Translational Regulation

Every cell in our body contains the same DNA, so why do they appear and work so differently from each other? Why do we have muscle cells and skin cells, and not only one type of cell? This is because not all the proteins our DNA codes for are synthesised in each cell. This is due to the regulation of transcription and translation, which controls the levels of proteins that are made from messenger RNA (mRNA).

Transcriptional vs. translational regulation

The whole process of protein synthesis needs to be regulated. Translation regulation is particularly important in maintaining homeostasis, controlling cell proliferation, growth and development, as well as defining the proteome. Many diseases result from issues with translational regulation, so by understanding the process, we can get one step closer to curing some diseases. The different types of gene regulation are explained in the table below.

Table 1. The different methods for regulating gene expression.

Translation regulation in eukaryotes vs. prokaryotes

In cells, metabolic activity is controlled by regulating which genes are transcribed and translated, as well as when this occurs. Only part of an organism's genome is translated. This is important as it allows cells to express only particular proteins and therefore allows them to become highly specialised.

Gene regulation is fundamentally the same in prokaryotes and eukaryotes. However, the stimuli that cause the changes in gene expression and the responses are more complex in eukaryotes. This is because eukaryotes are multicellular organisms with more organelles and a larger genome than prokaryotes. As a result, they need to respond to changes in both their external and internal environment.

Translation regulation in prokaryotes

Prokaryotes are single-celled organisms with DNA floating free in the cytoplasm. This is because they lack membrane-bound organelles, such as the nucleus. During the whole protein synthesis process, transcription and translation occur simultaneously. When the protein produced is no longer needed, transcription stops.

Bacteria can respond to changes in the environment because of gene regulation. They only express genes when the particular protein is needed, and this prevents vital resources from being wasted. This means that prokaryotic regulation mainly occurs through transcriptional regulation. When more protein is required, more transcription occurs.

In prokaryotes, the process of translation occurs simultaneously in the cytoplasm, which creates a rapid response to environmental cues such as increased nutrient content of the cell.

Translation regulation in eukaryotes

Eukaryotes are highly complex in comparison to prokaryotes. In eukaryotic cells, DNA is contained inside the nucleus. As DNA is too large to leave the nucleus, it must be transcribed into mRNA to enter the cytoplasm. Here, the ribosomes translate the mRNA into a protein.

Transcription and translation are separated due to the presence of membrane-bound organelles. This means that transcription only occurs in the confines of the nucleus, and translation occurs only in the cytoplasm. The regulation of gene expression occurs throughout the whole process until protein synthesis. Translational regulation occurs:

• When the RNA is translated into a protein (translational regulation)
• After the protein is produced (post-translational regulation)

Types of translational regulation

Once transcription is completed, and the mRNA has left the nucleus, it may or may not be translated to make proteins. The lifespan of the mRNA determines this. Additionally, how readily the mRNA can be attached to the ribosome influences how much protein is synthesized.

RNA interference (RNAi)

For RNAi to take place, short regulatory RNA molecules are transcribed from a cell's DNA. There are two main types:

• Micro-RNA (miRNA)

• Small-interfering RNA (siRNA)

miRNAs are single-stranded and not specific to an mRNA sequence, while siRNAs are double-stranded and highly specific to a particular mRNA sequence. When these molecules bind to particular proteins in the cytoplasm, they form a complex called an RNA-induced silencing complex (RISC). Once bound to mRNA, the RISC can either hydrolyse it by using the enzyme RNA hydrolase or prevent ribosomes from attaching to the mRNA, therefore preventing translation.

Below are the steps of RNAi:

• The mRNA leaves the nucleus once it has been transcribed and enters the cytoplasm.

• Double-stranded siRNA is synthesised and enters the cytoplasm. siRNA has a specific base sequence that is complementary to the target mRNA.

• One strand of the siRNA binds to RISC. This complex now acts as an enzyme.

• The siRNA can now bind to the target mRNA by complementary base pairing.

• The mRNAs phosphodiester bonds are hydrolysed and cleaved.

mRNA 5' capping

An altered nucleotide, termed a cap, is added to the 5' end of mRNA. This is extremely important in creating mature mRNA because only mature mRNA can bind to ribosomes. Capping ensures the stability of mRNA during translation and helps the mRNA bind to ribosomes.

The 5' cap has four main functions:

1. Prevents degradation of mRNA by exonucleases
2. Promotes translation
3. Regulates nuclear export of mRNA
4. Promotes intron excision

Post-translational regulation

This type of regulation occurs after the protein has been synthesised. The modifications made to the newly synthesised proteins can alter their function and, therefore, control the level of active proteins. For example, protein kinases can phosphorylate proteins to activate them.

Additionally, the level of active proteins is also regulated by degradation! Ubiquitin, a 76 amino acid protein, can 'tag' proteins to control which proteins need to be degraded by a complex called the proteasome. The process of adding ubiquitin is known as ubiquitination, and it is a highly regulated pathway.

Diseases caused by defects in translational and post-translational modification

It is becoming increasingly clear that defects with translation have consequences that relate to a variety of diseases. These include:

• Multiple sclerosis
• Neurodegeneration
• Mitochondrial myopathy
• Encephalopathy
• Lactic acidosis
• Stroke-like episodes
• Parkinson's disease
• Cancer

The proteome and genome

Recall that a gene is a length of DNA that codes for a protein. When we talk about the genome, we are referring to all the genes that are present in a cell. However, not every gene is expressed because different cells have different functions and require different proteins!

On the other hand, the proteome describes the full range of proteins that a particular cell can contain. Each gene can produce multiple different protein types, meaning the proteome is generally larger than the genome.

Scientists can study the human proteome and genome to understand gene function and interaction.

The Human Genome Project (HGP) was an international, collaborative research programme that sequenced our genome. It found that our genome was over 3 billion base pairs long. The project has been very useful in cure development and studying genes that lead to the development of certain diseases. An example of this is genes that are linked to an increased risk of certain cancer. Thanks to the HGP, we now know that if an individual's BRCA1 and BRCA2 genes are mutated, they are more likely to develop breast cancer. This can allow us to put early diagnostic measures in place.