Sigma factor

Definition

A sigma factor is a protein that plays a crucial role in the initiation of transcription in prokaryotes by guiding RNA polymerase to specific promoter regions of DNA. By recognizing and binding to specific sequences within the promoter, the sigma factor facilitates the formation of a stable RNA polymerase-DNA complex, ensuring the correct start point for transcription. Different sigma factors can direct RNA polymerase to different sets of genes, allowing bacteria to respond to various environmental changes.

5 Must Know Facts For Your Next Test

1. Sigma factors are essential for prokaryotic transcription because they determine which genes are expressed under specific conditions.
2. In bacteria, multiple sigma factors exist, each enabling the transcription of a distinct set of genes, such as those needed for stress responses or metabolic changes.
3. Once RNA polymerase is bound to the promoter with the help of a sigma factor, the sigma factor is released, allowing the polymerase to proceed with elongation of the RNA strand.
4. The consensus sequence recognized by sigma factors varies, and this variation allows different sigma factors to target different promoters.
5. Mutations in sigma factor genes can lead to altered patterns of gene expression, which can affect bacterial growth and survival.

Review Questions

• How does a sigma factor influence the specificity of RNA polymerase during transcription?
  • A sigma factor influences the specificity of RNA polymerase by binding to particular promoter sequences in DNA. This interaction ensures that RNA polymerase correctly identifies which genes to transcribe in response to environmental signals. Different sigma factors recognize distinct promoter sequences, allowing bacteria to fine-tune gene expression based on their needs.
• Discuss the role of different sigma factors in bacterial responses to environmental changes.
  • Different sigma factors play a vital role in how bacteria adapt to environmental changes by regulating the expression of specific sets of genes. For example, during heat shock or nutrient limitation, alternative sigma factors can be activated to initiate the transcription of genes that help the bacterium cope with stress. This ability to switch sigma factors is critical for survival in changing environments.
• Evaluate how mutations in sigma factor genes could impact bacterial survival and adaptation mechanisms.
  • Mutations in sigma factor genes can severely impact bacterial survival and adaptation by altering the regulation of gene expression. If a mutation leads to a dysfunctional sigma factor, it may prevent RNA polymerase from properly recognizing promoters, causing essential genes not to be expressed. This disruption can hinder the bacteria's ability to respond effectively to environmental stresses or challenges, potentially leading to reduced fitness and survival in competitive environments.

Related terms

RNA polymerase:

An enzyme that synthesizes RNA from a DNA template during the process of transcription.

Promoter: A specific DNA sequence where RNA polymerase binds to initiate transcription, often recognized by sigma factors.

Transcription initiation: The first stage of gene expression where RNA polymerase binds to the promoter and begins synthesizing RNA.