Sigma factor

A sigma factor is a prokaryotic protein that binds RNA polymerase and helps it recognize the correct promoter to start transcription. In General Biology I, it explains how bacteria turn specific genes on.

What is sigma factor?

A sigma factor is the part of bacterial transcription that tells RNA polymerase where to start. In General Biology I, you can think of it as the DNA-address label that gets attached to RNA polymerase before transcription begins.

RNA polymerase by itself can copy RNA, but in prokaryotes it does not always land on the correct promoter efficiently. When a sigma factor binds the polymerase, the pair becomes better at recognizing promoter sequences such as the -35 region and the Pribnow box near the start site. That recognition is what gets transcription initiated.

After RNA polymerase finds the promoter and starts building the RNA strand, the sigma factor usually detaches. That matters because sigma factor is mainly an initiation helper, not the enzyme that keeps extending the RNA. Once elongation is underway, the core RNA polymerase carries on without needing the sigma subunit in place.

Bacteria often have more than one sigma factor. The main one in E. coli is σ70, which directs transcription of many housekeeping genes, the genes needed for normal day-to-day cell function. Alternative sigma factors shift RNA polymerase toward different sets of promoters when conditions change, such as heat stress, nutrient limitation, or other environmental challenges.

That flexibility is one reason prokaryotes can respond so quickly. Instead of rebuilding the whole transcription machinery, the cell swaps in a different sigma factor and changes which genes get transcribed. So sigma factor is not just a binding protein, it is a control switch for which DNA sequences RNA polymerase can read first.

Why sigma factor matters in General Biology I

Sigma factor shows how bacteria regulate gene expression at the very first step of transcription. In General Biology I, that makes it one of the cleanest examples of gene control happening before an mRNA even exists.

It also connects directly to prokaryotic adaptation. When a bacterium hits heat shock, oxidative stress, or a new nutrient source, it can change sigma factors instead of changing every gene one by one. That gives the cell a fast way to redirect transcription toward the proteins it needs right now.

This term also helps you separate broad gene regulation from promoter-level control. Operons decide which genes are grouped together, but sigma factors help RNA polymerase recognize the promoter that starts the whole transcription unit. If you can track that starting step, it becomes much easier to explain why some genes are on, some are off, and how the cell changes that pattern under stress.

In labs, quizzes, and short-answer questions, sigma factor often shows up in diagrams of transcription initiation. If you can identify what binds first, what sequence gets recognized, and what happens after initiation, you can usually answer the question without memorizing extra detail.

How sigma factor connects across the course

RNA polymerase

Sigma factor works by binding to RNA polymerase and changing where it looks on the DNA. The core enzyme can make RNA, but the sigma factor gives it promoter specificity, so the polymerase starts at the right place instead of binding DNA randomly.

promoter

A promoter is the DNA region sigma factor helps RNA polymerase recognize. In bacteria, the promoter includes sequence features that signal the transcription start site, so sigma factor and promoter fit together like a recognition pair during initiation.

Basal transcription

Basal transcription is the low, basic level of transcription that happens without extra activation. Sigma factor is part of getting that baseline initiation started, especially for standard housekeeping genes that the cell needs all the time.

CAP protein

CAP protein is another bacterial regulator, but it works differently from sigma factor. Sigma factor helps RNA polymerase recognize promoters, while CAP helps increase transcription from certain promoters when glucose is low, so the two act at different regulatory steps.

Is sigma factor on the General Biology I exam?

A quiz item or short-answer prompt might give you a transcription diagram and ask which protein lets RNA polymerase recognize the promoter. Your job is to identify sigma factor as the initiation helper, then explain that it binds the core RNA polymerase, directs it to a specific promoter, and usually leaves after transcription begins. If you see a stress-response question, you may also need to say that alternative sigma factors let bacteria switch which genes get transcribed under different conditions. In lab or passage analysis, a change in sigma factor often shows up as a change in which RNA products are made, not as a change in the DNA itself. That is the big clue.

Sigma factor vs transcription factors

Sigma factor is a bacterial transcription protein, while transcription factors is the broader category that usually refers to eukaryotic regulatory proteins. Both affect gene expression, but sigma factor is the part that helps prokaryotic RNA polymerase find promoters and start transcription.

Key things to remember about sigma factor

• Sigma factor is a bacterial protein that binds RNA polymerase and helps it find the correct promoter to start transcription.

• It acts during initiation, then usually leaves after transcription begins, so it is not the part that keeps elongating the RNA strand.

• Different sigma factors let bacteria turn on different gene sets when conditions change, which is a fast way to adjust gene expression.

• In E. coli, σ70 is the main sigma factor for many housekeeping genes that support normal cell function.

• If you can trace promoter recognition and initiation in a diagram, you can usually explain sigma factor correctly.

Frequently asked questions about sigma factor

What is sigma factor in General Biology I?

Sigma factor is a bacterial protein that binds RNA polymerase and helps it recognize promoter DNA. It is part of transcription initiation, so it decides where RNA synthesis starts. In bacteria, that recognition step is a major control point for gene expression.

Does sigma factor stay attached during transcription?

Usually no. Sigma factor helps RNA polymerase start transcription, then it is released once elongation begins. The core RNA polymerase continues making RNA after initiation without needing sigma factor bound the whole time.

How is sigma factor different from a transcription factor?

Sigma factor is the bacterial protein that helps RNA polymerase recognize promoters in prokaryotes. Transcription factors is the broader term often used for regulatory proteins, especially in eukaryotes. They are similar in that they affect gene expression, but sigma factor has a very specific role in bacterial initiation.

Why do bacteria have different sigma factors?

Different sigma factors send RNA polymerase to different promoters, so the cell can change which genes get transcribed. That matters during stress or environmental change, because bacteria can quickly switch from routine gene expression to survival-mode gene expression.