An expression vector, otherwise known as an expression construct, is generally a plasmid that is used to introduce and express a specific gene into a target cell. Expression vector allows production of large amounts of stable mRNA. Once the expression vector is inside the cell, the protein that is encoded by the gene is produced by the cellular transcription and translation machinery. The plasmid is engineered such that it contains a highly active promoter which causes the production of large amounts of mRNA.
After expression of the gene product, the purification of the protein is required; but since the vector is introduced to a host cell, the protein of interest should be purified from the proteins of the host cell. Therefore, to make purification process easy, the cloned gene should have a tag. This tag could be histidine (His) tag or any other marker peptide.
Expression vectors are used for molecular biology techniques such as site-directed mutagenesis. In general, DNA vectors that are used in many molecular biology gene cloning experiments need not result in the expression of a protein. Expression vectors are often specifically designed to contain regulatory sequences that act as enhancer and promoter regions, and lead to efficient transcription of the gene that is carried on the expression vector. Expression vectors are basic tools for biotechnology and the production of proteins such as insulin that are important for medical treatments of specific diseases like diabetes.