Transcriptional state of line mouse mammary tumor virus promoter can affect topological domain size in vivo

Unrestrained DNA supercoiling and the number of topological domains were me asured within a 1.8 megabase pair chromosomal region consisting of about 20 0 tandem repeats of a mouse mammary tumor virus promoter-driven ha-v-ras ge ne. When uninduced, unrestrained negative supercoiling was organized into 3 2-kilobase pair (kb) topological domains. Upon induction, DNA supercoiling throughout the region was completely relaxed. Supercoiling was detected, ho wever, when elongation was blocked before or following induction. The forma tion of transcription initiation complexes upon addition of dexamethasone d ecreased the domain size to 16 kb. During transcription the domain size was 9 kb, the length of one repeat. These results suggest that topological dom ain boundaries can be "functional" in nature, being established by the form ation of activated and elongating transcription complexes.