DNA topoisomerase (topo) I is a nuclear enzyme that plays an important role
in DNA metabolism. Based on conserved nuclear targeting sequences, four cl
assic nuclear localization signals (NLSs) have been proposed at the N termi
nus of human topo I, but studies with yeast have suggested that only one of
them (amino acids (aa) 150-156) is sufficient to direct the enzyme to the
nucleus. In this study, we expressed human topo I fused to enhanced green f
luorescent protein (EGFP) in mammalian cells and demonstrated that whereas
aa 150-156 are sufficient for nuclear localization, the nucleolar localizat
ion requires as 157-199. More importantly, we identified a novel NLS within
aa 117-146. In contrast to the classic MLSs that are rich in basic amino a
cids, the novel NLS identified in this study is rich in acidic amino acids,
Furthermore, this novel NLS alone is sufficient to direct not only EGFP in
to the nucleus but also topo I; and the EGFP topo I fusion driven by the no
vel NLS is as active in vivo as the wild-type topo I in response to the top
o I inhibitor topotecan. Together, our results suggest that human topo I ca
rries two independent NLSs that have opposite amino acid compositions.