The pET plasmid is used for protein expression with T7 promotor in expression strains, such as E.coli BL21(DE3)
It contains a lacI gene which codes for the lac repressor protein, a protein of interest under the control of a T7 promoter for T7 RNA polymerase and a lac operator which can block transcription, directly behind the promotor. The lac operon is another control of transcription of the protein of interest.
Furthermore, T7 polymerase is located in the genome as well under the control of the lac operon.
When IPTG is added, the lac repressor is inactivated -> T7 polymerase and therefore the protein of interest will be expressed.
The Lac I gene is already present in the genome of the expression strain used, why is it also cloned into the pET vectors?