Chapter 7 & 9

Microbial Regulatory Systems

Introduction

This section explores the mechanisms by which microorganisms regulate gene expression and metabolic processes. Understanding these regulatory systems is essential for appreciating how microbes adapt to changing environments and control cellular functions.

Cell Size and Surface Area-to-Volume Ratio

Surface Area-to-Volume (S/V) Ratio

The surface area-to-volume ratio (S/V ratio) is a key factor influencing the efficiency of nutrient and waste exchange in microbial cells.

• Definition: The S/V ratio is the amount of cell surface area relative to its volume.

• Smaller cells have a higher S/V ratio, allowing for more efficient exchange of materials with the environment.

• Example: Pelagibacter ubique (smaller cell) has a higher S/V ratio than E. coli, making it more efficient at nutrient uptake and waste removal.

Key Point: Cells with a higher S/V ratio (i.e., smaller cells) are generally more efficient at exchanging nutrients and wastes with their environment.

Redox Biology in Microorganisms

Reduction Potential and Electron Flow

Microbial metabolism often involves redox reactions, where electrons are transferred between molecules. The tendency of a molecule to gain or lose electrons is described by its reduction potential ($E_0'$).

• Reduction Potential ($E_0'$): A measure (in volts) of a substance's tendency to accept or donate electrons.

• Electron Donor: Substance that loses electrons (is oxidized).

• Electron Acceptor: Substance that gains electrons (is reduced).

• The difference in reduction potential ($\Delta E_0'$) between donor and acceptor determines the amount of free energy released ($\Delta G_0'$).

Key Equation:

$\Delta G_0' = -nF\Delta E_0'$

• $n$ = number of electrons transferred

• $F$ = Faraday's constant (96,485 C/mol e-)

• $\Delta E_0'$ = difference in reduction potential between acceptor and donor

Key Point: The greater the $\Delta E_0'$, the more free energy is released during the redox reaction.

The Electron Tower

The electron tower is a visual representation of common redox couples arranged by their reduction potentials.

• Redox couples at the top (e.g., O2/H2O) have high (positive) $E_0'$ and are strong electron acceptors.

• Redox couples at the bottom (e.g., Fe2+/Fe3+, H2/H+) have low (negative) $E_0'$ and are strong electron donors.

• Electrons flow from donors (bottom) to acceptors (top), releasing energy.

Table: Selected Redox Couples and Their Reduction Potentials

Redox Couple	Reduction Potential ($E_0'$) [V]	Number of Electrons
O2/H2O	+0.82	4
Fe3+/Fe2+	+0.77	1
Mn4+/Mn2+	+0.47	2
Fumarate2-/Succinate2-	+0.03	2
Pyruvate/Lactate	-0.19	2
SO42-/H2S	-0.22	8
2H+/H2	-0.41	2
CO2/Glucose	-0.43	24
Fe3+/Fe2+ [10-6 M]	-0.62	2

Additional info: Table reconstructed from the electron tower image and text.

Gene Regulation in Microorganisms

Regulation by Noncoding RNAs (ncRNAs)

Microbes use various RNA molecules to regulate gene expression post-transcriptionally.

• Noncoding RNA (ncRNA): RNA molecules that do not code for proteins but regulate gene expression.

• Typically less than 400 nucleotides long.

• Can base-pair with mRNA to influence translation or stability.

Mechanisms of ncRNA Regulation:

• Translation Inhibition: ncRNA binds to the ribosome-binding site (RBS) of mRNA, blocking ribosome access and preventing translation.

• Translation Stimulation: ncRNA binding can sometimes expose the RBS, enhancing translation.

• mRNA Degradation: ncRNA can recruit ribonucleases, leading to mRNA degradation.

• mRNA Protection: ncRNA binding can also protect mRNA from degradation.

Example: Small RNAs (sRNAs) in bacteria regulate stress responses and metabolism by binding to target mRNAs.

Riboswitches

Riboswitches are regulatory segments of mRNA that bind small molecules, causing a conformational change that affects gene expression.

• Found in bacteria, archaea, and some plants.

• Binding of a metabolite to the riboswitch can cause premature transcription termination or inhibit translation.

• Example: The attenuation mechanism in the tryptophan operon of E. coli uses a riboswitch-like structure to regulate gene expression based on tryptophan levels.

Attenuation

Attenuation is a regulatory mechanism in prokaryotes where transcription is terminated prematurely in response to specific metabolite concentrations.

• Occurs in the leader region of certain operons (e.g., tryptophan operon).

• High levels of the metabolite (e.g., tryptophan) cause formation of a terminator hairpin in mRNA, halting transcription.

• Low levels allow transcription to continue, enabling gene expression.

Key Point: Attenuation links translation and transcription, which are coupled in prokaryotes but not in eukaryotes.

Genetics of Bacteria and Archaea

Key Vocabulary

• Strain: A genetic variant or subtype of a microorganism.

• Genotype: The nucleotide sequence of an organism's genome.

• Phenotype: The observable characteristics or traits of an organism.

Naming Conventions:

• Gene names: italicized, three lowercase letters + capital letter (e.g., hisC).

• Alleles: gene name followed by a number (e.g., hisC1).

• Phenotypes: capitalized, with + or - to indicate presence or absence (e.g., His+, His-).

Screening and Selection

Microbiologists use selective media and genetic screens to identify mutants or strains with desired traits.

• Screening: Identifying organisms with specific phenotypes by changing environmental conditions or using selective media.

• Example: Screening for antibiotic resistance by growing bacteria on media containing antibiotics.

DNA Repair and the SOS System

Bacteria have evolved mechanisms to repair DNA damage. The SOS system is a global response to extensive DNA damage.

• Triggered by stalled DNA replication or DNA lesions.

• Induces expression of DNA repair enzymes, some of which can synthesize DNA without a template (error-prone repair).

• Can lead to mutations but increases the chance of cell survival under stress.

Key Point: The SOS system is unique to bacteria and is a last-resort mechanism for DNA repair.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard microbiology textbooks.