Vectors are DNA molecules which are used as transporting vehicles which carries foreign gene of interest into a suitable host cell for the purpose of cloning and expression. There are two types of vectors: Cloning vectors which are used to clone foreign gene and expression vectors which are used to transcribe a foreign gene in RNA and then translated into protein.

Expression vector is a plasmid specially designed for expressing foreign genes in a cell. In addition to the basic features of a typical vector like origin of replication, insertion site, selectable marker gene, etc. they have certain regulatory elements like promoters, enhancers, termination sequence, initiation site, stop codon, etc. The most critical component of an expression vector is a promoter as it is the site where RNA polymerase binds. This region is also important in regulating the rate of transcription. It is always essential for an expression vector to carry a strong promoter so that highest rate of gene expression can be achieved. In E.coli cells a promoter can be easily regulated by an inducer or repressor. Most frequently used promoters for an E.coli expression vector includes:

• The lac promoter: This promoter is important for regulating the transcription of lac z gene that codes for β-galactosidase. This can be easily induced by isopropylthiogalactoside (IPTG) by fusing a lac promoter with a target gene one can achieve IPTG dependent expression of the foreign gene.


• The trp promoter: This promoter is responsible to transcribe a cluster of genes involved in tryptophan biosynthesis. It can be repressed by tryptophan itself and also it can be induced by 3-β-indoleacrylic acid.


• The tac promoter: This promoter is stronger than either of the above promoters as it is a hybrid of trp and lac promoter. It can also be induced by IPTG.


• The L promoter: It is a strong promoter involved in the transcription of molecule in E.coli. Mostly, the expression vector with this promoter is used with a mutant E. coli that synthesizes the λ repressor protein in a temperature-sensitive form.

The green fluorescent protein (GFP) encoding gene was first isolated from the bioluminescent jellyfish Aequorea victoria. It is a autofluorescent protein with eleven β-strands that makes up the β-barrel and an α-helix that runs through the centre. The chromophore is located in the centre of the β-barrel. This chromophore fluoresces with a peak wavelength of 508 nm (green light) when irradiated with UV or blue light (400 nm). GFP serves as an unique reporter and hence it is mostly used as a fusion tag for monitoring protein localization. GFP is usually tagged with a protein whose expression is to be tracked. In such cases the principle aim of the experiment is to investigate the sub-cellular localization of the protein of interest. Genetic engineering is used to produce vector in which the coding sequence of an uncharacterized protein say ‘X’ is coupled with a GFP coding sequence. The resulting GFP-X fusion construct is transfected into a suitable host cell, and the expression/sub-cellular location can be tracked.