Bacterial Genetics and Gene Expression

Bacterial Genetics Overview

• Focus on how bacteria utilize genetic processes compared to eukaryotic perspectives, which can vary by teaching faculty.

Bacterial Genetic Material

• Bacteria have genetic information coded in DNA, similar to eukaryotic cells.

• The genetic structures of viruses differ since they may also contain RNA.

Historical Context

• Key historical figures in the study of DNA include:

  • Rosalind Franklin and her husband, who collaborated with Arthur Kornberg.

  • Kornberg discovered DNA polymerase, an enzyme crucial for DNA replication.

Basics of Genes

• A gene is defined as a sequence of codons that codes for a protein.

• In our genetic structure:

  • One gene produces one protein, making the model monocistronic.

• In contrast, bacterial genes can be polycistronic, where one gene may encode multiple proteins due to efficiency in smaller genomes.

DNA Structure and Process

• Structure of DNA:

  • Composed of ribose sugar, phosphate, and nitrogenous bases (A, T, C, G).

  • DNA forms a double helix stabilized by hydrogen bonds.

• Bacterial DNA is circular and double-stranded, unlike eukaryotic linear DNA.

DNA Replication

• DNA replication is semiconservative: each new DNA strand consists of one parental and one new strand.

• Key steps in bacterial DNA replication include:

  • Origin of replication (ORI) is where replication begins.

  • Helicase unwinds DNA; single-stranded binding proteins prevent strands from rejoining.

  • DNA polymerase requires a primer to start synthesis; added nucleotides are complementary to the template strand.

  • Replication results in Okazaki fragments on the lagging strand.

  • DNA ligase seals gaps between fragments.

Gene Expression

• Gene expression involves two main processes: transcription and translation.

  • Transcription: Converts DNA to messenger RNA (mRNA).

  • RNA polymerase initiates RNA synthesis at the promoter region and does not need a primer.

  • Translation: Synthesizes proteins from mRNA using ribosomes and transfer RNA (tRNA).

• In bacteria, transcription and translation can occur simultaneously due to the absence of a nucleus, increasing efficiency.

RNA vs. DNA

• RNA differs from DNA in structure:

  • Sugar: Ribose vs. deoxyribose.

  • Base: Uracil replaces thymidine.

  • RNA is single-stranded.

Types of RNA

• Messenger RNA (mRNA): carries genetic information for protein synthesis.

• Transfer RNA (tRNA): brings amino acids to the ribosome for protein assembly.

• Ribosomal RNA (rRNA): component of ribosomes facilitating protein synthesis.

Ribosomes and Protein Synthesis

• Ribosomes consist of two subunits (30S and 50S for bacteria).

• The ribosome has three sites:

  • P site (peptidyl site): holds the tRNA with the growing polypeptide chain.

  • A site (aminoacyl site): where new tRNA enters bringing new amino acids.

  • E site (exit site): where empty tRNA exits.

• Initiation, elongation, and termination are the three phases of translation.

• Antibiotics such as tetracyclines and aminoglycosides target bacterial ribosomal function to inhibit protein synthesis.

Quorum Sensing in Bacteria

• Bacteria communicate through quorum sensing—detecting cell population density via signaling molecules.

• When a sufficient density of signals is detected, genes for collective behaviors (e.g., bioluminescence or toxin production) may be turned on.

• This is critical for coordinating group activities like biofilm formation and virulence in pathogenesis.

The phosphodiester bonds form the sugar-phosphate backbone of the DNA strands and stabilize the double helix. These bonds link the $5'$ phosphate group of one nucleotide to the $3'$ hydroxyl group of the next nucleotide.

Regarding the differences between RNA and DNA:

• Sugar: DNA contains deoxyribose, while RNA contains ribose.

• Base: DNA uses Thymine (T), but RNA replaces it with Uracil (U). So, in DNA, Adenine (A) pairs with Thymine (T), while in RNA, Adenine (A) pairs with Uracil (U).

• Strandedness: DNA is typically double-stranded, forming a double helix, whereas RNA is typically single-stranded.

The note contains information on both signal processing in bacteria and the process of DNA replication.

Regarding signal information, it is discussed in the section on Quorum Sensing in Bacteria. This process involves bacteria communicating by detecting cell population density via signaling molecules. Once a sufficient density of these signals is detected, genes for collective behaviors—such as bioluminescence, biofilm formation, or toxin production—can be activated.

For the process of DNA replication, the note details that it is a semiconservative process, meaning each new DNA strand contains one parental and one newly synthesized strand. The key steps involved are:

1. Replication starts at the Origin of replication (ORI).

2. Helicase unwinds the DNA, and single-stranded binding proteins prevent the separated strands from rejoining.

3. DNA polymerase synthesizes new DNA, which requires a primer to initiate. Nucleotides are added complementary to the template strand.

4. On the lagging strand, replication proceeds in segments, forming Okazaki fragments.

5. DNA ligase then seals the gaps between these Okazaki fragments.

Bacterial Genetics Overview

• Focus on how bacteria utilize genetic processes compared to eukaryotic perspectives, which can vary by teaching faculty.

Bacterial Genetic Material

• Bacteria have genetic information coded in DNA, similar to eukaryotic cells.

• The genetic structures of viruses differ since they may also contain RNA.

Historical Context

• Key historical figures in the study of DNA include:

  • Rosalind Franklin and her husband, who collaborated with Arthur Kornberg.

  • Kornberg discovered DNA polymerase, an enzyme crucial for DNA replication.

Basics of Genes and Genetic Code

• A gene is defined as a sequence of codons that codes for a protein.

• In our genetic structure:

  • One gene produces one protein, making the model monocistronic.

  • In contrast, bacterial genes can be polycistronic, where one gene may encode multiple proteins due to efficiency in smaller genomes.

• The genetic code is a set of rules by which information encoded in genetic material (DNA or mRNA sequences) is translated into proteins (amino acid sequences) by living cells.

  • It is degenerate, meaning most amino acids are specified by more than one codon.

  • It is universal, meaning the same codons specify the same amino acids in most organisms.

  • Stop codons (UAA, UAG, UGA) do not code for an amino acid but signal the termination of translation.

DNA Structure and Process

• Structure of DNA:

  • Composed of ribose sugar, phosphate, and nitrogenous bases (A, T, C, G).

  • DNA forms a double helix stabilized by hydrogen bonds.

  • Bacterial DNA is circular and double-stranded, unlike eukaryotic linear DNA.

• Phosphodiester bonds form the sugar-phosphate backbone of the DNA strands and stabilize the double helix. These bonds link the $5'$ phosphate group of one nucleotide to the $3'$ hydroxyl group of the next nucleotide.

DNA Replication

• DNA replication is semiconservative: each new DNA strand consists of one parental and one new strand.

• Key steps in bacterial DNA replication include:

  • Origin of replication (ORI) is where replication begins.

  • Helicase unwinds DNA; single-stranded binding proteins prevent strands from rejoining.

  • DNA polymerase requires a primer to start synthesis; added nucleotides are complementary to the template strand.

  • Replication results in Okazaki fragments on the lagging strand.

  • DNA ligase seals gaps between fragments.

Gene Expression

• Gene expression involves two main processes: transcription and translation.

Transcription

• Converts DNA to messenger RNA (mRNA).

• RNA polymerase initiates RNA synthesis at the promoter region and does not need a primer.

• Sigma factors:

  • Are bacterial proteins that enable specific binding of RNA polymerase to gene promoters.

  • Allow RNA polymerase to recognize and bind to specific DNA sequences, initiating transcription.

  • Different sigma factors can direct RNA polymerase to different sets of genes, allowing for alternative gene expression programs (e.g., in response to stress).

• Elongation of RNA transcript:

  • After initiation, the sigma factor usually dissociates from the RNA polymerase.

  • RNA polymerase moves along the DNA template strand, synthesizing an RNA molecule complementary to the DNA sequence in the $5'$ to $3'$ direction.

  • It unwinds the DNA ahead and rewinds it behind, forming a transcription bubble.

• Termination of transcription:

  • Signals RNA polymerase to stop synthesis and release the nascent RNA transcript and the DNA template.

  • Rho-independent termination:

  • Involves the formation of a stem-loop structure (a hairpin) in the mRNA, followed by a tract of uracils (U-rich region).

  • The hairpin causes RNA polymerase to pause, and the weak A-U base pairing in the U-rich region causes the RNA transcript to dissociate from the DNA template.

  • Rho-dependent termination (not detailed in the original prompt, but for completeness, it involves a Rho protein that recognizes specific sequences on the mRNA and dislodges RNA polymerase).

Translation

• Synthesizes proteins from mRNA using ribosomes and transfer RNA (tRNA).

• In bacteria, transcription and translation can occur simultaneously due to the absence of a nucleus, increasing efficiency.

• Role of mRNA with stop codons:

  • mRNA carries the genetic information from DNA to the ribosome.

  • The sequence of codons on mRNA dictates the sequence of amino acids in a protein.

  • Stop codons (UAA, UAG, UGA) in mRNA signal the termination of protein synthesis.

• Anticodon with ribosome details:

  • Each tRNA molecule has an anticodon loop which contains three nucleotides complementary to an mRNA codon.

  • The anticodon on tRNA binds to a complementary codon on mRNA within the ribosome during translation, ensuring the correct amino acid is added to the polypeptide chain.

RNA vs. DNA

• RNA differs from DNA in structure:

  • Sugar: DNA contains deoxyribose, while RNA contains ribose.

  • Base: DNA uses Thymine (T), but RNA replaces it with Uracil (U). So, in DNA, Adenine (A) pairs with Thymine (T), while in RNA, Adenine (A) pairs with Uracil (U).

  • Strandedness: DNA is typically double-stranded, forming a double helix, whereas RNA is typically single-stranded.

Types of RNA

• Messenger RNA (mRNA): carries genetic information for protein synthesis.

• Transfer RNA (tRNA): brings amino acids to the ribosome for protein assembly.

• Ribosomal RNA (rRNA): component of ribosomes facilitating protein synthesis.

Ribosomes and Protein Synthesis (Translation Steps)

• Ribosomes consist of two subunits (30S and 50S for bacteria).

• Role of ribosomes:

  • Ribosomes are the cellular machinery responsible for protein synthesis (translation).

  • They catalyze the formation of peptide bonds between amino acids, guided by the mRNA template.

• The ribosome has three sites:

  • P site (peptidyl site): holds the tRNA with the growing polypeptide chain.

  • A site (aminoacyl site): where new tRNA enters bringing new amino acids.

  • E site (exit site): where empty tRNA exits.

• Three phases of translation:

  1. Initiation of Translation (in Prokaryotes):

     • The 30S ribosomal subunit binds to the mRNA at the ribosomal binding site (Shine-Dalgarno sequence).

     • Initiation factors (IFs):

     • IF1: helps 30S subunit bind mRNA.

     • IF2: recruits initiator tRNA (carrying formylmethionine) to the P site.

     • IF3: prevents premature binding of the 50S subunit to the 30S subunit.

     • The 50S ribosomal subunit then joins, forming the intact 70S ribosome.

  2. Elongation:

     • A new aminoacyl-tRNA (carrying the next amino acid) enters the A site, guided by elongation factors (EFs) like EF-Tu (delivers tRNA to A site) and EF-G (promotes translocation).

     • A peptide bond is formed between the amino acid in the A site and the growing polypeptide chain in the P site, catalyzed by peptidyl transferase activity (a ribozyme in the 50S subunit).

     • The ribosome translocates, moving the mRNA and tRNAs, so the tRNA with the polypeptide shifts to the P site, the empty tRNA shifts to the E site and exits, and the A site becomes vacant for the next incoming tRNA.

  3. Termination (in Prokaryotes):

     • Occurs when a stop codon (UAA, UAG, or UGA) enters the A site.

     • Release factors (RFs):

     • RF1: recognizes UAA and UAG.

     • RF2: recognizes UAA and UGA.

     • RF3: assists RF1/RF2.

     • These release factors bind to the stop codon in the A site, leading to the hydrolysis of the bond between the polypeptide and the tRNA in the P site, releasing the completed polypeptide chain.

     • The ribosomal subunits then dissociate from the mRNA.

Antibiotics Targeting Bacterial Processes

• Antibiotics such as tetracyclines and aminoglycosides target bacterial ribosomal function to inhibit protein synthesis.

• Sulfonamides (e.g., sulfamethoxazole):

  • Relevance: Inhibit bacterial growth by competing with para-aminobenzoic acid (PABA), a precursor for folic acid synthesis.

  • Folic acid is essential for nucleotide synthesis in bacteria, so its inhibition prevents DNA and RNA production.

• Metronidazole:

  • Relevance: A prodrug activated in anaerobic bacteria and protozoa, forming reactive nitro radicals that damage DNA and other macromolecules, leading to cell death.

  • Effective against anaerobic bacterial infections and parasitic infections.

• Fluoroquinolones (e.g., ciprofloxacin):

  • Relevance: Inhibit bacterial DNA gyrase and topoisomerase IV, enzymes crucial for DNA replication, repair, and transcription.

  • Targeting these enzymes leads to DNA breaks and bacterial death, making them broad-spectrum antibiotics.

Quorum Sensing in Bacteria and Signal Transduction

• Bacteria communicate through quorum sensing—detecting cell population density via signaling molecules.

• When a sufficient density of signals is detected, genes for collective behaviors (e.g., bioluminescence or toxin production) may be turned on.

• This is critical for coordinating group activities like biofilm formation and virulence in pathogenesis.

• Mechanisms of Quorum Sensing in Gram-positive Bacteria:

  • Often involves oligopeptides as autoinducers.

  • These peptides are secreted and, upon reaching a threshold concentration, bind to a two-component regulatory system on the cell surface.

  • Relevance: Controls processes like competence, sporulation, and production of virulence factors (toxins, bacteriocins).

• Two-component regulatory system:

  • A common signal transduction pathway in bacteria.

  • Consists of two proteins:

  1. Sensor Kinase: A transmembrane protein that detects an external signal (e.g., autoinducer, nutrient levels).

  2. Response Regulator: A cytoplasmic protein phosphorylated by the sensor kinase, which then typically acts as a transcription factor, altering gene expression.

Bacterial Gene Regulation

• Mechanisms to control gene regulation ensure bacteria adapt to changing environments by switching genes on or off.

• Alternative sigma factors:

  • Bacteria possess multiple sigma factors, each recognizing different promoter sequences.

  • By altering the type of sigma factor produced or activated, bacteria can redirect RNA polymerase to transcribe specific sets of genes (e.g., heat shock response, flagella synthesis).

• DNA binding proteins:

  • Proteins that bind specifically to DNA sequences, often in regulatory regions (operators, enhancers/silencers).

  • Can act as repressors (blocking transcription) or activators (enhancing transcription).

• Transcriptional regulation by repressors:

  • Repressor proteins bind to an operator region, typically downstream of the promoter, physically blocking RNA polymerase from binding to the promoter or advancing along the DNA, thereby preventing transcription.

  • This often occurs in operons (e.g., Lac operon in the presence of glucose).

• The Lac Operon:

  • An inducible operon in E. coli responsible for the metabolism of lactose.

  • Contains genes for lactose uptake and breakdown (LacZ, LacY, LacA).

  • Lactose in the Lac Operon:

  • Serves as an inducer. When lactose is present, it is converted to allolactose, which binds to the Lac repressor protein.

  • This binding causes a conformational change in the repressor, making it unable to bind to the operator.

  • RNA polymerase can then bind to the promoter and transcribe the lac genes, allowing E. coli to utilize lactose.

  • Glucose in the Lac Operon:

  • Catabolite repression: Glucose is the preferred carbon source.

  • When glucose is present, cAMP levels are low. Low cAMP means the CAP (Catabolite Activator Protein)-cAMP complex does not form effectively.

  • The CAP-cAMP complex is required to bind upstream of the promoter and enhance RNA polymerase binding.

  • Therefore, even if lactose is present, the lac operon genes are transcribed at a low rate when glucose is abundant.

  • This ensures glucose is consumed first.

Differences between Eukaryotic and Prokaryotic Gene Expression

Eukaryotic Gene Regulation

• More complex than in prokaryotes due to larger genomes, multicellularity, and differentiation.

• Involves:

  • Chromatin remodeling: Altering DNA accessibility by modifying histones or DNA methylation.

  • Highly regulated transcription factors (activators and repressors).

  • Post-transcriptional control (mRNA splicing, stability).

  • Translational control.

  • Post-translational modifications.

• RNA interference (RNAi):

  • A biological process in which RNA molecules (e.g., small interfering RNA - siRNA, microRNA - miRNA) inhibit gene expression or translation, by neutralizing targeted mRNA molecules.

  • Plays roles in gene regulation, antiviral defense, and chromatin modification.

Genomics and Metagenomics

• Genomics:

  • The study of entire genomes, including the complete set of genes, their nucleotide sequences and organization, and their functions.

  • Involves sequencing, assembly, and annotation of genomes.

• Metagenomics:

  • The study of genetic material recovered directly from environmental samples (e.g., soil, water, human gut) without culturing microbial populations.

  • Allows for the study of the collective genomes of entire microbial communities, providing insights into microbial diversity, ecological roles, and potential new genes/functions that would be missed by traditional culture