“Sigma Cycle”

A new India–US study challenges the long-accepted “sigma cycle” model of bacterial transcription by showing that sigma factors can remain attached throughout transcription.

Key Findings of the Research

• Continuous Sigma Attachment: Researchers found that the principal sigma factor σA in Bacillus subtilis remains attached to RNA polymerase during the entire transcription process.
• Evidence Against Universal Sigma Cycle: A modified E. coli σ70 similarly stayed bound, indicating that sigma release after initiation is not a universal rule.
• Advanced Tracking Techniques: The team used biochemical assays, chromatin immunoprecipitation and fluorescence imaging to monitor sigma factor behaviour in real time.
• Revised Understanding of Gene Regulation: The findings show that different bacteria use distinct transcription mechanisms, reshaping textbook explanations of bacterial gene regulation.

About the “Sigma Cycle”

• The sigma cycle describes how sigma (σ) factors bind RNA polymerase (RNAP) to initiate transcription and then detach after RNA synthesis begins.
• Sigma factors are subunits of bacterial RNA polymerase (RNAP) that are essential for recognizing promoter DNA sequences and initiating transcription. 
• They bind to the core RNAP to form a holoenzyme, which then directs the polymerase to the correct start sites of genes
• Role of Sigma Factor
  • Competition for RNAP: Multiple sigma factors compete to bind the RNAP core, determining which gene sets will be transcribed.
  • Promoter Specificity: Sigma factor type dictates promoter recognition, enabling environmental or stress-specific gene expression responses.
  • Initiation and Release: Only the holoenzyme (RNAP + sigma) initiates transcription, after which sigma typically releases once the RNA chain reaches 8–9 nucleotides.
  • Reprogramming and Regulation: Sigma release allows new sigma factors to bind, with anti-sigma factors further regulating access and transcription timing.

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Potential Applications of the Findings

• The discovery may enable new antibiotic strategies targeting previously unknown transcription mechanisms.
• The findings can support synthetic biology innovations by engineering bacteria with altered transcriptional control.
• The results may improve understanding of bacterial evolution and stress responses.

Significance

• The study fundamentally revises a five-decade biological model, offering deeper insight into bacterial gene regulation across species.

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