For more than half a century, biological textbooks have narrated a story about how bacteria activate their genes. The mechanism popularly known as the sigma (σ) cycle was treated as a near-universal rule governing bacterial transcription. A recent scientific investigation has changed the view of this long-standing assumption, presenting evidence that the classical model does not apply across all bacterial systems.
Collaborative research involving scientists from the Bose Institute, an autonomous institute under the Department of Science and Technology, and Rutgers University in the United States, has now revealed that a crucial component of bacterial transcription behaves quite differently in certain organisms. Their findings suggest that the sigma cycle, once regarded as a universal framework, is in fact far more mature and variable than previously imagined.
Published in the proceedings of the National Academy of Sciences (PNAS), the study has not only challenged a foundational idea in molecular biology, but also has opened possibilities for biotechnology, antibiotic development and our broader understanding of microbial evolution.
The Traditional Sigma Cycle: A Textbook Paradigm
It is essential to understand the conventional model that dominated biological thought for decades. In case of bacteria, gene begins when an enzyme known as RNA polymerase attaches to DNA and initiates transcription, the process of converting genetic information into RNA molecules. RNA polymerase does not locate the starting point on DNA unaided. It requires assistance from specialised proteins known as sigma factors (σ factors).
The classical explanation proposed that a sigma factor would temporarily bind to RNA polymerase, thus guiding it to the appropriate promoter region on DNA. Once transcription began, the sigma factor would detach and allow RNA polymerase to continue elongating the RNA strand. Then the sigma factor could bind to another RNA polymerase molecule and repeat the cycle.
This elegant mechanism, which is widely described as the σ cycle, became an important concept in microbiology. Much of the evidence supporting this is from research on a familiar laboratory bacterium, Escherichia coli, specifically its principal sigma factor σ70. For decades, this model was considered broadly applicable to bacteria in general.
A Scientific Puzzle Emerges
There are certain observations in bacterial physiology that hint that the story might not be universally applicable. Different bacterial species possess distinct transcriptional architectures and regulatory behaviours. The underlying assumption remained that the sigma factor always detached from RNA polymerase after transcription initiation.
Researchers have also focused on Bacillus subtilis, a well-studied Gram-positive bacterium often used as a model organism for studying cellular processes. Instead of relying solely on indirect inference, the scientists monitored sigma factor behaviour using advanced experimental approaches, including chromatin immunoprecipitation techniques and fluorescence-based imaging systems. These methods allowed the team to track the molecular interactions occurring during transcription in real time.
Sigma Factors That Refuse to Leave
Contrary to the traditional sigma cycle model, the researchers discovered that the principal sigma factor in Bacillus subtilis, known as σA, does not detach from RNA polymerase after transcription begins. Instead, σA remains firmly associated with the enzyme throughout the entire transcription process. This observation fundamentally alters the narrative surrounding bacterial gene regulation.
Dr Jayanta Mukhopadhyay of the Bose Institute, the corresponding author of the study, explained that their findings demonstrate a markedly different behaviour from what textbooks have long suggested. In Bacillus subtilis, the sigma factor continues to accompany RNA polymerase from initiation through elongation, remaining bound during the transcription journey rather than disengaging early in the process. Such persistence challenges the assumption that sigma factors invariably operate in a cyclical attach-and-release pattern.
Revisiting E.coli: A Modified Approach
The investigation did not stop with Bacillus subtilis. To explore whether similar behaviour might occur in other bacteria, the researchers examined variants of the σ70 factor from Escherichia coli.
Their experiments revealed that a modified form of σ70, specifically one lacking a segment called region represented in the image, also remained attached to RNA polymerase during transcription. This behaviour contrasted with that of the full-length σ70 protein, which showed a stochastic release pattern during the elongation stage.
Thus, the sigma factor dissociation was not an inevitable step in transcription. Instead, the process appeared to depend on structural characteristics of the sigma factor itself. These findings introduce a layer of complexity by previously overlooking in the bacterial gene regulation.
Challenge to a Long-Standing Biological Doctrine
For decades, the sigma cycle has functioned as a cornerstone concept in microbiology. It shaped how scientists interpreted bacterial transcription, taught molecular biology to students and designed experiments investigating gene regulation.
The new findings indicate that the sigma cycle cannot be regarded as a universal mechanism. According to the study’s co-author, Aniruddha Tewari from the Bose Institute, the evidence suggests that the widely accepted cycle applies only to certain bacterial contexts. The behaviour of sigma factors varies across organisms, which means that the transcription mechanisms may have evolved along with different trajectories in different microbial lineages. These findings lead the researchers to reconsider long-held assumptions in order to explore bacterial transcription with a fresh perspective.
Insights into the Evolution of Gene Regulation
If sigma factors behave differently across species, it suggests that transcriptional regulation has diversified during bacterial evolution. Organisms retain the sigma factors throughout transcription as a regulatory strategy, while others release them during elongation.
This diversity would influence how bacteria respond to environmental stress, and would also control metabolic pathways or coordinate gene networks essential for survival. The persistence of sigma factors during transcription may also affect how genes are activated or suppressed under varying conditions. The findings also offer new clues about the evolutionary adaptability of microbial life.
Implications for Antibiotic Development
The research also influences the strategy aimed at combating bacterial infections. Many antibiotics target only essential bacteria by inhibiting transcription. Understanding how the RNA polymerase interacts with sigma factors could help scientists design molecules that disrupt these interactions.
If sigma factors remain bounded throughout the transcription phase in particular bacteria, by interfering with this persistent association might prove an effective strategy to block gene expression required for bacterial survival. By refining our knowledge of transcription dynamics, scientists can identify previously unrecognised vulnerabilities in pathogenic bacteria.
Opportunities in Biotechnology and Synthetic Biology
This discovery has potential applications in biotechnology and synthetic biology. Microorganisms will be utilised for the production of biofuels, biodegradable plastics and therapeutic compounds. This will require efficient gene expression in microbial cells.
The knowledge of the sigma factor will help in designing bacteria with predictable transcriptional activities. This will help in designing microorganisms, where desired genes can be expressed efficiently, and this will be possible by regulating the interactions of the sigma factor. This will also help in the development of industrial microorganisms for the production of valuable biomolecules.
Collaborative Science Across Continents
The study is a testament to the scientific community’s ability to come together in a collaborative effort that transcends geography and continents. The scientific community that contributed to this study includes Aniruddha Tewary, Shreya Sengupta, Soumya Mukherjee and Nilanjana Hazra from the Bose Institute. The study has collaborators from Rutgers University, including Yon W. Ebright, Richard H. Ebright and Jayanta Mukhopadhyay.
A New Chapter in Microbial Biology
Sometimes, scientific progress is not so much about new discoveries as it is about revising what we already know. The sigma cycle has always been considered an unchanging law of bacterial transcription.
Bacterial gene regulation is more complex, intricate and varied than thought. This study provides evidence that sigma factors can remain attached to RNA polymerase during transcription in certain bacteria, also redefining a fundamental part of molecular biology. It challenges scientists to look at transcription mechanisms across different bacterial species instead of focusing on a single model organism.
In the new age of research, questions like how genes are regulated in bacteria, how these processes evolved and how they can be used to benefit society. What began as a detailed study of sigma factor activity could potentially redefine our understanding of bacterial life itself.
