SUMMARYThe nucleosome remodelling complexes CHRAC and ACF of Drosophila are thought to play global roles in chromatin assembly and nucleosome dynamics. Disruption of the gene encoding the common ACF1 subunit compromises fly viability. Survivors show defects in chromatin assembly and chromatin-mediated gene repression at all developmental stages. We now show that ACF1 expression is under strict developmental control. The expression is strongly diminished during embryonic development and persists at high levels only in undifferentiated cells, including the germ cell precursors and larval neuroblasts. Constitutive expression of ACF1 is lethal. Cell-specific ectopic expression perturbs chromatin organisation and nuclear programmes. By monitoring heterochromatin formation during development, we have found that ACF1-containing factors are involved in the initial establishment of diversified chromatin structures, such as heterochromatin. Altering the levels of ACF1 leads to global and variegated deviations from normal chromatin organisation with pleiotropic defects.

The Chromatin Accessibility Complex (CHRAC) consists of the ATPase ISWI, the large ACF1 subunit and a pair of small histone-like proteins, CHRAC-14/16. CHRAC is a prototypical nucleosome sliding factor that mobilizes nucleosomes to improve the regularity and integrity of the chromatin fiber. This may facilitate the formation of repressive chromatin. Expression of the signature subunit ACF1 is restricted during embryonic development, but remains high in primordial germ cells. Therefore, we explored roles for ACF1 during Drosophila oogenesis. ACF1 is expressed in somatic and germline cells, with notable enrichment in germline stem cells and oocytes. The asymmetrical localization of ACF1 to these cells depends on the transport of the Acf1 mRNA by the Bicaudal-D/Egalitarian complex. Loss of ACF1 function in the novel Acf17 allele leads to defective egg chambers and their elimination through apoptosis. In addition, we find a variety of unusual 16-cell cyst packaging phenotypes in the previously known Acf11 allele, with a striking prevalence of egg chambers with two functional oocytes at opposite poles. Surprisingly, we found that the Acf11 deletion – despite disruption of the Acf1 reading frame – expresses low levels of a PHD-bromodomain module from the C-terminus of ACF1 that becomes enriched in oocytes. Expression of this module from the Acf1 genomic locus leads to packaging defects in the absence of functional ACF1, suggesting competitive interactions with unknown target molecules. Remarkably, a two-fold overexpression of CHRAC (ACF1 and CHRAC-16) leads to increased apoptosis and packaging defects. Evidently, finely tuned CHRAC levels are required for proper oogenesis.

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