ATF is important to late S phase-dependent regulation of DNA topoisomerase IIalpha gene expression in HeLa cells.

DNA topoisomerase IIalpha (Topo IIalpha) is regulated in late S phase-dependent manner. To identify late S phase-dependent cis-acting elements of Topo IIalpha gene, we have investigated the synchronized HeLa cells with chloramphenicol acetyltransferase and DNase I footprinting assays. The level of Topo IIalpha mRNA increased after release from aphidicolin block and reached a maximum in 8h (late S phase) in HeLa cells, and Topo II unknotting activity was also in parallel with the level of Topo IIalpha mRNA. The late S phase-regulatory element was found to be located in the region containing ATF-binding element between -290 and -90bp and the region was required for a maximal stimulation during late S phase. DNase I footprinting assay showed that ATF-binding element and novel cis-acting element (Topo IIalpha-specific sequence) were the principal protein-binding sites and the proteins interacting with these elements were induced during late S phase. One DNA-protein complex was formed by DNA mobility shift assay when ATF-binding site was incubated with nuclear extract prepared from late S phase cells, but no protein bound in non-S phase cells. Taken together, these results suggest that ATF may be essential transacting factor for maximal expression of Topo IIalpha gene during late S phase in HeLa cells.