Regulation of gene expression refers to a very broad term that may occur at various levels. Considering that gene expression results in the formation of a polypeptide, it can be regulated at several levels.
In eukaryotes, the regulation of gene expression could be exerted at four levels.
i) Transcriptional level: Formation of the primary transcript.
ii) Processing level: Formation of the primary transcript.
iii) Transport of mRNA from the nucleus to the cytoplasm.
iv) Translational level.
In prokaryotes, control of the rate of transcriptional initiation is the predominant site for control of gene expression. In a transcription unit, the activity of RNA polymerase at a given promoter is regulated by interaction with accessory proteins, which affects its ability to recognize start sites. These regulatory proteins can act both positively (activators) and negatively (repressors). The functioning of the operator depends upon the protein products.