THE EXPRESSION AND LOCALIZATION OF SCL-70 DNA TOPOISOMERASE-I VARY THROUGHOUT THE CELL-CYCLE/

Purpose and Methods, We investigated the expression and localization o f topoisomerase I by Western blot and indirect fluorescent antibody as say, respectively, using anti-Scl-70/topo I from patients with diffuse scleroderma, The contribution of topoisomerase I to DNA replication w as assessed using cells treated with the topoisomerase I inhibitor cam ptothecin, Results. Scl-topo I was detected at all cell cycle phases a s a single immunoreactive band of 100 kDa. Extracts from cells in the S phase contained the largest amount of immunoreactive Scl-70/topo I, Variations in the subcellular distribution of Scl-70/topo I were seen throughout the cell cycle, with a speckled nucleoplasmic distribution during G1 contrasting with concentration within the nucleolus during S , Camptothecin exposure blocked topoisomerase I expression and caused a significant decrease in DIVA production, Conclusion. These data sugg est (1) that topomerase I is active mainly during the S phase and cont ributes to DNA replication, and (2) that topoisomerase I may be involv ed in ribosomal gene transcription.