Transcriptional Repression Mediated by Polycomb Group Proteins and Other Chromatin-associated Repressors Is Selectively Blocked by Insulators

Johan Van Der Vlag,Jan L Den Blaauwen,Richard G.A.B Sewalt,Roel Van Driel,Arie P Otte

doi:10.1074/jbc.275.1.697

Johan Van Der Vlag, Jan L Den Blaauwen + Show 3 more

Open Access

https://doi.org/10.1074/jbc.275.1.697

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Abstract

Polycomb group (PcG) proteins repress gene activity over a considerable distance, possibly by spreading along the chromatin fiber. Insulators or boundary elements, genetic elements within the chromatin, may serve to terminate the repressing action of PcG proteins. We studied the ability of insulators to block the action of chromatin-associated repressors such as PcG proteins, HP1, and MeCP2. We found that the Drosophila special chromatin structure insulator completely blocks transcriptional repression mediated by all of the repressors we tested. The Drosophila gypsy insulator was able to block the repression mediated by the PcG proteins Su(z)2 and RING1, as well as mHP1, but not the repression mediated by MeCP2 and the PcG protein HPC2. The 5'-located DNase I-hypersensitive site in the chicken beta-globin locus displayed a limited ability to block repression, and a matrix or scaffold attachment region element was entirely unable to block repression mediated by any repressor tested. Our results indicate that insulators can block repression mediated by PcG proteins and other chromatin-associated repressors, but with a high level of selectivity. This high degree of specificity may provide a useful assay to define and characterize distinct classes of insulators.

Highlights

Polycomb group (PcG)1 proteins form chromatin-associated multimeric protein complexes that are involved in maintaining the transcriptional repressive state of genes during Drosophila and vertebrate development [1,2,3]
We concentrated on the following chromatin-associated repressors: the human PcG protein HPC2 [19], the Drosophila PcG protein Su(z)2 [22], the human PcG-associated protein RING1 [20, 21, 30], M31 or mHP1, a murine homolog of the Drosophila heterochromatin protein HP1 [25, 26], and the methyl-CpG-binding protein, MeCP2 [27, 28]
Drosophila Insulators Block Repression Mediated by Chromatin-associated Repressors—In this paper, we report that insulators or boundary elements are able to block repression of gene activity in human cells

Summary

Introduction

Polycomb group (PcG) proteins form chromatin-associated multimeric protein complexes that are involved in maintaining the transcriptional repressive state of genes during Drosophila and vertebrate development [1,2,3]. When the white gene is flanked by the scs and scsЈ insulators, the expression of white becomes less variable between transformants [6] This has been interpreted as an indication that the insulators shield the transgene from either activating or repressing effects, emerging from the surrounding chromatin. We tested whether insulators protect against repression, mediated by the PcG proteins HPC2 [19], RING1 [20, 21], and Su(z) (suppressor of zeste) [22, 23] We compared these proteins with two other, chromatin-associated repressors, the mouse homolog of heterochromatin protein 1, mHP1 (24 –26), and the methylCpG-binding protein, MeCP2 [27, 28]. Our results suggest that different insulators can be classified according to their ability to block repression mediated by specific chromatin-associated repressors

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