Enhancer (English Enhance " amplify ", dt rarely also called transcription enhancer ) are sections with characteristic base sequence in eukaryotic DNA (as well as silencers) which belong to the cis- elements. They affect the deposition of the transcription complex on the promoter and thus increase the transcriptional activity of a gene. Critical to this transcription amplification, the spatial orientation of the enhancer to the promoter ( enhancer-promoter interaction). Thus, the enhancer can be up to several thousand bases upstream (upstream ) or after (downstream) the promoter lie and still be brought by the three-dimensional structure of supercoiled DNA in the proximity of the promoter sequence. On the formation of these structures, the histones are also involved. The DNA structural features that are involved in the regulation of transcription, is complex and diverse.
Examples of the enhancer activity is the DNA rearrangement in certain tumor cells or in the Ig class switching of B-lymphocytes. Here DNA portions are removed from the DNA enzymatically. In this way, ' advances ' of the enhancer in the proximity of the promoter. The result is an increased gene transcription and thus, for example, the amplification of the oncogene ( in the case of tumor cells) or production of IgG ( in the case of plasma cells). Enhancers are therefore key elements in the regulation of gene expression.
1981 one of the first enhancer has been described. It comes from the polyomavirus SV40. It contains two identical 72 bp long sections, the hot 72 - repeat sequences. Each of the two sections has itself already a weak enhancing effect on the promoter, and together they enhance the activity several times.