Introduction- Heterologous expression system offers a cheap and reproducible production of the protein in large quantities for applications. Various prokaryotic and eukaryotic expression system are developed for this purpose with distinct features. E.coli as a expression system is cost effective and manipulation procedures are easy but it has limitation to express proteins with post-translation modification. Similarly, eukaryotic expression system can be able to perform post-translational modifications but it may not be cost effective. In the current lecture, we will discuss different expression system and the underlying strategies to produce foreign protein in large quantities.

Criteria to choose a expression system- Choosing a host expression system is the first step to decide the strategies to clone the gene in a suitable vector and subsequent down-stream procedures. The number of factors need to consider to choose the host expression-vector system suited for over-expression of a protein.

1. Quantity of the desired protein- if a protein is required in small amount, any host expression system may be suitable for the purpose but if a large quantity of the protein is required, a e.coli, yeast or baculo expression system might be more suitable than mammalian expression system.

2. Size of the protein- E.Coli expression system is not preferred for large size of the protein but an eukaryotic expression system is more suitable.

3. Compatibility between source organism and expression system- In general a close distance between source organism and expression system is preferred as it may increases the chances of getting the expression of cloned gene and presence of the protein in soluable fraction.

4. Down-stream application- This is the most important criteria to choose a host-vector system. If the protein production is for generating antibodies, any expression system may suit well for this purpose but if the protein is required for activity or for ELISA, then a compatible expression system is preferred.

In this series of lecture, we will discuss 4 different expression system:

1. E.COLI as a Expression System-

2. Yeast as a Expression System-

3. Insect Cell line as a Expression System-