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The Role of Polycomb Group and Trithorax Group Chromatin Complexes in the Maintenance of Determined Cell States

Renato Paro, Peter J. Harte


In a developing organism, determined states of cells must be maintained over many cell generations. In Drosophila the genes of the Polycomb group (PcG) and trithorax group (trxG) are part of a cellular memory system that maintains the differential expression patterns of genes encoding developmental regulators involved in defining the determined states of cells. The PcG is responsible for maintaining the inactive expression states of regulatory genes such as the homeotic selector genes. Evidence increasingly suggests that the PcG proteins act by an epigenetic mechanism to keep genes inactive in a stable and heritable manner. The PcG genes share some similarities with another class of Drosophila genes, modifiers of position-effect variegation (PEV), some of which encode structural components of heterochromatin (see Henikoff, this volume). These similarities suggest that the permanent silencing of developmental regulatory genes might involve heterochromatin-like repressive complexes.

The products of the trxG genes are required for long-term maintenance of the active transcriptional states of developmental regulatory genes. They appear to counter the formation of repressive PcG-chromatin structures to render the genes accessible to transcription factors. Accumulating evidence suggests that an intricate interplay between the PcG and trxG proteins determines the distinct higher-order chromatin structures responsible for long-term maintenance of stable transcriptional states of developmental regulatory genes.

Mutations in any of the PcG genes (see Table 1) cause embryonic lethality and exhibit homeotic transformations that result from widespread ectopic expression of homeotic selector genes outside their normal realms of action. However, this...

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