Transcription in Prokaryotes

Transcription in Prokaryotes

Which terminator type does NOT require Rho?

Intrinsic terminators, also called rho-independent, consist of GC-rich palindrome transcribed into RNA that folds into stable hairpin stem-loop followed by uridine-rich tract of four to eight U residues. Hairpin forms within RNA exit channel causing polymerase pausing, while weak rU-dA hybrid in active center destabilizes elongation complex without requiring additional factors. Spontaneous release of transcript and polymerase occurs due to low thermodynamic stability. Rho-dependent extrinsic terminators lack this hairpin and require Rho hexameric helicase ATPase tracking along cytosine-rich rut sites to pull RNA out.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: Intrinsic vs Rho-dependent termination; Alberts, Molecular Biology of the Cell, Bacterial termination models

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Transcription in Prokaryotes

Coupling of transcription and translation occurs in

Because bacteria lack a nuclear envelope, transcription and translation occur in same compartment and are physically coupled. As soon as Shine-Dalgarno sequence and start codon emerge from RNA polymerase, 30S ribosomal subunit binds and initiates protein synthesis while polymerase continues elongating. This coupling allows NusG to bridge RNA polymerase and leading ribosome, enhancing processivity and synchronizing rates. It also underpins transcriptional attenuation, polarity, and antitermination. In eukaryotes, nuclear membrane separates mRNA synthesis in nucleus from translation in cytoplasm, precluding such simultaneous coupling.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Bacteria couple transcription and translation; NCBI Bookshelf prokaryotic gene regulation

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Transcription in Prokaryotes

Rifampicin inhibits transcription by targeting which subunit?

Rifampicin, an ansamycin antibiotic used against tuberculosis, binds within a deep pocket of the RNA polymerase beta subunit encoded by rpoB gene, lined by residues of rifampicin resistance determining region. Located about twelve angstroms from active site magnesium, it does not block NTP binding but physically obstructs path of growing RNA chain longer than two to three nucleotides, acting as steric block. Binding creates highly stable drug-enzyme complex with nanomolar affinity. Eukaryotic polymerases II, I, III lack this pocket and remain insensitive. Mutations in RRDR cause clinical resistance.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 9: Rifampicin targets beta subunit; Nature Struct Biol, Rifampicin inhibition mechanism

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Transcription in Prokaryotes

Which region of σ70 acts as RNA mimic and must be ejected for elongation?

Sigma70 comprises domains sigma1.1 through sigma4 connected by linkers. Region 3.2, approximately amino acids 510-520 forming acidic loop between domains sigma2 and sigma4, threads through RNA polymerase active site cleft into RNA exit channel. It acts as a molecular mimic of nascent RNA, positioning initiating nucleotides and stabilizing abortive initiation complexes while preventing extension beyond five to six nucleotides. When RNA grows longer, it collides sterically with this loop, necessitating ejection. Removal of 3.2 linker clears exit path and permits promoter clearance and transition to stable elongation.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: Sigma region 3.2 as RNA mimic; PNAS 2013 Structure of sigma70 holoenzyme

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Transcription in Prokaryotes

Which sigma factor is also called housekeeping sigma factor?

Sigma70, encoded by rpoD, is the primary housekeeping sigma factor of Escherichia coli and most Gram-negative bacteria active during exponential growth. It directs RNA polymerase to the majority of promoters driving essential genes for central metabolism, ribosome synthesis, and cell envelope production, recognizing consensus -35 TTGACA and -10 TATAAT sequences. Housekeeping activity ensures continuous expression of vegetative functions. Under stress or developmental change, alternative sigma factors such as sigma32 for heat shock, sigma38 for general stress, sigma28 for flagella, and sigma54 for nitrogen assimilation displace sigma70.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 9: Prokaryotic transcription and housekeeping sigma70 function

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Transcription in Prokaryotes

During transcription initiation, sigma factor is released

In bacteria, sigma factor combines with core RNA polymerase to form holoenzyme that recognizes -10 and -35 promoter motifs and unwinds DNA. During early elongation, the sigma 3.2 linker remains lodged inside the RNA exit channel, obstructing growth beyond a few nucleotides. Once transcript reaches 9 to 11 nucleotides, steric clash ejects region 3.2, disrupts sigma-DNA and sigma-core contacts, and triggers conformational change to processive elongation complex. Sigma is released after promoter escape, recycles to new core enzymes, while NusA, NusG and other regulators bind elongating polymerase.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Bacterial transcription initiation, sigma cycle and promoter escape

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Transcription in Prokaryotes

NusG protein couples transcription with

NusG represents conserved transcription factor homologous to eukaryotic Spt5, bridging transcription and translation machineries in bacteria. N-terminal NG domain binds clamp helices of RNA polymerase increasing elongation processivity, while C-terminal KOW domain interacts with ribosomal protein NusE/S10 linking lead ribosome to polymerase during coupled transcription-translation. This physical tether prevents Rho from accessing nascent RNA, suppresses premature termination, and maintains reading frame coupling. NusG also influences DNA repair through interaction with UvrD. Thus primary described function remains coupling transcription apparatus with ribosomal translation ensuring mRNA surveillance and efficient gene expression coordination.

Ref: Alberts Chapter 6: NusG couples transcription with translation coupling mechanism; Nature 2020 Structure NusG bridging RNAP-ribosome; Lodish Chapter 8 Coupling

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Transcription in Prokaryotes

NusA protein primarily

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

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Transcription in Prokaryotes

Intrinsic termination involves

Intrinsic termination, also termed Rho-independent, encoded purely by DNA sequence producing terminator structure in nascent RNA. Sequence comprises GC-rich inverted repeat capable forming stable hairpin stem-loop of 7-20 base pairs followed immediately by run of four to eight uridines that pair weakly with template adenines. Hairpin nucleation within polymerase exit channel causes pausing and allosteric destabilization while weak rU-dA hybrid facilitates transcript release without ATP. Mutations disrupting hairpin stability or replacing U-tract reduce termination efficiency. This architecture widely used for bacterial gene boundaries and engineered expression vectors for efficient transcription cessation.

Ref: Watson Molecular Biology Gene Chapter 13: Intrinsic termination hairpin U-rich tract mechanism; Berg Biochemistry Terminator hairpin stability model

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Transcription in Prokaryotes

Rho-dependent termination requires

Rho-dependent termination requires specific cis-acting RNA element termed Rho utilization site, rut site, located upstream of termination point within nascent transcript. Rut characterized as cytosine-rich, guanosine-poor, unstructured stretch approximately 80 nucleotides lacking stable stem-loops allowing Rho hexamer loading. After binding, Rho hydrolyzes ATP translocating along RNA catching polymerase paused at downstream region rich in GC sequences. Essentiality distinguishes this mechanism from intrinsic termination which needs hairpin plus polyU tract and operates without Rho. Deletion or structure sequestration of rut abolishes termination highlighting requirement.

Ref: Alberts Molecular Biology Cell Chapter 6: Rho-dependent termination requires rut site C-rich unstructured; Watson Chapter 13 Rho mechanism rut loading

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Transcription in Prokaryotes

Rho protein is best described as

Rho protein functions as homohexameric motor translocating along nascent RNA toward polymerase, coupling ATP hydrolysis to RNA displacement. Structural analysis reveals each subunit contains N-terminal oligonucleotide-binding domain forming primary C-rich rut binding site and C-terminal RecA-like ATPase domain providing RNA-dependent ATPase and 5' to 3' helicase activities. Mechanism resembles ring helicase threading RNA through central pore. Rather than simple endonuclease or ligase, Rho acts mechanical translocase unwinding RNA-DNA hybrid within elongation complex. Inhibitor bicyclomycin targets ATPase pocket, validating RNA-dependent ATPase helicase classification essential for factor-dependent termination pathway.

Ref: Lodish Molecular Cell Biology Chapter 8: Rho RNA-dependent ATPase helicase structure function; Alberts Chapter 6: Rho hexameric translocase model

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Transcription in Prokaryotes

Average rate of transcription elongation in E. coli is about

Quantitative in vitro transcription and live-cell imaging using fluorescent reporter systems measure bacterial RNA polymerase elongation speed. Under physiological ionic strength, temperature 37°C, and saturating rNTP concentrations, average velocity approximately 40 nucleotides per second, though variable due to ubiquitous pausing induced by DNA sequences, regulatory proteins NusA, and nascent RNA secondary structures. This rate accommodates coupling with translation elongation ~15 amino acids per second maintaining ribosome behind polymerase. Compared to DNA polymerase 800 nt/s, slower speed allows proofreading and regulation. Value widely accepted in molecular microbiology textbooks describing E. coli transcription kinetics.

Ref: Watson Chapter 13: Average transcription elongation rate E coli 40 nt/s; Alberts Chapter 6: Transcription kinetics measurements; Berg Chapter 28

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