Q.28 In bacteria, the σ factor that plays a major role in transcription during the
stationary phase is
(A) σ70
(B) σ54
(C) σ28
(D) σ32
σ32 is the sigma factor that plays a major role in bacterial transcription during stationary phase, particularly for stress response genes. This is key for CSIR NET Life Sciences preparation, as it distinguishes alternative sigma factors in E. coli.
Option Analysis
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(A) σ70: The housekeeping sigma factor (RpoD) directs transcription of most genes during exponential growth phase, recognizing -10 and -35 promoter regions. Its activity decreases in stationary phase due to competition from other sigmas like σS.
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(B) σ54: An alternative sigma (RpoN) involved in nitrogen metabolism, motility, and biofilm formation; requires enhancer-binding proteins for activation. It does not primarily regulate stationary phase transcription.
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(C) σ28: Also called σF or FliA, this flagellar-specific sigma factor controls genes for flagella biosynthesis, chemotaxis, and motility in exponential or specific growth conditions. No major stationary phase role.
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(D) σ32: Correct answer. Encoded by rpoH, σ32 directs heat shock response and activates chaperones/proteases (e.g., dnaK, groEL). Levels rise during transition to stationary phase via increased stability, aiding survival under nutrient limitation and stress.
In bacterial gene regulation, the sigma factor stationary phase bacteria transition is crucial for survival under nutrient scarcity. Bacteria like E. coli switch sigma factors to reprogram transcription, with σ32 emerging as the key player during stationary phase for heat shock and stress genes. This concept is vital for CSIR NET Life Sciences, testing understanding of alternative sigma factors beyond housekeeping σ70.
Sigma Factors in Bacterial Transcription
Sigma factors bind RNA polymerase core, enabling promoter-specific initiation. During exponential growth, σ70 dominates, but stationary phase activates alternatives like σS (RpoS, σ38) for general stress and σ32 for proteotoxic stress.
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σ70: Exponential phase housekeeping.
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σS: General stationary phase/starvation (not in options).
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σ32: Heat shock, elevated at stationary entry.
Why σ32 in Stationary Phase?
As cells enter stationary phase, σ32 protein stabilizes (half-life doubles), transcribing chaperones like DnaK and GroEL despite high temperatures or misfolded proteins. Unlike σ70 (repressed) or motility-focused σ28/σ54, σ32 ensures adaptation.
| Sigma Factor | Primary Role | Phase Association |
|---|---|---|
| σ70 | Housekeeping genes | Exponential |
| σ54 | Nitrogen/motility | Specific conditions |
| σ28 | Flagella/chemotaxis | Motility |
| σ32 | Heat shock/chaperones | Stationary/stress |
CSIR NET aspirants: Memorize σ32 for stationary phase questions—distinguish from σS (general stress).
