8 public questions tagged with this topic.
The sigma (Ï) factor helps RNA polymerase recognize and bind to the promoter during transcription initiation in bacteria.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
The sigma (Ï) factor of prokaryotic RNA polymerase recognizes the promoter and facilitates transcription initiation.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
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
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
Flagellar biosynthesis requires ordered cascade regulated by multiple transcription factors. After early class I master regulator FlhDC activating class II genes encoding hook-basal body, gene fliA encoding sigma28 also known as σ28 or FliA induced. σ28 holoenzyme recognizes promoters of class III late operons encoding filament flagellin, motor proteins, and chemotaxis components after completion basal structure, ensuring proper temporal assembly. FlgM anti-sigma inhibits σ28 until secreted upon hook completion. Mutants lacking σ28 non-motile. By contrast σ70 exponential, σ32 heat shock, σ54 nitrogen regulate unrelated regulons, highlighting σ28 specificity for flagellar expression.
Ref: Helmann & Chamberlin 1988 Annu Rev Biochem Sigma factors; Lodish Chapter 8: σ28 flagellar gene expression control; NCBI flagellar cascade FliA
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
Sigma70 polypeptide organized into four conserved regions comprising helical domains. Region 2, particularly subregion 2.3-2.4, responsible for recognition and melting of -10 Pribnow box element. Crystal structures show aromatic residues tryptophan from region 2 intercalate DNA, flipping adenine at -11 and thymine at -7 into protein pockets stabilizing unwound transcription bubble. Region 4.2 recognizes -35, region 1 autoinhibits DNA binding. Mutations within region 2 abolish -10 specific binding while leaving -35 interaction intact, demonstrating modular mapping. Hence region 2 primarily confers -10 element specificity essential for open complex nucleation.
Ref: Lodish Chapter 8: σ70 region 2 recognizes -10 element Pribnow box; Alberts Chapter 6 Sigma domain structure region mapping
Sigma factor constitutes dissociable specificity subunit converting promoter-non-specific core polymerase into holoenzyme able to recognize -10 and -35 promoter elements and initiate transcription at defined sites. During initiation, sigma domains 2-4 contact DNA, facilitate melting, and stabilize open complex. After RNA chain reaches about ten nucleotides, sigma releases from core, reducing affinity for promoter and allowing transition to elongation. Core continues elongation independently. Housekeeping sigma70 used exponentially, alternative sigmas redirect polymerase to stress promoters. Therefore sigma primarily governs initiation step, not elongation, termination, or proofreading functions which are core or accessory factor mediated.
Ref: Lodish Chapter 8: Sigma factor required mainly for initiation promoter recognition; Alberts Chapter 6 Sigma cycle release
