Microbial Regulatory Systems — Gene Expression, Quorum Sensing & Signal Transduction Explained | Chapter 7 of Brock Biology of Microorganisms

Welcome to another in-depth chapter breakdown from Last Minute Lecture. In this post, we summarize Chapter 7 of Brock Biology of Microorganisms, which explores the elegant regulatory mechanisms microbes use to sense their environment and adjust gene expression. From two-component systems to small RNAs and enzyme feedback inhibition, this chapter illustrates the precision of microbial gene regulation.

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This comprehensive guide is ideal for students in microbiology, molecular biology, and AP Biology, offering insight into how microorganisms maintain adaptability and survival.

Transcriptional Regulation and DNA-Binding Proteins

Most microbial gene regulation occurs at the transcriptional level. Key components include:

• DNA-binding proteins: Repressors and activators that influence RNA polymerase activity
• Negative control: Repressors block transcription at operator regions
• Positive control: Activators enhance RNA polymerase binding
• Effectors: Small molecules (inducers or corepressors) that modulate regulator activity
• Operons: Common in prokaryotes, allow coordinated regulation of multiple genes

Regulation in Archaea

Archaea also use repressors and activators, though their promoters resemble those of eukaryotes. Proteins like TrmBL1 can function as both activators and repressors depending on environmental conditions. Transcriptional regulation may involve interference with TBP and TFB binding.

Two-Component Signal Transduction Systems

These systems detect external signals and alter gene expression through phosphorylation:

• Sensor kinase: Detects environmental signal and autophosphorylates
• Response regulator: Receives phosphate group and modulates transcription

Examples include:

• EnvZ-OmpR: Osmotic regulation in E. coli
• NRII-NRI: Nitrogen regulation system

Chemotaxis and Adaptive Behavior

Chemotaxis allows microbes to move toward or away from chemical signals using:

• MCPs (Methyl-accepting Chemotaxis Proteins): Detect environmental stimuli
• CheA: Kinase that phosphorylates CheY
• CheY-P: Alters flagellar motor rotation (CW = tumbling, CCW = smooth swimming)

Adaptation mechanisms involve CheR and CheB modifying MCP methylation status.

Quorum Sensing and Population Coordination

Quorum sensing enables bacteria to assess their population density via autoinducers and coordinate group behaviors:

• Autoinducers: Accumulate and bind to regulatory proteins when threshold is reached
• Trigger biofilm formation, virulence, bioluminescence, and more

Examples:

• Aliivibrio fischeri: LuxR–AHL system for light emission
• Staphylococcus aureus: AIP and Agr system for pathogenicity
• E. coli: Senses AI-3 and host hormones

Global Regulatory Systems

Lac Operon

• Repressed by LacI unless lactose (allolactose) is present
• Requires CRP–cAMP for full expression
• Demonstrates catabolite repression and diauxic growth

Stringent Response

• Triggered by amino acid starvation
• ppGpp and pppGpp alarmones suppress rRNA and tRNA synthesis
• Promotes amino acid biosynthesis and stress resistance

Pho Regulon

• Activates phosphate acquisition genes under low phosphate
• Mediated by PhoR–PhoB system
• Also influences virulence and nitrogen regulation

Heat Shock Response

• Induces heat shock proteins (Hsps) to repair damaged proteins
• Controlled by sigma factor RpoH, regulated by DnaK chaperone binding

RNA-Based Regulation

Small RNAs (sRNAs)

• Base pair with mRNAs to affect translation/stability
• Require Hfq for proper function
• Examples: RyhB (iron stress), SgrS (glucose-phosphate stress)

Riboswitches

• Located in 5′ UTRs; bind metabolites to change RNA structure
• Control translation initiation or transcription termination
• Examples: Thiamine, lysine biosynthesis regulation

Attenuation

• Regulates transcription based on translation speed of a leader peptide
• Best-known example: trp operon in E. coli
• Unique to prokaryotes where transcription and translation are coupled

Post-Translational Control and Enzyme Regulation

Feedback Inhibition

• End product of a pathway binds allosterically to inhibit an early enzyme
• Isoenzymes allow nuanced regulation

Post-Translational Modifications

• Include phosphorylation, methylation, adenylylation, uridylylation
• Example: GlnD uridylylates enzymes involved in nitrogen regulation

Anti-Sigma Factors

• Bind sigma factors to block transcription initiation
• Examples: RseA (blocks RpoE), SpoIIAB (blocks σF during sporulation)

Glossary Highlights

• Activator / Repressor: DNA-binding proteins regulating transcription
• Two-Component System: Sensor kinase + response regulator
• Quorum Sensing: Gene regulation based on cell density
• RpoH / RpoS: Sigma factors for heat shock and general stress
• sRNA / Riboswitch: RNA-based mechanisms for gene control
• Attenuation: Translation-dependent transcription termination
• Feedback Inhibition: Metabolic product inhibits enzyme activity
• Isoenzymes: Multiple forms of an enzyme with distinct regulation

Conclusion

Chapter 7 of Brock Biology of Microorganisms reveals how bacteria and archaea expertly regulate their genes, proteins, and behavior through a web of finely tuned mechanisms. Whether through operon repression, quorum sensing, or post-translational modification, these systems help microbes adapt, compete, and survive in changing environments.

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