7.23D: Mammalian Gene Expression in Bacteria
- Describe the sequence of events in a genetically engineered expression system
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins and are produced after the process of translation. An expression system that is categorized as a genetic engineering product is a system specifically designed for the production of a gene product of choice. This is normally a protein, although may also be RNA, such as tRNA or a ribozyme.
The genetically engineered expression system contains the appropriate DNA sequence for the gene of choice which is engineered into a plasmid that is introduced into a bacteria host. The molecular machinery that is required to transcribe the DNA is derived from the innate and naturally occurring machinery in the host. The DNA is then transcribed into mRNA and then translated into protein products.
In a genetically engineered system, this entire process of gene expression may be induced depending on the plasmid used. In the broadest sense, mammalian gene expression includes every living cell but the term is more normally used to refer to expression as a laboratory tool. An expression system is therefore often artificial in some manner. Viruses and bacteria are an excellent example of expression systems.
The oldest and most widely used expression systems are cell-based. Expression is often done to a very high level and therefore referred to as overexpression. There are many ways to introduce foreign DNA to a cell for expression, and there are many different host cells which may be used for expression. Each expression system also has distinct advantages and liabilities.
Expression systems are normally referred to by the host and the DNA source or the delivery mechanism for the genetic material. For example, common bacterial hosts are E.coli and B. subtilis. With E. coli, DNA is normally introduced in a plasmid expression vector. The techniques for overexpression in E. coli work by increasing the number of copies of the gene or increasing the binding strength of the promoter region so as to assist transcription.
Key Points
- Recently improved methods of DNA chemical synthesis, combined with recombinant DNA technology, permit the design and relatively rapid synthesis of modest-sized genes that can be incorporated into prokaryotic cells for gene expression using genetic engineering.
- The feasibility of this general approach was first demonstrated by the synthesis and expression of the mammalian peptide somatostatin in Escherichia coli.
- Mammalian gene expression can be achieved in many expression hosts by utilizing the host’s naturally occurring machinery.
Key Terms
- ribozyme : A fragment of RNA that can act as an enzyme.
- plasmid : A circle of double-stranded DNA that is separate from the chromosomes, which is found in bacteria and protozoa.