A nuclear single-stranded-DNA binding factor interacts with the long terminal repeats of the 1731 Drosophila retrotransposon.

Using gel mobility assays, we have detected two proteins that bind in the U3 region of the 1731 retrotransposon long terminal repeats (between positions -110 and -73) in nuclear extracts from Drosophila melanogaster cultured cells. The first one binds double-stranded DNA, whereas the other binds the mRNA-like strand in a sequence-specific manner. We report here the characterization of the latter protein, named NssBF for nuclear single-stranded-DNA binding factor. Gel filtration shows an apparent molecular mass of 95 kDa for NssBF. The points of contact between NssBF and its single-stranded DNA target were determined. This protein binds neither the complementary strand nor the corresponding RNA sequence. A possible role of NssBF in transcription is discussed.