Genes and regulation necessity
The qualitative and quantitative composition of synthesized proteins constantly changes depending on the tissue type and the stage of organism development. At any given time, the majority of the genome is inactive. Depending on their activity level, genes are divided into two groups:
- Constitutive genes (housekeeping genes, housekeeping genes). They function continuously in all cells, ensuring their basic viability. They are responsible for the synthesis of histones, ribosomal proteins, tubulins, as well as tRNA and rRNA.
- Inducible genes (luxury genes, luxury genes). They are activated only in specific tissues or under the influence of the external/internal environment. They provide tissue specificity.
This activity is controlled by special regulatory proteins encoded by regulatory genes.
Classification of transcription factors
Transcription factors are divided into two major groups depending on how universal their action is.
1. General transcription factors They are required for the function of absolutely all genes in any cell. Their main task is to recognize the promoter (most often its region called the TATA box) and position the enzyme on it. For example, to initiate RNA polymerase II, a complex of seven basic factors is required: TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, and TFIIJ. RNA polymerases I and III also have their own specific sets of general factors.
2. Specific transcription factors These proteins provide differential expression—they determine which specific gene needs to be activated in a particular tissue. They bind to special regulatory sequences—enhancers.
Their mechanism of action is as follows:
- Enhancers can be located at a considerable distance from the promoter.
- Specific proteins attach to the enhancer.
- The DNA strand bends, forming a spatial loop.
- Due to this loop, the enhancer proteins make physical contact with RNA polymerase II at the promoter, initiating the process.
Functional domains
Transcription factors have a complex structure. The molecule of such a protein is usually divided into three functional parts (domains):
- DNA-binding domain. Directly contacts the enhancer or promoter regions.
- Activation domain. Necessary for interaction with other coregulatory proteins.
- Ligand-binding (phosphorylating) domain. Perceives signals from other molecules. For example, if a steroid or sex hormone acts as a ligand, it binds to this domain, leading to the transcriptional activation of an entire complex of dependent genes.
Families of DNA-binding proteins
DNA-binding domains recognize the necessary sites because their surface perfectly matches the relief of the DNA double helix. Based on structural features, three main motifs are distinguished:
- Zinc fingers (Zinc finger). This is a short peptide loop whose shape is supported by a zinc ion (Zn²⁺). It binds two cysteine residues on one side and two histidine residues on the other. The helix of such a "finger" fits into the major groove of DNA, binding securely to it over several turns. This alters the conformation of DNA and opens access for enzymes. The motif is extremely important for embryonic development.
- Leucine zippers (Leucine zipper). These are dimers in whose α-helices the amino acid leucine repeats strictly every 7 positions. They interact with adjacent major grooves of DNA.
- Helix-loop-helix (Helix-loop-helix). Two helical regions of the protein are separated by a non-helical loop, which ensures their dimerization. Adjacent to this motif is a region of positively charged (basic) amino acids. Upon dimer formation, they are attracted to the negatively charged DNA and initiate transcription.
Homeotic genes
Homeotic genes are genes encoding critical transcription factors that act as key regulators of embryonic development. They control the entire program of organ and tissue formation.
These regions are strikingly conserved and are found in all animals: from the Drosophila fruit fly to humans. Mutations in them lead to catastrophic anatomical disruptions. For example, in humans they can cause tooth hypoplasia, while in Drosophila—the formation of legs instead of antennae or the appearance of extra body segments with wings.
Briefly: answers to topical questions
What human anomalies and diseases arise from mutations in homeotic genes?
Mutations (structural alterations) in homeotic genes lead to disturbances in body structure formation.
For humans, the sources indicate the following example: tooth hypoplasia.
Homeotic mutations phenotypically manifest as the transformation of some organs into others, i.e., the transformation of body parts.
To remember the three domains of a transcription factor, use the acronym DAF: DNA-binding (hooks onto the gene), Activation (gathers a team of helpers), Ligand-binding (catches a chemical signal, such as a hormone).
Class and exam questions
How do constitutive genes differ from inducible ones?
Constitutive genes provide basic functions and operate continuously in all cells (for example, genes for ribosomal proteins). Inducible genes are active only in specialized tissues or are turned on in response to specific signals.
How can an enhancer affect a promoter if they are located far apart?
When specific transcription factors bind to an enhancer, the DNA strand bends into a loop. Thanks to this spatial approximation, the proteins at the enhancer can physically interact with RNA polymerase at the promoter.
What is a "zinc finger"?
This is a structural motif of a DNA-binding protein. It is a peptide loop stabilized by a zinc ion that fits into the major groove of the DNA helix and alters its conformation to initiate transcription.
What else to study on this topic
- Specific features of RNA polymerases I and III and their general factors
- Mechanisms of post-translational modification of finished proteins
- The role of homeotic genes in human embryogenesis
- Hormonal regulation of gene expression via ligand-binding domains
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