Ectopic HP1 promotes chromosome loops and variegated silencing in Drosophila

Abstract

A transgene inserted in euchromatin exhibits mosaic expression when targeted by a fusion protein made of the DNA-binding domain of GAL4 and the heterochromatin-associated protein HP1. The silencing responds to the loss of a dose of the dominant modi®ers of position-effect variegation Su(var)3-7 and Su(var)2-5, the locus encoding HP1. The genomic environs of the insertion site at 87C1 comprise the dispersed repetitive elements micropia and ag. In the presence of the GAL4±HP1 chimera, the polytene chromosomes of this line form loops between the insertion site of the transgene and six other sections of chromosome 3R, as well as, rarely, with pericentric and telomeric heterochromatin. In contrast to the insertion site of the transgene at 87C, the six loopforming sites in the euchromatic arm were each previously described as intercalary heterochromatin. Moreover, GAL4±HP1 tethering on one homologue trans-inactivates the reporter on the other. HP1, probably together with other partners, could thus facilitate the coalescence of dispersed middle repetitive sequences, and organize the heterochromatic structure responsible for the variegated silencing of nearby euchromatic genes.

Effects of tethering HP1 to euchromatic regions of theDrosophilagenome

“…When we compare silencing with lacI-HP1 to a study using a Drosophila Gal4-HP1 tethering system (Seum et al, 2001), some interesting differences emerge. In the Gal4-HP1 studies, tethered HP1 silenced a white reporter gene at only one of six genomic locations tested.…”

Dynamic Regulation of Effector Protein Binding to Histone Modifications: The Biology of HP1 Switching

“…[34][35][36][37] On a molecular level, HP1 might induce heterochromatin formation via cross-linking of nucleosomes in combination with the recruitment of other chromatin-modifying proteins. 38,39 The few instances where artificial HP1 targeting did not induce silencing 40 or the startling situations where HP1 has been implicated in gene activation 16,41,42 might be due to unique behavior of HP1 isoforms, distinct local chromatin environments, and the differential and regulated interaction with additional factors. 17 …”

Does heterochromatin protein 1 always follow code?

“…A second example is SU(VAR)3-7, a zinc finger protein that associates with satellite DNA sequences (49). HP1 and SU(VAR)3-7 colocalize in the Drosophila embryo and on polytene chromosomes (50,51) and interactions between the two proteins have been demonstrated by yeast two-hybrid analysis and coimmunoprecipitation from embryonic extracts (35,50). More specifically, the CSD of HP1 interacts with multiple regions of SU(VAR)3-7, but it is not yet clear how these two proteins collaborate to form and͞or spread heterochromatin.…”

“…A Gal4-HP1 fusion protein tethered upstream of a reporter gene caused silencing at only one of six genomic locations tested (51,81). Interestingly, the site that supported silencing was flanked by middle repetitive DNA sequences, reminiscent of heterochromatin domains.…”