Chapter 7: Microbial Regulatory Systems | Brock Biology of Microorganisms (Podcast Summary)

Chapter 7 explores the sophisticated systems microbes use to regulate gene expression and protein activity in response to environmental conditions. Bacteria and archaea use transcriptional control, signal transduction, quorum sensing, RNA-based regulation, and enzyme feedback to maintain metabolic balance and adaptability.

✅ DNA-Binding Proteins and Transcriptional Regulation
🔸 Most gene regulation occurs at the transcription level
🔸 DNA-binding proteins (e.g., repressors, activators) control transcription
🔸 Common motifs include helix-turn-helix structures that bind inverted repeats
🔸 Repressors block RNA polymerase at operator regions (negative control)
🔸 Activators help RNA polymerase bind at promoters (positive control)
🔸 Effectors (inducers or corepressors) bind to regulatory proteins and modify their activity
🔸 Operons: multiple genes under control of a single regulatory region (common in prokaryotes)

✅ Archaeal Transcriptional Regulation
🔸 Archaea use activators and repressors but lack classical bacterial operons
🔸 Regulation may involve blocking TBP and TFB binding at the promoter
🔸 Some regulators (e.g., TrmBL1) function as activators or repressors depending on context

✅ Two-Component Signal Transduction Systems
🔸 Composed of:
 🔸 Sensor kinase in membrane
 🔸 Response regulator in cytoplasm
🔸 Sensor detects stimulus and autophosphorylates
🔸 Phosphate is transferred to the response regulator → alters gene expression
🔸 Examples:
 🔸 EnvZ-OmpR: osmolarity in E. coli
 🔸 Ntr (NRII-NRI): nitrogen regulation

✅ Chemotaxis
🔸 Movement in response to chemical gradients
🔸 Uses methyl-accepting chemotaxis proteins (MCPs)
🔸 MCPs interact with CheA (kinase) and CheY (motor response)
🔸 Attractant → inhibits CheA → counterclockwise flagellar rotation → smooth swimming
🔸 Repellent → activates CheA → CheY-P → clockwise rotation → tumbling
🔸 Adaptation: CheR methylates MCPs; CheB demethylates after CheA-P activation

✅ Quorum Sensing and Cell-to-Cell Communication
🔸 Monitors population density via autoinducers
🔸 Autoinducers bind regulatory proteins at threshold concentration
🔸 Triggers group behaviors: biofilm formation, virulence, bioluminescence
🔸 Examples:
 🔸 Aliivibrio fischeri: LuxR–AHL for light emission
 🔸 E. coli: AI-3 and host hormones
 🔸 Staphylococcus aureus: AIP and Agr system

✅ Global Control Systems
🔸 Regulate multiple operons simultaneously
🔸 Respond to environmental signals

🔸 Lac Operon:
 🔸 Induced by lactose (via allolactose); repressed by LacI
 🔸 Catabolite repression blocks lac expression in presence of glucose
 🔸 Requires CRP–cAMP for full activation
 🔸 Produces diauxic growth pattern

🔸 Stringent Response:
 🔸 Triggered by amino acid starvation
 🔸 Mediated by ppGpp / pppGpp (alarmones)
 🔸 Downregulates rRNA/tRNA synthesis; upregulates amino acid biosynthesis
 🔸 Linked to persister cell formation and stress survival

🔸 Pho Regulon (Phosphate Starvation):
 🔸 Controlled by PhoR–PhoB two-component system
 🔸 Activates genes for phosphate acquisition and storage
 🔸 Can repress nitrogen-related genes and regulate virulence/biofilms

🔸 Heat Shock Response:
 🔸 Induces heat shock proteins (Hsps) for refolding/destroying damaged proteins
 🔸 Regulated by RpoH sigma factor
 🔸 DnaK binds RpoH under normal conditions but stabilizes it under heat stress

✅ RNA-Based Gene Regulation
🔸 sRNAs (small RNAs):
 🔸 Base pair with mRNA to affect translation or stability
 🔸 Require chaperone Hfq
 🔸 Examples:
  🔸 RyhB (iron starvation)
  🔸 SgrS (glucose-phosphate stress)

🔸 Riboswitches:
 🔸 RNA domains in 5′ UTR that bind small metabolites
 🔸 Change structure to block RBS or cause early transcription termination
 🔸 Example: thiamine or lysine biosynthesis control

🔸 Attenuation:
 🔸 Transcription termination based on leader peptide translation
 🔸 Seen in E. coli trp operon
 🔸 Not used in eukaryotes (due to separation of transcription and translation)

✅ Post-Translational and Enzyme Regulation
🔸 Feedback Inhibition:
 🔸 End product inhibits an early enzyme via allosteric binding
 🔸 Some use isoenzymes to fine-tune regulation

🔸 Post-Translational Modifications:
 🔸 Phosphorylation, methylation, adenylylation, uridylylation
 🔸 Example: GlnD regulates glutamine synthetase via uridylylation
🔸 Anti-sigma factors inactivate specific sigma factors
 🔸 RseA inhibits RpoE
 🔸 SpoIIAB inhibits σF during sporulation

📚 Glossary Highlights
🔸 Activator / Repressor Proteins – Bind DNA to turn transcription on or off
🔸 Effector Molecules – Inducers or corepressors that modify regulator activity
🔸 Quorum Sensing – Population-density dependent gene regulation
🔸 Two-Component System – Sensor kinase + response regulator
🔸 RpoS / RpoH – Sigma factors for stress responses
🔸 sRNA / Riboswitch – Noncoding RNAs regulating gene expression
🔸 Attenuation – Premature transcription termination control
🔸 Feedback Inhibition – End product inhibits enzyme activity
🔸 Isoenzyme – Alternate enzyme versions regulated separately

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