INVERSE RELATIONSHIP BETWEEN CATENATION AND SUPERHELICITY IN NEWLY REPLICATED SIMIAN-VIRUS-40 DAUGHTER CHROMOSOMES

High resolution gel electrophoresis was used to demonstrate an inverse relationship between catenation linking number and superhelicity in h ighly catenated simian virus 40 chromosomes caused by exposure to the topoisomerase II inhibitor ICRF-193. Since ICRF-193 does not unwind DN A, we conclude that the decreased superhelicity in catenated SV40 daug hter chromosomes is a direct result of increased catenation. It is lik ely that catenation decreases superhelicity by interfering with the fo rmation of nucleosomes. The absence of normal chromatin structure in r egions of catenation may facilitate access to topoisomerase II under n ormal conditions. ICRF-193 does not prevent initiation of SV40 DNA rep lication. (C) 1994 Academic Press, Inc.