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The long terminal repeat (LTR)-containing retrotransposon Tf1 propagates within the fission yeast Schizosaccharomyces pombeas the result of several mechanisms that are typical of both retrotransposons and retroviruses. To identify host factors that contribute to the transposition process, we mutagenized cultures of S. pombeand screened them for strains that were unable to support Tf1 transposition. One such strain contained a mutation in a gene we named nup124. The product of this gene contains 11 FXFG repeats and is a component of the nuclear pore complex. In addition to the reduced levels of Tf1 transposition, the nup124-1allele caused a significant reduction in the nuclear localization of Tf1 Gag. Surprisingly, the mutation in nup124-1did not cause any reduction in the growth rate, the nuclear localization of specific nuclear localization signal-containing proteins, or the cytoplasmic localization of poly(A) mRNA. A two-hybrid analysis and an in vitro precipitation assay both identified an interaction between Tf1 Gag and the N terminus of Nup124p. These results provide evidence for an unusual mechanism of nuclear import that relies on a direct interaction between a nuclear pore factor and Tf1 Gag.

ABSTRACTThe Tf2 retrotransposon, found in the fission yeastSchizosaccharomyces pombe, is nearly identical to its sister element, Tf1, in its reverse transcriptase-RNase H and integrase domains but is very divergent in the gagdomain, the protease, the 5′ untranslated region, and the U3 domain of the long terminal repeats. It has now been demonstrated that aneo-marked copy of Tf2 overexpressed from a heterologous promoter can mobilize into the S. pombegenome and produce true transposition events. However, the Tf2-neomobilization frequency is 10- to 20-fold lower than that of Tf1-neo, and 70% of the Tf2-neoevents are homologous recombination events generated independently of a functional Tf2 integrase. Thus, the Tf2 element is primarily dependent on homologous recombination with preexisting copies of Tf2 for its propagation. Finally, production of Tf2-neoproteins and cDNA was also analyzed; surprisingly, Tf2 was found to produce its reverse transcriptase as a single species in which it is fused to protease, unlike all other retroviruses and retrotransposons.

ABSTRACTThe long terminal repeat (LTR)-containing retrotransposon Tf1 propagates within the fission yeast Schizosaccharomyces pombeas the result of several mechanisms that are typical of both retrotransposons and retroviruses. To identify host factors that contribute to the transposition process, we mutagenized cultures ofS. pombeand screened them for strains that were unable to support Tf1 transposition. One such strain contained a mutation in a gene we named nup124. The product of this gene contains 11 FXFG repeats and is a component of the nuclear pore complex. In addition to the reduced levels of Tf1 transposition, thenup124-1allele caused a significant reduction in the nuclear localization of Tf1 Gag. Surprisingly, the mutation innup124-1did not cause any reduction in the growth rate, the nuclear localization of specific nuclear localization signal-containing proteins, or the cytoplasmic localization of poly(A) mRNA. A two-hybrid analysis and an in vitro precipitation assay both identified an interaction between Tf1 Gag and the N terminus of Nup124p. These results provide evidence for an unusual mechanism of nuclear import that relies on a direct interaction between a nuclear pore factor and Tf1 Gag.