In vitro transcribed SV40 minichromosomes, as the bulk minichromosomes, have a low level of unconstrained negative supercoils.

The method for detecting the small fraction (1%) of transcriptionally active SV40 minichromosomes, in the presence of the bulk minichromosomes, (14) has been applied to directly analyze the topology of transcribed and non-transcribed minichromosomal DNA. We show here that DNA of both transcribed and non-transcribed minichromosomes have the same number of supercoils which are constrained by nucleosomes. In addition, minichromosomal DNA of both fractions have a low level of unconstrained supercoils (1-2 extra supercoils) which can be relaxed in vitro by topoisomerase I.