A LINE-like transposable element in Drosophila, the I factor, encodes a protein with properties similar to those of retroviral nucleocapsids.

I factors are members of the LINE-like family of transposable elements and move by reverse transcription of an RNA intermediate. Complete I factors contain two open reading frames. The amino acid sequence encoded by the first of these, ORF1, includes the motif CX2CX4HX4C that is characteristic of the nucleocapsid domain of retroviral gag polypeptides followed by a copy of the slightly different sequences CX2CX4HX6C and CX2CX9HX6C. The function of this protein is unknown. We have expressed this protein in Escherichia coli and Spodoptera frugiperda cells and have shown that it binds both DNA and RNA but without any evidence for sequence specificity. The properties of deletion derivatives of the protein indicate that more than one region is responsible for DNA binding and that the CCHC motif is not essential for this. The ORF1 protein expressed in either E. coli or Spodoptera cells forms high molecular weight structures that require the region of the protein including the CCHC motif for their formation. This protein can also accelerate the annealing of complementary single-stranded oligonucleotides. These results suggest that this protein may associate with the RNA transposition intermediates of the I factor to form particles that enter the nucleus during transposition and that it may stimulate both the priming of reverse transcription and integration. This may be generally true for the product of the first open reading frame of LINE-like elements.