Drosophila chromosome condensation proteins topoisomerase II and Barren colocalize with Polycomb and maintain Fab-7 PRE silencing

Mechanisms of cellular memory control the maintenance of cellular identity at the level of chromatin structure. We have investigated whether the conve rse is true; namely, if functions responsible for maintenance of chromosome structure play a role in epigenetic control of gene expression. We show th at Topoisomerase II (TOPOII) and Barren (BARR) interact in vivo with Polyco mb group (PcG) target sequences in the bithorax complex of Drosophila, incl uding Polycomb response elements. In addition, we find that the PcG protein Polyhomeotic (PH) interacts physically with TOPOII and BARR and that BARR is required for Fab-7-regulated homeotic gene expression. Conversely, we fi nd defects in chromosome segregation associated with ph mutations. We propo se that chromatin condensation proteins are involved in mechanisms acting i n interphase that regulate chromosome domain topology and are essential for the maintenance of gene expression.