Molecular analysis of the responder satellite DNA in Drosophila melanogaster: DNA bending, nucleosome structure, and Rsp-binding proteins.

The Responder (Rsp) locus of Drosophila melanogaster, the target locus of segregation distortion, is a satellite DNA array. This repeat array imparts some fitness advantage to the chromosomes bearing it. In this paper, we report the following three related molecular properties of this satellite repeat: (1) Sequence-directed curvature--On a polyacrylamide gel, Rsp-containing fragments migrate slower than would be predicted on the basis of their physical sizes. The extent of migration retardation correlates with the size and position of the Rsp sequence in a DNA fragment, suggesting that Rsp DNA is bent. The bending is shown to be affected by a DNA-binding drug (Hoechst 33258). (2) Nucleosome structure--Nucleosomes associated with Rsp repeats have an unusual spacing pattern. Instead of being spaced at approximately 190-bp intervals as is the bulk chromatin, they are separated at approximately 240-bp intervals, roughly the size of a dimeric Rsp repeat. The nucleosomal structure in the Rsp region is preferentially disrupted by Hoechst 33258, whereas the bulk chromatin appears to be insensitive to the drug. (3) Rsp-DNA binding proteins--Gel mobility-shift assays using nuclear extracts from pupae and end-labeled Rsp repeat demonstrate the presence of three distinct DNA-protein complexes. Competition assays suggest that these complexes are specific to the Rsp sequence, and two of these nucleoprotein complexes seem to be influenced by the presence of Hoechst 33258. The observed complexes are formed by nonhistone proteins of somatic origin and may be related to the normal functions of Rsp, rather than to the germ-line segregation distortion activities.