Topoisomerase II inhibitors influence simian virus 40 chromatin structure in vivo accompanied with inhibition of replication, transcription and changes in DNA supercoiling.

The association of topoisomerase II enzymes with papovavirus-chromatin has been described recently. We used cells infected with simian virus 40 (SV40) to investigate the in vivo effect of the topoisomerase II inhibitors nalidixic acid and novobiocin on the viral chromatin template. Blocking of topoisomerases inhibits DNA and RNA synthesis. The block with the inhibitors resulted in a conversion of a 95- to 75-svedberg chromatin to a 180- to 150-svedberg component accompanied with a variation in the topological linking number of the DNA. The condensed chromatin fractions (180-150 S) from inhibited cells resemble encapsidation intermediates and their DNA has a linking number as the DNA extracted from purified virions. Moreover, the virion DNA has a higher superhelical density compared to intracellular chromatin isolated from untreated cells and argues for a supercoiling activity in the cell.