bacterial transcription, RNA polymerase, promoters, and stress response

Sigma factor

A sigma factor is a bacterial transcription-initiation protein that helps RNA polymerase find the right promoters and start copying DNA into RNA.

Main job

Guides bacterial RNA polymerase to promoter sequences before transcription begins.

Core partnership

Combines with RNA polymerase core enzyme to form a promoter-recognizing holoenzyme.

Regulatory use

Different sigma factors can redirect transcription toward stress, growth, or developmental gene sets.

Diagram of sigma 70 domains, promoter recognition, and RNA polymerase transcription initiation complex organization — Sigma 70 domain organization and promoter recognition in a bacterial transcription-initiation complex.Wikimedia Commons

What a sigma factor is

A sigma factor is a bacterial protein that helps RNA polymerase begin transcription at the correct DNA sites. The RNA polymerase core enzyme can make RNA, but sigma gives it promoter specificity. Together they form a holoenzyme that can recognize promoter patterns and start a transcript in the right place.

Promoter recognition

Many bacterial promoters include recognizable sequence elements upstream of the transcription start site, often described around the -10 and -35 positions. Sigma-factor regions contact these promoter elements and help position RNA polymerase. This is why sigma factors are central to bacterial transcription initiation.

The holoenzyme cycle

Sigma first associates with the RNA polymerase core enzyme, helping it bind promoter DNA. After promoter opening and early RNA synthesis, sigma often becomes less tightly associated while RNA polymerase continues elongation. The same core enzyme can later pair with another sigma factor for a different transcription program.

Housekeeping and alternative sigma factors

Bacteria commonly have a primary sigma factor for genes needed during ordinary growth, such as sigma 70 in Escherichia coli. Many also encode alternative sigma factors that become important under particular conditions, including heat shock, nutrient limitation, cell-envelope stress, stationary phase, or sporulation.

Changing the transcriptome quickly

Swapping sigma factors lets a bacterium change many transcription targets without redesigning RNA polymerase itself. A stress-responsive sigma factor can favor promoters for protective genes, while the primary sigma factor continues to support essential growth functions. This gives bacteria a fast way to reshape gene expression.

Anti-sigma factors

Sigma activity is often controlled by anti-sigma factors, proteins that bind sigma factors and prevent them from working until a signal arrives. In some systems, stress changes the anti-sigma factor, releases the sigma factor, and allows a new promoter set to be transcribed.

Not the same as eukaryotic factors

Sigma factors are characteristic of bacterial transcription. Eukaryotic cells use a larger collection of general transcription factors and RNA polymerase-specific machinery to start transcription. The functional problem is similar, finding the right start sites, but the molecular systems are different.

Why it matters

Sigma factors show how one enzyme can be retargeted to many genetic programs. They help explain bacterial adaptability, promoter design, stress responses, and the logic behind many operons. They are also important in microbiology because disrupting transcription initiation can strongly affect bacterial survival.

Key concepts

RNA polymerase core enzymethe catalytic bacterial enzyme complex that synthesizes RNA.
HoloenzymeRNA polymerase core plus a sigma factor during promoter recognition.
Promoter specificitythe ability to select correct transcription start regions.
Alternative sigma factora sigma factor used for a specialized gene-expression program.

Common examples

Sigma 70 is the main housekeeping sigma factor in E. coli.
Sigma 32 is associated with heat-shock responses in E. coli.
Sigma 38, also called RpoS, is linked to stationary-phase and stress responses.
Extracytoplasmic-function sigma factors often respond to cell-envelope or environmental stress.

Common misconceptions

Sigma factors do not synthesize RNA by themselves; they guide RNA polymerase.
A promoter is not recognized equally well without the right sigma factor.
Sigma factors are not the same thing as all transcription factors in every organism.
Alternative sigma factors do not replace all regulation; they work alongside repressors, activators, and RNA-based controls.

Open questions

How do bacteria balance competition among sigma factors for limited RNA polymerase core?
How do promoter sequences evolve to tune sigma-factor preference?
How do anti-sigma systems integrate multiple environmental signals?
Which sigma-factor networks are most vulnerable to antimicrobial intervention?