Sigma factor

Definition

A sigma factor is a protein that is essential for the initiation of transcription in prokaryotes by binding to RNA polymerase and directing it to specific promoter regions on the DNA. It plays a crucial role in recognizing the start sites of genes, thus enabling the RNA polymerase to properly initiate the synthesis of RNA from the DNA template.

5 Must Know Facts For Your Next Test

1. Sigma factors are not permanently attached to RNA polymerase; they dissociate after the initiation phase of transcription is complete.
2. Different sigma factors can recognize different sets of promoters, allowing bacteria to respond to various environmental conditions by regulating gene expression.
3. The primary sigma factor in Escherichia coli is σ70, which is responsible for the majority of gene expression during normal growth.
4. Sigma factors can be classified into different families based on their amino acid sequences and functions, including those that control heat shock genes or nitrogen utilization.
5. Mutations in sigma factor genes can lead to altered expression of specific genes, impacting the bacterium's ability to adapt to changes in its environment.

Review Questions

• How does the sigma factor interact with RNA polymerase during transcription initiation?
  • The sigma factor associates with RNA polymerase to form a holoenzyme, which is necessary for the initiation of transcription. This complex allows the enzyme to recognize and bind to specific promoter regions on the DNA. Once bound, the sigma factor facilitates the proper positioning of RNA polymerase at the start site, which is crucial for accurate and efficient transcription initiation.
• Discuss the significance of having multiple sigma factors in prokaryotic organisms and how this affects gene regulation.
  • The presence of multiple sigma factors in prokaryotic organisms provides a means for dynamic regulation of gene expression in response to changing environmental conditions. Each sigma factor can recognize different promoter sequences, enabling the bacterium to activate or repress specific sets of genes as needed. For example, during stress conditions such as heat shock or nutrient deprivation, alternative sigma factors can be utilized to prioritize the expression of genes essential for survival, showcasing an adaptive response at the molecular level.
• Evaluate the impact of sigma factor mutations on bacterial adaptability and survival.
  • Mutations in sigma factor genes can significantly affect a bacterium's adaptability and survival by altering its gene expression patterns. For instance, a mutation may lead to a loss of function in a sigma factor that controls stress response genes, making the organism more susceptible to environmental challenges. Conversely, some mutations might enhance fitness by enabling faster responses to specific stimuli. Overall, these mutations can have profound consequences on a bacterium's ability to thrive in diverse environments, influencing its evolutionary trajectory.

Related terms

RNA polymerase: An enzyme responsible for synthesizing RNA from a DNA template during transcription.

Promoter: A specific DNA sequence where RNA polymerase and sigma factors bind to initiate transcription.

Transcription initiation complex: A multi-protein assembly that forms at the promoter region of a gene, which includes RNA polymerase and transcription factors necessary for initiating RNA synthesis.