Producing foreign proteins in bacteria
Bacteria can produce foreign proteins from introduced genes, using their own gene expression machinery. Producing proteins in bacteria has greatly simplified the study of how proteins work. It has also made it possible to make large amounts of medically important proteins, such as insulin, within bacteria.
Fermentation vessel for bacteria
Many proteins, particularly medicines, are produced by expressing the genes that encode them in bacteria. To obtain the proteins in industrial amounts, bacteria are grown in large fermentation vessels. They are then harvested, and the protein of interest is purified away from bacterial components.
How to make a foreign protein in bacteria
To produce a foreign protein in bacteria, you first need to clone the gene that encodes it, then introduce the vector containing your gene into bacteria.
For further information, see the article: How to add foreign DNA to bacteria.
It’s important that the vector you clone it into is an ‘expression vector’ – that is, a vector that includes a bacterial promoter sequence in front of your gene of interest. By including a bacterial promoter, you are giving the bacterium instructions to make a protein from your gene of interest – essentially, you are ‘tricking’ bacteria into producing a foreign protein.
For further information, see the article: Proteins – what they are and how they’re made.
Which promoter?
To produce large amounts of high-quality protein, the appropriate promoter should be chosen – which one is best depends on the introduced gene and on the bacterium that is hosting it. Often, scientists use strong promoters that can be switched on and off. This means that bacteria won’t begin to make the foreign protein until their environment is changed in some way (for instance, a chemical is added to the bacterial culture or the temperature is changed).
Producing proteins in bacteria aids scientific research
X-ray crystallography of a protein
X-ray crystallography is an important technique for working out the structure of a protein (the three-dimensional organisation of its amino acids). This is the crystal structure of the protein myoglobin from sperm whale – the first protein to have its structure determined by X-ray crystallography.
Before foreign proteins were first produced in bacteria, scientists had to collect their protein of interest from its natural source. This process was long, and it was difficult to collect large amounts of protein. Now, scientists routinely clone the gene that encodes ‘their’ protein and express large amounts of it in bacteria. They can then explore the protein’s function – either by isolating it from the bacteria and carrying out tests or by looking at how the presence of the protein changes how the bacteria behave.
Being able to access large amounts of a single protein has also made it much easier to work out the complex three-dimensional shape of a protein. X-ray crystallography – the most important technique for studying protein structure – requires large amounts of pure protein.
Proteins produced in bacteria are an important source of medicines
Many medicines and drugs – particularly hormones – are proteins. These include insulin (for treating diabetes), erythropoietin (for treating anaemia), growth hormone (for treating growth disorders) and others. Today, bacteria (and other organisms) are used routinely as biological ‘factories’ to produce protein medicines in large amounts.
Using bacteria has essentially replaced older methods of obtaining the proteins, which included harvesting protein from the pancreas of pigs or cattle (insulin) or from the pituitary gland of deceased humans (human growth hormone). Proteins harvested from these sources carried the risk of disease from impurities in the preparation. It was also difficult to obtain enough of the protein, as supply depended on the availability of pigs, cattle and cadavers.
How insulin started a revolution
Insulin is a hormone that controls the level of sugar (glucose) in the bloodstream. It is released from the pancreas when the glucose concentration in blood gets too high. Individuals with type 1 diabetes do not produce insulin, so they cannot control their blood glucose levels. They take insulin on a daily basis to stop their blood glucose levels becoming dangerously high.
Insulin was the first protein drug to be produced commercially in bacteria. In 1978, a version of the human gene that encodes insulin was cloned and introduced into E. coli. The bacteria were shown to produce a form of human insulin. Within 4 years, bacteria-produced insulin was commercially available as a treatment for diabetes.
Type 1 diabetes
Bob Elliott, Medical Director for Living Cell Technologies (LCT), explains the cause and symptoms of type 1 diabetes.
Select here for transcript and copyright information.
Useful link
More information about diabetes in New Zealand.
