Microbial Genomics, Evolution, and Systematics: Study Notes

I. Investigating Genomes

Introduction to Genomics

Genomics is the study of the entire genetic material of an organism, including genes, regulatory sequences, and noncoding DNA. This field encompasses mapping, sequencing, analyzing, and comparing genomes, providing insights into microbial diversity and function.

• Genome: The complete set of genetic information in an organism, including all genes and noncoding regions.

• Genomics: The discipline focused on the comprehensive analysis of genomes.

• Historical Milestones:

  • RNA virus MS2: First genome sequenced in 1976 (3,569 bp).

  • Haemophilus influenzae: First cellular genome sequenced in 1995 (1,830,137 bp).

Applications of Genomics:

• Metagenomics: Assessing the gene content of entire microbial communities.

• Transcriptomics, Proteomics, Metabolomics: Mapping and modeling gene expression, protein function, and metabolic pathways.

• Diagnostics: Detecting pathogens and monitoring disease outbreaks.

• Insights into metabolism and virulence: Identifying nutrient requirements and pathogenicity factors.

PCR (Polymerase Chain Reaction)

PCR is a technique used to amplify specific DNA sequences, enabling detailed genetic analysis.

• Key Steps:

  1. Denaturation: DNA is heated to separate strands.

  2. Annealing: Primers bind to target sequences.

  3. Extension: DNA polymerase synthesizes new DNA.

• Each cycle doubles the number of target DNA copies, resulting in exponential amplification.

• Equation: $N = N_0 \times 2^n$ (where $N$ is the number of DNA copies after $n$ cycles)

Restriction Enzymes and Nucleic Acid Separation

Restriction enzymes cut DNA at specific sequences, and gel electrophoresis separates DNA fragments by size.

• Gel Electrophoresis: Uses an electric field to move negatively charged DNA toward the positive electrode through an agarose gel.

• DNA fragments are visualized using ethidium bromide under UV light.

Sequencing Genomes

Determining the precise order of nucleotides in DNA or RNA is essential for genomics.

• Sanger Dideoxy Method:

  • Uses dideoxynucleotides (ddNTPs) to terminate DNA synthesis at specific bases.

  • Results in DNA fragments of varying lengths, which are separated by gel electrophoresis.

  • Each base is labeled (radioactively or with fluorescent dye) for detection.

• Automated Sequencing: Uses fluorescent dyes for high-throughput sequencing.

Modern Sequencing Techniques

• Shotgun Sequencing: Genome is fragmented, cloned, and sequenced; computer algorithms assemble the sequence.

• Second-Generation Sequencing: Massively parallel, high-throughput methods; much faster than Sanger sequencing.

• Third-Generation Sequencing: Sequences single DNA molecules in real time (e.g., HeliScope, SMRT sequencing).

• Fourth-Generation Sequencing: Uses semiconductor or nanopore technology to detect DNA sequence without optical detection.

II. Microbial Genomes

Genome Size and Content

Microbial genomes vary in size and gene content, reflecting their ecological roles and evolutionary history.

• Prokaryotic genes average ~1,000 bp; about 1,000 genes per megabase.

• Genome size correlates with the number of open reading frames (ORFs).

• Smallest genomes are found in obligate parasites and endosymbionts; largest in free-living bacteria.

• Minimum gene set for a viable cell is estimated at 250–300 genes.

III. Functional Genomics

Microarrays and the Transcriptome

Functional genomics explores gene expression and function using high-throughput techniques.

• Transcriptome: The complete set of RNA transcripts produced under specific conditions.

• Microarrays: DNA chips with immobilized gene sequences; hybridized with labeled mRNA to assess gene expression patterns.

• Applications: Global gene expression profiling, identification of gene function, comparison of related organisms, and pathogen detection.

RNA-seq

• All RNA molecules from a cell are sequenced, providing quantitative data on gene expression.

• Measures both coding (mRNA) and noncoding RNA.

• Allows determination of which genes are transcribed and the abundance of each transcript.

Proteomics and the Interactome

• Proteomics: Study of the entire protein complement of a cell, including structure, function, and regulation.

• 2-D PAGE: Separates proteins by isoelectric point (horizontal) and size (vertical).

Metabolomics and Systems Biology

• Metabolome: The complete set of small-molecule metabolites in a cell.

• Mass Spectrometry: Key technique for metabolite analysis (e.g., MALDI-TOF).

• Systems Biology: Integrates genomics, transcriptomics, proteomics, and metabolomics to model biological systems.

Single-Cell Genomics

• Analyzes the genome, transcriptome, and proteome of individual cells from environmental samples.

• Enables study of uncultured microorganisms and rare cell types.

Metagenomics

• Metagenome: The collective genome of all organisms in an environmental sample.

• Used to study microbial diversity in complex environments (e.g., soil, ocean, acid mine runoff).

Core Genome versus Pan Genome

• Chromosomal Islands: Regions of foreign origin in bacterial chromosomes, often containing genes for virulence, symbiosis, or biodegradation.

• Contribute specialized functions not essential for basic growth.

V. Early Earth and the Origin and Diversification of Life

Formation and Early History of Earth

Life on Earth began under anoxic and high-temperature conditions, with the first biochemical compounds formed abiotically.

• Subsurface Origin Hypothesis: Life originated at hydrothermal vents on the ocean floor, where conditions were stable and energy sources abundant.

Self-Replication and the RNA World

• Prebiotic chemistry led to self-replicating systems, possibly based on RNA (RNA world theory).

• RNA can bind small molecules and has catalytic activity, potentially catalyzing its own synthesis.

Early Metabolism

• Primitive cells were anaerobic chemolithotrophs, obtaining carbon from CO2 and energy from H2 (produced by reactions involving H2S or UV light).

• Organic material accumulation led to the evolution of chemoorganotrophic metabolisms.

Photosynthesis and the Oxidation of Earth

• Development of oxygenic photosynthesis by cyanobacteria (~2.7 billion years ago) led to atmospheric oxygen accumulation.

• Oxygen enabled new metabolic pathways and the formation of the ozone layer, protecting life from UV radiation.

Endosymbiotic Origin of Eukaryotes

• Endosymbiosis Hypothesis: Mitochondria and chloroplasts originated from symbiotic associations between prokaryotes and ancestral eukaryotic cells.

• Two main models: acquisition of organelles after nucleus formation, or simultaneous association of an archaeal host and a bacterial symbiont.

VI. Living Fossils: DNA Records the History of Life

Molecular Phylogeny and the Tree of Life

• Carl Woese: Used SSU rRNA genes to establish the three domains of life: Bacteria, Archaea, and Eukarya.

• SSU rRNA genes are highly conserved, functionally constant, and present in all organisms, making them ideal for phylogenetic studies.

Phylogenetic Analysis

• Comparative rRNA sequencing involves amplification, sequencing, and analysis of SSU rRNA genes.

• Sequence alignment and phylogenetic tree construction reveal evolutionary relationships.

• Branch length in phylogenetic trees represents the number of genetic changes.

VII. Microbial Evolution

Genetic Variation and Evolutionary Processes

• Mutations: Changes in DNA sequence due to replication errors, radiation, or other factors; can be adaptive or neutral.

• Other mechanisms: gene duplication, horizontal gene transfer, gene loss.

• Genetic Drift: Random changes in gene frequencies over time, not driven by selection.

VIII. Microbial Systematics

Species Concepts and Taxonomy

• No universally accepted species concept for prokaryotes; phylogenetic species concept is commonly used.

• Prokaryotic species are defined by high DNA-DNA hybridization (≥70%) and ≥97% SSU rRNA gene sequence identity.

• New species are proposed if SSU rRNA gene sequence differs by >3% from known strains.

Taxonomic Methods

• Polyphasic Taxonomy: Combines phylogenetic, genotypic, and phenotypic analyses.

• Multilocus Sequence Typing (MLST): Sequences multiple housekeeping genes for high-resolution strain differentiation.

• Ribotyping: DNA fingerprinting based on restriction enzyme digestion of rRNA genes; used in clinical and environmental microbiology.

• Phenotypic Analysis: Examines cell morphology, metabolism, and chemical composition (e.g., fatty acid methyl ester analysis).

Classification and Nomenclature

• Prokaryotes are named using the binomial system (genus and species), following Linnaean taxonomy.

IX. Functional and Morphological Diversity of Bacteria

Functional Diversity

• Functional traits are distributed among bacteria and archaea due to gene loss, convergent evolution, and horizontal gene transfer.

Examples of Functional Groups

• Cyanobacteria: Oxygenic phototrophs with thylakoid membranes; many fix nitrogen and produce toxins.

• Purple and Green Sulfur Bacteria: Anoxygenic phototrophs using H2S as an electron donor; store sulfur as globules.

• Predatory Bacteria: e.g., Bdellovibrio (preys on other bacteria), myxobacteria (form fruiting bodies).

Morphological Diversity

• Spirochetes: Motile, spiral-shaped bacteria with endoflagella.

• Spirilla: Spiral-shaped, motile proteobacteria.

• Budding and Stalked Bacteria: e.g., Caulobacter (forms stalks for attachment).

• Bioluminescent Bacteria: e.g., Vibrio species.

X. Phylogenetic Overview of Bacteria

• Major phyla: Proteobacteria, Actinobacteria, Firmicutes, Bacteroidetes.

• Proteobacteria: Metabolically and morphologically diverse; includes many common bacteria.

• Firmicutes: Includes lactic acid bacteria (Lactobacillus, Streptococcus), spore-formers (Bacillus, Clostridium), and others.

• Actinobacteria: Filamentous, soil-dwelling bacteria (e.g., Streptomyces), many produce antibiotics.

XI. Diversity of Archaea

• Includes thermophiles, hyperthermophiles, and extreme halophiles.

• Halophilic archaea maintain water balance by accumulating K+ ions and have highly acidic proteins.

• Some haloarchaea use bacteriorhodopsin for light-driven ATP synthesis.

XII. Evolution and Life at High Temperatures

• Thermophiles and hyperthermophiles thrive at high temperatures; upper limit for life is around 150°C (ATP degrades above this temperature).