The Two hybrid system uses this approach for detecting interaction among different proteins. I t has been found that there is no need of covalent attachment between the two domains for the transcriptional activity. Transcription can be activated if the two domains can be brought together, which is done with the help of two interacting proteins attached with each domain. This aspect is also used in order to check whether two different proteins interact among themselves or not, if they interact then there will be the expression of reporter gene otherwise no expression.

The Construction of Two hybrid system includes creation of two different gene containing vectors i.e

1. Gene of DNA binding domain fused Bait protein (X- Bait protein )

2.  Gene of Transcriptional activation domain fused Prey protein (Y-Prey protein)

These hybrid constructs are expressed in Y east cells containing Reporter genes. If the Protein X ( Bait protein) and Protein Y (Prey protein) interact, they will bring the transcription a ctivation domain into close vicinity with the DNA binding domain, thus triggering the transcriptional process by allowing the RNA polymerase to bind and initiate transcription which is detected by the expression of the specific r eporter gene ( e.g. color change in case of gal1-lacZ - the beta- galactosidase gene ).