Long-range gene activation by an enhancer

Enhancers control gene activity at a distance.
An enhancer is a distant site involved in initiation
of transcription (see p. 206). It may be located
either upstream or downstream of the
same DNA strand (cis-acting) or on a different
DNA strand (trans-acting). Enhancer elements
provide tissue-specific or time-dependent regulation.
It is unclear how enhancers can exert
their effect from a considerable distance. One
model suggests that DNA forms a loop between
enhancer and promoter. Activator proteins
bound to the enhancer, e.g., a steroid hormone,
could then come into contact with the general
transcription factor complex at the promoter.
Others might function as repressors