General and specific transcription factors. Transcription initiation complex & looping. Combinatorial regulation.
- Transcription factors are proteins that help turn specific genes "on" or "off" by binding to nearby DNA.
- Transcription factors that are activators boost a gene's transcription. Repressors decrease transcription.
- Groups of transcription factor binding sites called enhancers and silencers can turn a gene on/off in specific parts of the body.
- Transcription factors allow cells to perform logic operations and combine different sources of information to "decide" whether to express a gene.
Do you have any transcription factors in your body? I sure hope the answer is yes, because otherwise, you're going to have a hard time keeping your cells running!
Transcription factors are proteins that regulate the transcription of genes—that is, their copying into RNA, on the way to making a protein.
The human body contains many transcription factors. So does the body of a bird, tree, or fungus! Transcription factors help ensure that the right genes are expressed in the right cells of the body, at the right time.
Transcription: The key control point
Transcription is the process where a gene's DNA sequence is copied (transcribed) into an RNA molecule. Transcription is a key step in using information from a gene to make a protein. If you're not familiar with those ideas yet, you might consider watching the central dogma video for a solid intro from Sal.
Gene expression is when a gene in DNA is "turned on," that is, used to make the protein it specifies. Not all the genes in your body are turned on at the same time, or in the same cells or parts of the body.
For many genes, transcription is the key on/off control point:
- If a gene is not transcribed in a cell, it can't be used to make a protein in that cell.
- If a gene does get transcribed, it is likely going to be used to make a protein (expressed). In general, the more a gene is transcribed, the more protein that will be made.
Various factors control how much a gene is transcribed. For instance, how tightly the DNA of the gene is wound around its supporting proteins to form chromatin can affect a gene's availability for transcription.
Proteins called transcription factors, however, play a particularly central role in regulating transcription. These important proteins help determine which genes are active in each cell of your body.
What has to happen for a gene to be transcribed? The enzyme RNA polymerase, which makes a new RNA molecule from a DNA template, must attach to the DNA of the gene. It attaches at a spot called the promoter.
In humans and other eukaryotes, there is an extra step. RNA polymerase can attach to the promoter only with the help of proteins called basal (general) transcription factors. They are part of the cell's core transcription toolkit, needed for the transcription of any gene.
However, many transcription factors (including some of the coolest ones!) are not the general kind. Instead, there is a large class of transcription factors that control the expression of specific, individual genes. For instance, a transcription factor might activate only a set of genes needed in certain neurons.
How do transcription factors work?
A typical transcription factor binds to DNA at a certain target sequence. Once it's bound, the transcription factor makes it either harder or easier for RNA polymerase to bind to the promoter of the gene.
Some transcription factors activate transcription. For instance, they may help the general transcription factors and/or RNA polymerase bind to the promoter, as shown in the diagram below.
Other transcription factors repress transcription. This repression can work in a variety of ways. As one example, a repressor may get in the way of the basal transcription factors or RNA polymerase, making it so they can't bind to the promoter or begin transcription.
The binding sites for transcription factors are often close to a gene's promoter. However, they can also be found in other parts of the DNA, sometimes very far away from the promoter, and still affect transcription of the gene.
The flexibility of DNA is what allows transcription factors at distant binding sites to do their job. The DNA loops like cooked spaghetti to bring far-off binding sites and transcription factors close to general transcription factors or "mediator" proteins.
In the cartoon above, an activating transcription factor bound at a far-away site helps RNA polymerase bind to the promoter and start transcribing.
How is this different from E. coli?
So far, human and other eukaryotic transcription factors don't seem all that different from the transcription factors we've seen in bacteria. They bind DNA and make it easier or harder for RNA polymerase to do its job—just like the lac repressor protein of E. coli
In general, this is a pretty good takeaway. Proteins that control transcription tend to act in similar ways, whether they're in your own cells or in the bacteria that live in your nose. The main differences are mechanical—how far away regulatory sites are, whether basal transcription factors are needed, etc.
However, there are also some meaningful differences in how transcription factors are used in humans. Humans and other eukaryotes are complex: we're made up of trillions of cells organized into unique tissues and body structures. Each cell in your body must run its own "program" of gene expression.
Turning genes on in specific body parts
Some genes need to be expressed in more than one body part or type of cell. For instance, suppose a gene needed to be turned on in your spine, skull, and fingertips, but not in the rest of your body. How can transcription factors make this pattern happen?
A gene with this type of pattern may have several enhancers (far-away clusters of binding sites for activators) or silencers (the same thing, but for repressors). Each enhancer or silencer may activate or repress the gene in a certain cell type or body part, binding transcription factors that are made in that part of the body.
Example: Modular mouse
As an example, let's consider a gene found in mice, called Tbx4. This gene is important for the development of many different parts of the mouse body, including the blood vessels and hind legs
During development, several well-defined enhancers drive Tbx4 expression in different parts of the mouse embryo. The diagram below shows some of the Tbx4 enhancers, each labeled with the body part where it produces expression.
Evolution of development
Enhancers like those of the Tbx4 gene are called tissue-specific enhancers: they control a gene's expression in a certain part of the body. Mutations of tissue-specific enhancers and silencers may play a key role in the evolution of body form.
How could that work? Suppose that a mutation, or change in DNA, happened in the coding sequence of the Tbx4 gene. The mutation would inactivate the gene everywhere in the body, and a mouse without a normal copy would likely die. However, a mutation in an enhancer might just change the expression pattern a bit, leading to a new feature (e.g., a shorter leg) without killing the mouse.
Transcription factors and cellular "logic"
Can cells do logic? Not in the same way as your amazing brain. However, cells can detect information and combine it to determine the correct response—in much the same way that your calculator detects pushed buttons and outputs an answer.
We can see an example of this "molecular logic" when we consider how transcription factors regulate genes. Many genes are controlled by several different transcription factors, with a specific combination needed to turn the gene on; this is particularly true in eukaryotes and is sometimes called combinatorial regulation.
For instance, a gene may be expressed only if activators A and B are present, and if repressor C is absent.
The use of multiple transcription factors to regulate a gene means that different sources of information can be integrated into a single outcome. For instance, imagine that:
- Activator A is present only in skin cells
- Activator B is active only in cells receiving "divide now!" signals (growth factors) from neighbors
- Repressor C is produced when a cell's DNA is damaged
In this case, the gene would be "turned on" only in skin cells that are receiving division signals and have undamaged, healthy DNA. This pattern of regulation might make sense for a gene involved in cell division in skin cells. In fact, the loss of proteins similar to repressor C can lead to cancer.
Real-life combinatorial regulation can be a bit more complicated than this. For instance, many different transcription factors may be involved, or it may matter exactly how many molecules of a given transcription factor are bound to the DNA.
Want to join the conversation?
- Alright but I still don't understand what is the difference between a general transcription factor and a specific one.(22 votes)
- General, or basal, transcription factors simply assist in the binding of RNA polymerase to the promoter. Other types of transcription factors include activators and repressors. These transcription factors affect transcription in different ways; activators assist in the binding of RNA polymerase and repressors stop transcription.(25 votes)
- can a single mRNA strand be translated multiple times?(9 votes)
- Are enhancers required for transcription to occur?(7 votes)
- Generally, enhancers can be bound by activators to increase the likelihood that a particular gene will be transcribed. Therefore, they are not strictly required.(8 votes)
- does prokaryotes have any transcription factors?
if yes, kindly mention their names??(4 votes)
- Yes, prokaryotes have transcription factors. Think about E. coli and the lac operon. The activator and repressor proteins involved in lac operon expression are the transcription factors. However, the mechanisms by which transcription factors work are simpler than those in eukaryotes.(10 votes)
- How do transcription factors differ from sigma factors?(8 votes)
- Well, apart from being proteins to control transcription in Prokaryotes, they are homologous to archaeal transcription factor B and to eukaryotic factor TFIIB.
Sigma factors are also needed at the promoter to initiate transcription, while transcription factors regulate the gene expression.(1 vote)
- are all transcriptional factors proteins? if not what are different transcription factors?(3 votes)
- Yes, all transcription factors are proteins. They are coded for by regulatory genes, which are genes that encode a protein involved in regulation of gene expression (such as a transcription factor). However, recently people are discovering that transcription factors can have bits of sugar and other non-protein stuff added to them to regulate their activity. But yes, all transcription factors are proteins.(4 votes)
- Does general transcription factors always bind to proximal control elements, and specific transcription factors to distal?
Also, are the bindings to specific transcription factors essential for that individual gene to start transcription?
I would very much appreciate the help.(3 votes)
- Good question!
While I believe the pattern you describe (with the general transcription factors binding to proximal elements) is common, many promoters (possibly most) don't follow that pattern.
For example, according to a 2014 review† only ~20% of RNA polymerase II promoters contain a TATA box (which means that ~80% aren't bound by TATA binding protein) and ~30% have no recognizable promoter elements!
Another example is that many (but not all) genes transcribed by RNA polymerase III have promoters within the gene§.
§Note: See the first figure in this review for details:
As for your second question, it appears that some "housekeeping"¶ genes (including many of the TATA-less pol II promoters) lack specific factor binding sites.
¶Note: "housekeeping" genes (e.g. translation factors and ribosomal proteins) are expressed everywhere and at a so their expression doesn't require a lot of fine tuning.(3 votes)
- Which ways would you test if a mutant gene was affecting a transcription factor?(3 votes)
- Knock-out a gene.Targeted gene deletion in order to study the efefct of gene mutation.
- For the Tbx4 gene, how does RNAP transcribe the whole gene for a lung cell? The lung cell enhancer is right in the middle of the gene.(3 votes)
- Do molecules that bind to repressors to change their shape count as transcription factors too? If not, what are they called?(4 votes)