Involvement of DNA topoisomerase I in transcription of human ribosomal RNA genes

Enrichment and depletion of Hela topoisomerase I recognition sites among specific types of DNA elements

The SDS-induced nicking of DNA helix by Hela topoisomerase I in vitro has been studied by using 2.9 kb of cloned human DNA as the substrate. The frequency of nicking is increased from 1/23 (nick/nt) to 1/19 (nick/nt) when camptothecin is present in the nicking reaction. The cytotoxic drug also induces DNA nicks without the addition of SDS. Although the consensus built from DNA sequences from -20 to +20 of more than one hundred of the nicking sites only shows a preference for T at position -1, the distributions of the topoisomerae I-cleavable sites among different categories of specific DNA sequences are apparently non- random. Long stretches of tandem (CA), A, or T residues, and the GC-rich promoter region of alpha 1 globin gene are all refractory to the nicking reaction. However, the nicking frequencies of short direct repeats flanking different Alu type sequences are as high as 1/6 (nick/nt). Finally, several tandemly arranged minirepeats of the form (TxAy)z, that are usually found at the 3' ends of the primate Alu family or Kpnl family repeats, can be cleaved efficiently in a regular pattern by the enzyme. These data are discussed in terms of the mode of recognition of DNA sequences/structures by topoisomerase I, and its possible roles in the nonhomologous insertion of repetitive DNA sequences.

Association of DNA topoisomerase I and RNA polymerase I: a possible role for topoisomerase I in ribosomal gene transcription

RNA polymerase I preparations purified from a rat hepatoma contained DNA topoisomerase activity. The DNA topoisomerase associated with the polymerase had an Mr of 110,000, required Mg2+ but not ATP, and was recognized by anti-topoisomerase I antibodies. When added to RNA polymerase I preparations containing topoisomerase activity, anti-topoisomerase I antibodies were able to inhibit the DNA relaxing activity of the preparation as well as RNA synthesis in vitro. RNA polymerase II prepared by analogous procedures did not contain topoisomerase activity and was not recognized by the antibodies. The topoisomerase I: polymerase I complex was reversibly dissociated by column chromatography on Sephacryl S200 in the presence of 0.25 M (NH4)2SO4. Topoisomerase I was immunolocalized in the transcriptionally active ribosomal gene complex containing RNA polymerase I in situ. These data indicate that topoisomerase I and RNA polymerase I are tightly complexed both in vivo and in vitro, and suggest a role for DNA topoisomerase I in the transcription of ribosomal genes.