Insulation of Enhancer-Promoter Communication by a Gypsy Transposon Insert in the Drosophila cut Gene: Cooperation between Suppressor of Hairy-wing and Modifier of mdg4 Proteins

Abstract

The Drosophila mod(mdg4) gene products counteract heterochromatin-mediated silencing of the white gene and help activate genes of the bithorax complex. They also regulate the insulator activity of the gypsy transposon when gypsy inserts between an enhancer and promoter. The Su(Hw) protein is required for gypsymediated insulation, and the Mod(mdg4)-67.2 protein binds to Su(Hw). The aim of this study was to determine whether Mod(mdg4)-67.2 is a coinsulator that helps Su(Hw) block enhancers or a facilitator of activation that is inhibited by Su(Hw). Here we provide evidence that Mod(mdg4)-67.2 acts as a coinsulator by showing that some loss-of-function mod(mdg4) mutations decrease enhancer blocking by a gypsy insert in the cut gene. We find that the C terminus of Mod(mdg4)-67.2 binds in vitro to a region of Su(Hw) that is required for insulation, while the N terminus mediates self-association. The N terminus of Mod(mdg4)-67.2 also interacts with the Chip protein, which facilitates activation of cut. Mod(mdg4)-67.2 truncated in the C terminus interferes in a dominant-negative fashion with insulation in cut but does not significantly affect heterochromatin-mediated silencing of white. We infer that multiple contacts between Su(Hw) and a Mod(mdg4)-67.2 multimer are required for insulation. We theorize that Mod(mdg4)-67.2 usually aids gene activation but can also act as a coinsulator by helping Su(Hw) trap facilitators of activation, such as the Chip protein.

Roles of the sister chromatid cohesion apparatus in gene expression, development, and human syndromes

“…Reducing Nipped-B dosage would slow the rate of cohesin mobilization, thus hindering activation. It is unknown how cohesin would hinder enhancer-promoter communication, but one possibility is similar to that proposed for the gypsy transposon insulator, in which the insulator blocks a spread of homeoprotein binding between the enhancer and promoter (Dorsett 1999;Gause et al 2001;Torigoi et al 2000). …”

Functional links between Drosophila Nipped-B and cohesin in somatic and meiotic cells

“…Effects of Nipped-B mutations on ct K and su(Hw) e2 bx 34e mutant phenotypes Males with wild-type or balanced mutant alleles of Nipped-B were crossed to ct K females at 25 and 27°C, and the wing margin nicks in the male progeny heterozygous for the Nipped-B allele being tested were quantified as previously described (Gause et al 2001;Rollins et al 2004). Nipped-B*/+; su(Hw) e2 bx 34e flies were generated by crosses, and the bithorax phenotypes were quantified as previously described (Morcillo et al 1996).…”

“…It causes partial loss of the adult wing margin by partially blocking the remote wing margin enhancer that drives cut expression during wing margin development. Decreases in ct K expression further reduce the amount of wing margin as quantified by counting the number of nicks or gaps in the bristle row along the adult wing margin (Dorsett et al 2005;Gause et al 2001;Rollins et al 2004). Null Nipped-B alleles and in vivo Nipped-B RNAi increase the wing nicking displayed by ct K (Rollins et al 2004).…”

An endogenous Suppressor of Hairy-wing insulator separates regulatory domains in Drosophila

“…The function of this insulator depends on two proteins, the zinc finger DNA-binding protein, Suppressor of Hairy-wing [Su(Hw)], and the BTB͞POZ domain protein Modifier of mdg4 [Mod-(mdg4)]67.2 that is recruited to chromosomes through proteinprotein interactions with the Su(Hw) protein (19)(20)(21)(22). The gypsy insulator proteins colocalize at hundreds of genomic sites, with Ͻ1% of these sites representing positions of the gypsy retrotransposon (23).…”