Cell cycle-specific and transcription-related phosphorylation of mammalian topoisomerase I

Eukaryotic DNA topoisomerase I has been recently shown to be associated with the transcriptional machinery and has also been implicated to function in DNA replication and perhaps other DNA transactions. We have identified several differentially phosphorylated forms of mammalian topoisomerase I as electrophoretic variants by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These differently phosphorylated forms cleave chromosomal DNA in cells and also relax supercoiled DNA with about equal activity, suggesting that they primarily function other than to activate catalysis. One of the phosphorylated forms is specifically present during mitosis. Upon transition from mitosis into G1 phase, two forms differing in phosphorylation state appear and persist throughout the remainder of interphase. When cells are incubated in a pellet, one of the interphase phosphorylated forms disappears coincidentally with an increase in the abundance of the other; if the cell pellet is disrupted and the cells are reincubated in suspension, the forms rapidly shift back to their original abundance levels. Finally, a shift in relative abundance of the differently phosphorylated interphase forms is observed when transcription is inhibited. These results suggest that dynamic phosphorylation and dephosphorylation regulate topoisomerase I during RNA transcription and cell cycle progression.