9. 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
Bacterial Sigma Factors: Which Sigma Factor Regulates Transcription During the Stationary Phase?
Introduction
Bacterial cells continuously adjust gene expression in response to environmental changes. One of the most important mechanisms controlling this adaptation is the use of different sigma (σ) factors. Sigma factors are specialized proteins that bind to the core RNA polymerase and direct it to specific promoter sequences, allowing the transcription of particular groups of genes.
Different sigma factors become active under different physiological conditions such as exponential growth, heat shock, nitrogen starvation, flagellar assembly, sporulation, and stationary phase. During the stationary phase, bacteria experience nutrient limitation, oxidative stress, osmotic stress, and other environmental challenges. To survive these conditions, they activate a specialized sigma factor known as σ38 (RpoS), often called the stationary-phase sigma factor.
Correct Answer
The biologically correct answer is: σ38 (RpoS).
Important Note: The correct stationary-phase sigma factor, σ38 (RpoS), is not present among the given options. Therefore, the question appears to contain a typographical or printing error.
Detailed Explanation
During exponential growth, the primary sigma factor σ70 directs transcription of housekeeping genes required for normal cellular metabolism. As nutrients become limited and cells enter the stationary phase, the concentration of σ38 (encoded by the rpoS gene) increases dramatically. This alternative sigma factor redirects RNA polymerase to promoters of genes involved in stress resistance, starvation survival, oxidative stress protection, osmotic stress response, acid resistance, and long-term cell survival.
Because σ38 specifically regulates stationary-phase gene expression, it is universally recognized as the stationary-phase sigma factor in bacteria such as Escherichia coli.
Explanation of Option (A): σ70
This option is incorrect.
σ70 is the major housekeeping sigma factor. It controls transcription of genes required during normal exponential growth rather than stationary phase.
Explanation of Option (B): σ54
This option is incorrect.
σ54 regulates genes involved primarily in nitrogen metabolism and nitrogen limitation. It requires specialized activator proteins and ATP hydrolysis for transcription initiation.
Explanation of Option (C): σ28
This option is incorrect.
σ28, also known as FliA, regulates the expression of genes involved in bacterial flagellar biosynthesis and motility.
Explanation of Option (D): σ32
This option is incorrect.
σ32 is the heat-shock sigma factor. It activates transcription of molecular chaperones and proteases that help refold or degrade damaged proteins during heat stress.
Major Bacterial Sigma Factors
| Sigma Factor | Primary Function |
|---|---|
| σ70 (RpoD) | Housekeeping genes during exponential growth. |
| σ38 (RpoS) | Stationary phase and general stress response. |
| σ32 (RpoH) | Heat-shock response. |
| σ54 (RpoN) | Nitrogen metabolism. |
| σ28 (FliA) | Flagellar biosynthesis and motility. |
Why σ38 Is Important
As bacteria exhaust nutrients, they must conserve energy and activate genes that promote survival rather than rapid growth. σ38 coordinates the transcription of hundreds of stress-response genes, allowing bacteria to withstand starvation, oxidative damage, osmotic stress, acidic environments, and other adverse conditions. This regulatory network is essential for long-term survival.
Final Answer
The correct biological answer is σ38 (RpoS).
Since σ38 is absent from the given options, the question contains a printing/typographical error.
