A long and complex enhancer activates transcription of the gene coding for the highly abundant immediate early mRNA in murine cytomegalovirus Medizin - Open Access LMU - Teil 04/22

Using the simian virus 40 "enhancer trap"
approach, we have identified a transcription enhancer located
just upstream of the major immediate early gene of murine
cytomegalovirus. This enhancer has several striking properties.
(.) Together with the enhancer ofhuman cytomegalovirus,
it is the strongest transcription enhancer found to date. (ö) It
is an extremely long enhancer, spanning >700 base pairs. (ÜI)
It consists of a rather complex pattern of sequence repeats, the
longest of which is 181 base pairs. Also, several types of short
sequence motifs are scattered throughout the enhancer in
monomeric, heterodimeric, or homodimeric (palindromic)
form. These motifs have been identified to be components of
other enhancers and promoters, and they are presumably
binding sites for specific nuclear factors. Our analysis suggests
that enhancers are composed of a modular arrangement of
short conserved sequence motifs and that enhancer strength is
correlated with the redundancy of these motifs.