SKIPPER, AN LTR RETROTRANSPOSON OF DICTYOSTELIUM

The complete sequence of a retrotransposon from Dictyostelium discoide um, named skipper, was obtained from cDNA and genomic clones. The sequ ence of a nearly full-length skipper cDNA was similar to that of three other partially sequenced cDNAs. The corresponding retrotransposon is represented in similar to 15-20 copies and is abundantly transcribed. Skipper contains three open reading frames (ORFs) with an unusual seq uence organization, aspects of which resemble certain mammalian retrov iruses. ORFs 1 and 3 correspond to gag and pol genes; the second ORF, pro, corresponding to protease, was separated from gag by a single sto p codon followed shortly thereafter by a potential pseudoknot. ORF3 (p ol) was separated from pro by a +1 frameshift. ORFs 2 and 3 overlapped by 32 bp. The computed amino acid sequences of the skipper ORFs conta in regions resembling retrotransposon polyprotein domains, including a nucleic acid binding protein, aspartyl protease, reverse transcriptas e and integrase. Skipper is the first example of a retrotransposon wit h a separate pro gene. Skipper is also novel in that it appears to use stop codon suppression rather than frameshifting to modulate pro expr ession. Finally, skipper and its components may provide useful tools f or the genetic characterization of Dictyostelium.