Promoters - A promoter sequence is the site to which RNA polymerase first binds during the initiation of transcription. Therefore, a promoter sequence is absolutely essential for the transcription of a DNA segment; the promoter must be located at that end of the gene, which has the initiation codon AUG, i.e., upstream of the coding region of the gene.

In contrast, an enhancer sequence is itself not the site of RNA polymerase binding, but it enhances the activity of promoter located in its neighbourhood often up to several kilobases away. The promoter sequence determines not only the level of transcription of a gene but also the tissue or cell type where the gene will be expressed; it also determines the developmental stage at which the gene will be expressed and, if applicable, to which stimuli the expression will respond.

A suitable enhancer sequence may also be used along with the promoter either to enhance the promoter activity or to achieve a degree of specificity in terms of the tissue in or the conditions under which the gene is expressed.

Some enhancers are quite general in their effects, e.g., SV40 enhancer. SV40 enhancer increases the activity of all the promoters transcribed by RNA polymerase II located in either direction upto several kilobases away.

Some other enhancers are either tissue specific or stimulus specific in their action; this property can be used to achieve enhanced expression of a trans gene in specific tissues/under specific conditions.