The promoter causes the expression of the protein and includes the promoter (the position of RNA polymerase) and the operator. The promoter gene sequence controls the binding of RNA polymerase and transcription factors. The promoter in the plasmid should be compatible with the type of RNA supposed to be synthesized. For example, The RNA Polymerase II promoter is used to synthesize mRNA, and the RNA Polymerase III Promoter is used to synthesize shRNA. Also, the promoter you choose should be compatible with the host cell, because the transcription system varies in cells and organisms, and the promoter uses the host transcription system.
Some of the most common bacterial promoters used in cloning ;