INVADING THE YEAST NUCLEUS - A NUCLEAR-LOCALIZATION SIGNAL AT THE C-TERMINUS OF TY1 INTEGRASE IS REQUIRED FOR TRANSPOSITION IN-VIVO

Retrotransposon Ty1 faces a formidable cell barrier during transpositi on-the yeast nuclear membrane which remains intact throughout the cell cycle, We investigated the mechanism by which transposition intermedi ates are transported from the cytoplasm (the presumed site of Ty1 DNA synthesis) to the nucleus, where they are integrated into the genome, Ty1 integrase has a nuclear localization signal (NLS) at its C terminu s, Both full-length integrase and a C-terminal fragment localize to th e nucleus, C-terminal deletion mutants in Ty1 integrase were used to m ap the putative NLS to the last 74 amino acid residues of integrase, M utations in basic segments within this region decreased retrotransposi tion at least 50-fold in vivo, Furthermore, these mutant integrase pro teins failed to localize to the nucleus, Production of virus-like part icles, reverse transcriptase activity, and complete in vitro Ty1 integ ration resembled wild-type levels, consistent with failure of the muta nt integrases to enter the nucleus.