7 public questions tagged with this topic.
Enhancers do not decrease gene transcription; they enhance it by increasing the efficiency of RNA polymerase binding.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
Alternative sigma factors redirect RNA polymerase holoenzyme to distinct promoter classes under stress conditions. Sigma32 encoded by rpoH in Escherichia coli accumulates during temperature upshift because heat-induced misfolded proteins titrate chaperones DnaK, DnaJ, GroEL, releasing sigma32 from sequestration and from FtsH-mediated proteolysis. Free sigma32 binds core polymerase, activating heat-shock promoters defined by -35 CTTGAA and -10 CCCCATNT consensus driving transcription of chaperones GroEL, DnaK, proteases ClpB, Lon, and repair proteins. This rapid reprogramming restores proteostasis, increases refolding capacity, and provides transient thermotolerance until homeostasis returns and sigma32 degraded via feedback regulation.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8: Heat Shock Sigma Factor σ32 Regulation
6S RNA is highly abundant ~184 nucleotide noncoding RNA accumulating during stationary phase and nutrient limitation in Escherichia coli that structurally mimics open promoter DNA bubble. Its conserved elongated duplex with central bulge and terminal loop fits within active site channel of sigma70 holoenzyme, specifically contacting beta, beta' subunits and sigma70 region 4.2. Sequestration globally downregulates transcription from strong sigma70 promoters, freeing core polymerase to associate with alternative sigma factors like sigmaS and sigma32. Upon nutrient upshift, 6S RNA serves as template for short product RNAs pRNA, causing conformational change releasing polymerase and restoring exponential growth transcriptional program rapidly.
Ref: NCBI Bookshelf, Molecular Biology: Bacterial Small RNAs – 6S RNA Control of Sigma70 RNA Polymerase
Basal transcription refers to low constitutive synthesis occurring without induction due to imperfect repressor-operator occupancy governed by binding kinetics. LacI occasionally dissociates permitting brief promoter vacancies where RNA polymerase initiates transcription producing few molecules per generation. Leakiness yields two to five copies of LacY permease and beta-galactosidase per cell even under repression. Such sentinel molecules are crucial because they allow initial lactose import and intracellular allolactose generation enabling rapid autocatalytic activation upon lactose appearance, preventing regulatory system from becoming irreversibly locked off despite future inducer availability.
Ref: Alberts Molecular Biology of the Cell 6th ed. Chapter 7 – leaky basal transcription provides permease for initial inducer import.
TFIIH includes CAK subcomplex cyclin activating kinase comprising CDK7 kinase, cyclin H, MAT1 assembly factor stabilizing kinase module. After promoter melting, CDK7 phosphorylates serine five residues within YSPTSPS heptad repeats of RPB1 C-terminal domain, converting initiation complex to early elongation state. Serine5 phosphorylation reduces affinity for Mediator, facilitating promoter escape, and creates binding site for capping enzymes and Set1 methyltransferase depositing H3K4 trimethylation. Later transcription factor P-TEFb containing CDK9 phosphorylates serine two to promote productive elongation and recruitment of polyadenylation and splicing machineries.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: TFIIH kinase phosphorylates CTD Ser5; Lodish 9th ed., CTD code
NusA is essential elongation factor universally conserved associating with RNA polymerase via interaction with β-flap tip helix and α-CTD as well as nascent RNA exit channel. It increases dwell time at pause sites, enhances formation terminal hairpin structures, and stimulates Rho-independent and Rho-dependent termination by stabilizing paused elongation complex and promoting RNA folding kinetics favoring terminator over antiterminator conformations. NusA also modulates attenuation in trp and his operons and interacts with λ N antitermination complex. Rather than initiating transcription or melting promoter, primary functional impact lies in pausing and termination stimulation regulating transcriptional polarity.
Ref: Alberts Chapter 6: NusA stimulates termination and pausing function; Lodish Chapter 8: NusA antitermination regulation mechanism; Nature NusA-RNAP complex
Thermal stress induces accumulation unfolded proteins triggering activation alternate sigma factor σ32 encoded by rpoH gene, also known as heat shock sigma. Under normal growth, DnaK chaperone sequesters σ32 targeting FtsH proteolysis maintaining low levels. Upon heat shock, misfolded proteins titrate chaperones freeing σ32 to associate with core polymerase redirecting holoenzyme to promoters of heat shock regulon including groEL, dnaK, clpB chaperones and proteases. Promoters possess distinct consensus recognized preferentially by σ32. Therefore σ32 specifically orchestrates heat shock response, contrasting housekeeping σ70, σ54 nitrogen response, σ28 flagellar synthesis.
Ref: Alberts Molecular Biology Cell Chapter 7: Sigma32 heat shock response E coli RpoH regulation; Watson Chapter 14 Alternative sigma factors stress
