The expression and localization of Scl-70/DNA topoisomerase I vary throughout the cell cycle.

PURPOSE AND METHODS

We investigated the expression and localization of topoisomerase I by Western blot and indirect fluorescent antibody assay, respectively, using anti-Scl-70/topo I from patients with diffuse scleroderma. The contribution of topoisomerase I to DNA replication was assessed using cells treated with the topoisomerase I inhibitor camptothecin.

RESULTS

Scl-topo I was detected at all cell cycle phases as 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 DNA production.

CONCLUSION

These data suggest (1) that topomerase I is active mainly during the S phase and contributes to DNA replication, and (2) that topoisomerase I may be involved in ribosomal gene transcription.