Papovavirus chromatin associated cellular endonuclease which introduces one double-strand cut in superhelical...

Nuclear extracts from SV40-infected CV-1 monkey kidney cells and from polyoma-infected 3T3 mouse cells contain an endonucleolytic activity which cleaves circular viral DNA within the chromatin to full-length linear rods [Waldeck, W., Föhring, B., Chowdhury, K., Gruss, P., & Sauer, G, (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 5964-5968; Scott, W. A., & Wigmore, D. J. (1978) Cell (Cambridge, Mass.) 15, 1511-1518]. Sedimentation of the nuclear extracts through sucrose density gradients revealed a preferential binding of the endonuclease to the viral chromatin. Deproteinized exogenous covalently closed superhelical DNA substrates such as SV40 and polyoma as well as Col E1 and PM2 DNAs were linearized by the endonuclease by introduction of one double-strand break per molecule. The reaction products, FOIII unit length rods, were shown to be devoid of single-strand nicks by electrophoresis in denaturing agarose gels. The double-strand break was randomly located within the various substrates since redigestion of the FOIII with single-cut restriction endonucleases failed to generate discrete pairs of reaction products. Neither linear double-stranded nor nicked circular FOII DNA structures were accepted as substrates. The endonucleolytic activity does not require the presence of ATP but is sensitive to EDTA. The enzyme activity is of cellular origin since nuclear extracts from uninfected CV-1 cells converted exogenous superhelical DNA to FOIII structures with the same properties as those described above. The biological properties of the endonuclease are discussed in the light of its possible function in permitting genetic exchange between different circular genomes. Further, it may play an essential role late during the replication of papovavirus DNA when the catenated daughter molecules are liberated from each other by an as yet unidentified mechanism.