Study of the Ability of the gypsy Insulator to Stabilize Amplification of the chorion Replication Origin of Drosophila melanogaster

The role of the gypsy insulator in the replication origin (RO) activity in the presence and absence of one and two copies of this insulator in several genomic sites was studied. Due to the fact that the prepared model system makes it possible to study the activity of this element in a given genomic site, it was shown that the RO stabilization, indeed, is determined by the activity of the insulator rather than by the construct integration site into the genome. The role of the Su(Hw) protein in this process was also studied in detail.

Effective blocking of the white enhancer requires cooperation between two main mechanisms suggested for the insulator function

Chromatin insulators block the action of transcriptional enhancers when interposed between an enhancer and a promoter. In this study, we examined the role of chromatin loops formed by two unrelated insulators, gypsy and Fab-7, in their enhancer-blocking activity. To test for this activity, we selected the white reporter gene that is activated by the eye-specific enhancer. The results showed that one copy of the gypsy or Fab-7 insulator failed to block the eye enhancer in most of genomic sites, whereas a chromatin loop formed by two gypsy insulators flanking either the eye enhancer or the reporter completely blocked white stimulation by the enhancer. However, strong enhancer blocking was achieved due not only to chromatin loop formation but also to the direct interaction of the gypsy insulator with the eye enhancer, which was confirmed by the 3C assay. In particular, it was observed that Mod(mdg4)-67.2, a component of the gypsy insulator, interacted with the Zeste protein, which is critical for the eye enhancer–white promoter communication. These results suggest that efficient enhancer blocking depends on the combination of two factors: chromatin loop formation by paired insulators, which generates physical constraints for enhancer–promoter communication, and the direct interaction of proteins recruited to an insulator and to the enhancer–promoter pair.

The mechanism underlying enhancer blocking by insulators is unclear. Current models suggest that insulator proteins block enhancers either by formation of chromatin loops or by direct interaction with protein complexes bound to the enhancers and promoters. Here, we tested the role of a chromatin loop in blocking the activity of two Drosophila insulators, gypsy and Fab-7. Both insulators failed to effectively block the interaction between the eye enhancer and the white promoter at most of genomic sites. Insertion of an additional gypsy copy either upstream of the eye enhancer or downstream from the white gene led to complete blocking of the enhancer–promoter communication. In contrast, flanking of the eye enhancer by Fab-7 insulators only weakly improved enhancer blocking. Such a difference in enhancer blocking may be explained by finding that Mod(mdg4)-67.2, a component of gypsy insulator, directly interacts with the Zeste protein, which is critical for enhancer–promoter communication in the white gene.

New properties of Drosophila scs and scs' insulators

Insulators are defined as a class of regulatory elements that delimit independent transcriptional domains within eukaryotic genomes. The first insulators to be identified were scs and scs', which flank the domain including two heat shock 70 genes. Zw5 and BEAF bind to scs and scs', respectively, and are responsible for the interaction between these insulators. Using the regulatory regions of yellow and white reporter genes, we have found that the interaction between scs and scs' improves the enhancer-blocking activity of the weak scs' insulator. The sequences of scs and scs' insulators include the promoters of genes that are strongly active in S2 cells but not in the eyes, in which the enhancer-blocking activity of these insulators has been extensively examined. Only the promoter of the Cad87A gene located at the end of the scs insulator drives white expression in the eyes, and the white enhancer can slightly stimulate this promoter. The scs insulator contains polyadenylation signals that may be important for preventing transcription through the insulator. As shown previously, scs and scs' can insulate transcription of the white transgene from the enhancing effects of the surrounding genome, a phenomenon known as the chromosomal position effect (CPE). After analyzing many independent transgenic lines, we have concluded that transgenes carrying the scs insulator are rarely inserted into genomic regions that stimulate the white reporter expression in the eyes.

Identification and mapping of ten new potential insulators in the FXYD5-COX7A1 region of human chromosome 19q13.12

A positive-negative selection system revealed 10 potential insulators able to block enhancer interaction with promoter in the 10(6) bp human chromosome 19 region between genes FXYD5 and COX7A1. Relative positions of insulators and genes are in accord with the hypothesis that insulators subdivide genomic DNA into independently regulated loop domains.