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Molecular and Cellular Biology, January 2004, p. 105-122, Vol. 24, No. 1
0270-7306/04/$08.00+0 DOI: 10.1128/MCB.24.1.105-122.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Kashiwa, Chiba 277-8562, Japan,1 Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, Maryland 212052
Received 16 June 2003/ Returned for modification 5 August 2003/ Accepted 25 September 2003
Non-long terminal repeat (non-LTR) retrotransposons, most of which carry two open reading frames (ORFs), are abundant mobile elements that are distributed widely among eukaryotes. ORF2 encodes enzymatic domains, such as reverse transcriptase, that are conserved in all retroelements, but the functional roles of ORF1 in vivo are little understood. We show with green fluorescent protein-ORF1 fusion proteins that the ORF1 proteins of SART1, a telomeric repeat-specific non-LTR retrotransposon in Bombyx mori, are transported into the nucleus to produce a dotted localization pattern. Nuclear localization signals N1 (RRKR) and N2 (PSKRGRG) at the N terminus and a highly basic region in the center of SART1 ORF1 are involved in nuclear import and the dotted localization pattern in the nucleus, respectively. An in vivo retrotransposition assay clarified that at least three ORF1 domains, N1/N2, the central basic domain, and CCHC zinc fingers are required for SART1 retrotransposition. The nuclear import activity of SART1 ORF1 makes it clear that the ORF1 proteins of non-LTR retrotransposons work mainly in the nucleus, in contrast to the cytoplasmic action of Gag proteins of LTR elements. The functional domains found here in SART1 ORF1 will be useful for developing a more efficient and target-specific LINE-based gene delivery vector.
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