Insulators in vertebrates: regulatory mechanisms and chromatin structure

Insulators were first identified as genomic elements either blocking communication between promoters and enhancers (enhancerblocking activity) or restricting heterochromatin spreading (barrier activity). There are several types of insulators in Drosophila which utilize different proteins. All insulators identified in vertebrates work with the help of the multifunctional transcription factor CTCF. Biological functions of vertebrate insulators are not clear yet. They are supposed to separate chromatin domains albeit there is almost none direct evidence of this fact. The most significant is the participation of insulators in maintenance of centers of imprinting (imprinting choice regions). The results of a number of recently published articles indicate that isolation of a gene by placement of this gene into a separate topological domain (loop) is crucial to establishing imprinting. In this particular case as well as in many other cases insulators serve as architectural elements supporting the three-dimensional structure of genome. Moreover, interaction between pairs of insulators where cohesin plays a pivotal role along with CTCF folds genome into various loops.