Bacterial Genome Replication, Gene Expression, Regulation, and Modern Microbial Genomics

Chapter 13: Bacterial Genome Replication and Expression

DNA, RNA, and Protein Structure

Understanding the structure of nucleic acids and proteins is foundational to microbiology and molecular genetics.

• DNA Structure: Double helix polymer composed of two antiparallel strands of nucleotides. Each nucleotide contains a deoxyribose sugar, phosphate group, and nitrogenous base (A, T, C, G).

• RNA Structure: Single-stranded nucleic acid with ribose sugar, phosphate, and bases (A, U, C, G). RNA can fold into complex three-dimensional structures.

• Protein Structure: Linear chains of amino acids (polypeptides) fold into precise three-dimensional arrangements, determining protein function.

Central Dogma: Describes the flow of genetic information: DNA → RNA (transcription) → Protein (translation).

• Cistron: A DNA or RNA segment coding for a specific polypeptide; functional unit of a gene.

• Polysome: Multiple ribosomes translating a single mRNA simultaneously, increasing protein synthesis efficiency.

Organization of Bacterial Genetic Information

• Bacterial genomes are typically a single, circular, double-stranded DNA chromosome located in the nucleoid region of the cytoplasm.

DNA Replication in Bacteria

Bacterial DNA replication is a rapid, accurate, semi-conservative process starting at a single origin and proceeding bidirectionally.

• Key Enzymes and Functions:

  • DNA Helicase: Unwinds the double helix at the replication fork.

  • Topoisomerase: Relieves supercoiling ahead of the fork.

  • Single-Stranded Binding Proteins: Stabilize separated DNA strands.

  • DNA Primase: Synthesizes short RNA primers for DNA polymerase initiation.

  • DNA Polymerase III: Main enzyme for new DNA synthesis (5'→3').

  • DNA Polymerase I: Removes RNA primers and fills gaps with DNA.

  • DNA Ligase: Seals nicks, joining Okazaki fragments on the lagging strand.

  • Sliding Clamp: Holds DNA polymerase III in place for efficient synthesis.

• Direction of Synthesis: Always 5' to 3'.

• Complementarity Rule: A pairs with T (or U in RNA), C pairs with G via hydrogen bonds.

• Leading vs. Lagging Strand: Leading strand synthesized continuously; lagging strand synthesized discontinuously as Okazaki fragments.

• Proofreading: DNA polymerases correct errors during replication, ensuring fidelity.

• Termination: Occurs when replication forks meet; issues like catenation and dimerization require resolution by specific enzymes.

• End Replication Problem: In eukaryotes, inability to fully replicate chromosome ends leads to shortening; not an issue in circular bacterial chromosomes.

Example: Escherichia coli replicates its 4.6 million base pair genome in about 40 minutes.

Transcription in Bacteria

Transcription is the synthesis of RNA from a DNA template, catalyzed by RNA polymerase.

• RNA Polymerase: Reads DNA 3'→5', synthesizes RNA 5'→3', binds to promoter regions, and does not require a primer.

• Stages of Transcription:

  • Initiation: RNA polymerase binds promoter, unwinds DNA, starts RNA synthesis.

  • Elongation: RNA polymerase moves along template, synthesizing RNA.

  • Termination: RNA polymerase releases DNA and RNA at terminator sequences.

• Sigma Factor: Directs RNA polymerase to specific promoters, enabling initiation.

• Bacterial mRNA Components: 5' untranslated region (with ribosome binding site), coding sequence (start to stop codon), 3' untranslated region.

• Types of RNA: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).

• Gene Organization: Bacteria: compact, circular, often in operons, no introns. Eukaryotes: large, linear, with introns, no operons.

Translation in Bacteria

Translation is the process of synthesizing proteins from mRNA templates, occurring on ribosomes in the cytoplasm.

• Codon: Three-nucleotide mRNA sequence specifying an amino acid or stop signal.

• Reading Frame: Division of nucleotide sequence into consecutive, non-overlapping triplets.

• Sense Codons: Specify amino acids; Stop Codons: UAA, UAG, UGA signal termination.

• Start Codon: AUG; codes for methionine (eukaryotes) or formyl methionine (prokaryotes).

• Codon Bias: Preference for certain codons, optimizing translation efficiency.

• tRNA: Adaptor molecule matching amino acids to mRNA codons via anticodons.

• rRNA: Structural and catalytic component of ribosomes.

• Wobble Base Pairing: Flexible pairing at the third codon position allows one tRNA to recognize multiple codons.

• Aminoacyl-tRNA Synthetase: Enzyme that attaches specific amino acids to their corresponding tRNAs, forming "charged" tRNAs.

• Ribosome Structure: 70S ribosome (30S + 50S subunits) in bacteria; site of protein synthesis and antibiotic action.

• Translation Steps:

  • Initiation: Small subunit binds mRNA at Shine-Dalgarno sequence, initiator tRNA binds start codon, large subunit joins.

  • Elongation: Codon recognition, peptide bond formation, translocation.

  • Termination: Stop codon enters A site, release factors promote polypeptide release and ribosome disassembly.

• Bacteria vs. Eukaryotes: Bacteria couple transcription and translation; eukaryotes separate these processes and process mRNA (splicing, capping, tailing).

• Introns: Present in eukaryotic genes, absent in bacteria.

• 16S rRNA Gene: Used for bacterial identification due to its conserved and variable regions.

Feature	Bacteria	Eukaryotes
Genome Structure	Circular, compact, operons	Linear, large, introns
Transcription & Translation	Coupled in cytoplasm	Separated (nucleus/cytoplasm)
Introns	Absent	Present
Ribosome Size	70S (30S+50S)	80S (40S+60S)

Chapter 14: Gene Regulation

Constitutive vs. Induced Genes

Gene regulation allows bacteria to adapt to environmental changes efficiently.

• Constitutive Genes: Always expressed; encode essential "housekeeping" functions.

• Induced Genes: Expressed only in response to specific stimuli or substrates.

Lac Operon in E. coli

The lac operon is a classic example of inducible gene regulation in bacteria.

• Composed of three structural genes (lacZ, lacY, lacA) for lactose metabolism.

• Regulated by the Lac repressor (inhibits transcription) and CAP protein (activates transcription in absence of glucose).

• Expressed only when lactose is present and glucose is absent.

Positive and Negative Regulation

• Positive Regulation: Activators enhance RNA polymerase binding, increasing gene expression.

• Negative Regulation: Repressors bind operator sites, blocking transcription.

Regulation by Glucose and Lactose

• High lactose, low glucose: lac operon ON (repressor removed, CAP-cAMP activates transcription).

• High glucose: lac operon OFF (catabolite repression, low cAMP, CAP inactive).

Lactose	Glucose	Lac Operon Expression
Absent	Any	OFF
Present	Present	Low
Present	Absent	High

Chapter 17: Recombinant DNA Technology

Genetic Engineering and Recombinant DNA

Genetic engineering manipulates microbial DNA to create organisms with new or improved traits.

• Recombinant DNA Technology: Combines DNA from different species, inserted into host organisms for amplification.

• Cloning: Isolating and amplifying specific DNA segments using vectors (e.g., plasmids) in bacteria.

• Biotechnology: Application of microbes to develop products and processes (e.g., pharmaceuticals, food, environmental management).

• Industrial Microbiology: Large-scale use of microbes to manufacture valuable products.

Restriction Enzymes and Cloning Process

• Restriction Enzymes: Bacterial proteins that cut DNA at specific palindromic sequences.

• Cloning Process: Involves cutting DNA, ligating into vectors, and transforming into host cells for replication.

Polymerase Chain Reaction (PCR)

• Amplifies specific DNA or RNA segments exponentially.

• Reverse Transcription PCR (RT-PCR): Converts RNA to cDNA, then amplifies by PCR.

Genetically Engineered Products and Microbial Production

• Microbes engineered to produce pharmaceuticals, enhance food production, or manage environmental issues (e.g., CRISPR applications).

• Microbial production uses bacteria, fungi, and yeasts as "living factories" for high-value compounds.

Chapter 18: Metagenomics and Omics Technologies

Genomics and DNA Sequencing

• Genomics: Study of the entire DNA content of an organism, including gene structure, function, and evolution.

• DNA Sequencing: Determines the order of nucleotides in DNA.

• Sanger Sequencing: High accuracy, suitable for small fragments.

• Next-Generation Sequencing (NGS): High-throughput, parallel sequencing for whole genomes and metagenomics.

Feature	Sanger Sequencing	Next-Gen Sequencing
Scale	Small	Massive, parallel
Accuracy	High	High
Cost per Sample	High	Low
Applications	Fragments, plasmids	Whole genomes, metagenomics

Whole Genome and RNA Sequencing

• Whole Genome Sequencing: Determines the complete DNA sequence of a microorganism.

• RNA Sequencing: Analyzes the entire set of RNA transcripts (transcriptome).

Metagenomics, Meta-transcriptomics, Proteomics, and Metabolomics

• Shotgun Metagenomics: Sequences all DNA/RNA from complex samples, revealing microbial diversity without culturing.

• Meta-transcriptomics: Studies total RNA (especially mRNA) from microbial communities.

• Proteomics: Large-scale study of all proteins expressed by microbes under specific conditions.

• Metabolomics: Comprehensive analysis of all small-molecule metabolites in a cell or community.

Bioinformatics and Microarrays

• Bioinformatics: Uses computational tools to analyze and interpret large-scale biological data (DNA, RNA, protein sequences).

• Microarrays: Miniaturized platforms with ordered DNA, RNA, or protein probes for high-throughput analysis.

Chapters 10 & 11: Microbial Metabolism and Catabolism

Microbial Metabolism

Microbial metabolism encompasses all chemical reactions (catabolism and anabolism) in a microorganism, essential for growth, reproduction, and adaptation.

• Catabolism: Breakdown of molecules to release energy (ATP, NADH).

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy.

Glucose Catabolism and Glycolysis

• Glucose Catabolism: Converts glucose to pyruvate or lactic acid, generating ATP and NADH.

• Glycolysis: Anaerobic process converting one glucose to two pyruvate, net gain of 2 ATP and 2 NADH.

Equation for Glycolysis:

$\mathrm{Glucose} + 2\ \mathrm{NAD}^+ + 2\ \mathrm{ADP} + 2\ \mathrm{P_i} \rightarrow 2\ \mathrm{Pyruvate} + 2\ \mathrm{NADH} + 2\ \mathrm{ATP} + 2\ \mathrm{H_2O}$

TCA Cycle (Krebs Cycle)

• Central metabolic pathway in aerobic microbes; oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and GTP/ATP.

Equation for TCA Cycle (per acetyl-CoA):

$\mathrm{Acetyl{-}CoA} + 3\ \mathrm{NAD}^+ + \mathrm{FAD} + \mathrm{GDP} + \mathrm{P_i} + 2\ \mathrm{H_2O} \rightarrow 2\ \mathrm{CO_2} + 3\ \mathrm{NADH} + \mathrm{FADH_2} + \mathrm{GTP} + 2\ \mathrm{H^+} + \mathrm{CoA{-}SH}$

Electron Transport Chain (ETC) and Phosphorylation

• ETC: Series of membrane-bound proteins transferring electrons from NADH/FADH2 to O2 (aerobic), generating a proton gradient.

• Oxidative Phosphorylation: ATP synthesis driven by proton gradient via ATP synthase.

• Substrate-Level Phosphorylation: Direct enzymatic transfer of phosphate to ADP during glycolysis or TCA cycle.

Equation for Oxidative Phosphorylation:

$\mathrm{ADP} + \mathrm{P_i} + \text{proton motive force} \rightarrow \mathrm{ATP}$

• Comparison: Substrate-level phosphorylation yields less ATP than oxidative phosphorylation, which is the main energy source in aerobic respiration.

Example: Escherichia coli can switch between aerobic and anaerobic metabolism depending on oxygen availability.

Additional info: Where details were brief, standard academic context was added for clarity and completeness (e.g., equations, definitions, and examples).