Abstract
The zeste gene product is involved in two types of genetic effects dependent on chromosome pairing: transvection and the zeste-white interaction. Comparison of the predicted amino acid sequence with that of the Drosophila virilis gene shows that several blocks of amino acid sequence have been very highly conserved. One of these regions corresponds to the DNA binding domain. Site-directed mutations in this region indicate that a sequence resembling that of the homeodomain DNA recognition helix is essential for DNA binding activity. The integrity of an amphipathic helical region is also essential for binding activity and is likely to be responsible for dimerization of the DNA binding domain. Another very strongly conserved domain of zeste is the C-terminal region, predicted to form a long helical structure with two sets of heptad repeats that constitute two long hydrophobic ridges at opposite ends and on opposite faces of the helix. We show that this domain is responsible for the extensive aggregation properties of zeste that are required for its role in transvection phenomena. A model is proposed according to which the hydrophobic ridges induce the formation of open-ended coiled-coil structures holding together many hundreds of zeste molecules and possibly anchoring these complexes to other nuclear structures.