fails to undergo recombination is typically degraded by the cell.

Transformation, Transduction, and Conjugation

Transformation

• A donor cell dies and releases "naked DNA" into the environment.

• The recipient cell must be competent, possessing specific competence proteins on its surface to bind and transport the naked DNA across the membrane.

• Maintenance involves homologous recombination.

Transduction

• Transfer mediated by a bacteriophage (virus).

• Generalized Transduction: Results from an assembly mistake where bacterial DNA is accidentally packaged into a viral protein coat instead of viral DNA.

• Specialized Transduction: Occurs when a phage incorrectly excises itself from the bacterial genome, taking adjacent bacterial genes with it. All resulting new virus particles carry the bacterial gene.

Conjugation

• Requires a Fertility (F) Plasmid.

  • Contains the tra gene which encodes the sex pili.

  • Contains an Ori sequence for independent replication.

  • May contain an R factor (antibiotic resistance gene).

• Cell Types:

  • $F^-$ Cell: Recipient; lacks the F plasmid.

  • $F^+$ Cell: Donor; contains an independent F plasmid.

  • Hfr Cell: High Frequency Recombination cell; the F plasmid is integrated into the bacterial chromosome.

• Mating Scenarios:

  • $F^+$ x $F^-$: The F plasmid is copied and transferred. The recipient becomes either an $F^+$ cell or an Hfr cell (if the plasmid integrates).

  • Hfr x $F^-$: The Hfr cell copies part of its chromosome starting at the F factor. The mating bridge usually breaks before the entire chromosome (and the full F factor/tra gene) is transferred. The recipient cell remains $F^-$ but often acquires new chromosomal genes via recombination.