Compositional and functional analysis of polycomb target chromatin

Dissertation presented to obtain the Ph.D degree in Molecular Biology

In metazoans, chromatin-mediated gene silencing phenomena fall into three main categories: Heterochromatin-mediated gene silencing, Telomeric gene silencing and Polycomb Group (PcG)-mediated silencing. The function of the Drosophila PcG system is best known at the homeotic gene clusters, where it keeps the transcriptional state of developmental regulators repressed, contributing to the maintenance of cell identity during embryo development. This system is also present at the TAS repeat sequences, which are responsible for Telomeric silencing in Drosophila, although the mechanisms may differ. Here I report on the discoveries from different approaches to study the function of PcG-regulated chromatin at two genomic locations: the bxd Polycomb Response Element, from the Ubx locus, and the TAS repeat sequences. I show that Zeste, one of the PRC1 PcG complex subunits, interacts specifically with the Polyhomeotic protein, likely as a dimer. Using an in vitro system with purified factors I show that the PRC1-mediated chromatin compaction over a bxd sequence is enhanced by Zeste in a dimerization- and DNA binding- dependent way and that other factors known to interact with the PcG machinery also display an enhancement effect. These results are consistent with a targeting function and suggestive of a resilient mechanism for the targeting of PRC1 function.(...)

O autor desta tese beneficiou do apoio financeiro da FCT e do FSE no âmbito do Quadro Comunitário de apoio, tendo recebido a bolsa de estudo com a referência nº SFRH/BD/11800/2003