Distinct effects of topoisomerase I and RNA polymerase I inhibitors suggest a dual mechanism of nucleolar/nucleoplasmic partitioning of topoisomerase I

Topoisomerase I is mostly nucleolar, because it plays a preeminent role in ribosomal DNA ( rDNA) transcription. It is cleared from nucleoli following exposure to drugs stabilizing covalent DNA intermediates of the enzyme ( e. g. camptothecin) or inhibiting RNA polymerases ( e. g. actinomycin D), an effect summarily attributed to blockade of rDNA transcription. Here we show that two distinct mechanisms are at work: ( i) Both drugs induce inactivation and segregation of the rRNA transcription machinery. With actinomycin D this leads to a co- migration of RNA- polymerase I and topoisomerase I to the nucleolar perimeter. The process has a slow onset (> 20 min), is independent of topoisomerase I activity, but requires the N- terminal domain of the enzyme to colocalize with RNA polymerase I. ( ii) Camptothecin induces, in addition, immobilization of active topoisomerase I on genomic DNA resulting in rapid nucleolar clearance and spreading of the enzyme to the entire nucleoplasm. This effect is independent of the state of rRNA transcription, involves segregation of topoisomerase I from RNA polymerase I, has a rapid onset (< 1 min), and requires catalytic activity but neither the N- terminal domain of topoisomerase I nor its major sumoylation site. Thus, nucleolar/ nucleoplasmic partitioning of topoisomerase I is regulated by interactions with RNA polymerase I and DNA but not by sumoylation.