Transcriptional and Structural Analyses of Isolated SV40 Chromatin

An ideal biochemical source of a defined chromatin template assembled in vivo is the SV40 minichromosomes (see Note 1 ). At all stages in the viral lytic cycle, SV40 DNA is complexed with cellular histone and nonhistone proteins to form the episomal chromatin structure called a minichromosome (MC). MCs are therefore the viral template for both the host replication and host transcription machinery. The chromatin properties of MCs reflect transcriptionally competent host chromatin in every respect that has been examined. They contain more highly acetylated histones, high-mobility group proteins, and DNase I hypersensitive sites (1 –4 ). For these reasons, SV40 MCs have long been used as a model system for transcriptionally active chromatin. Because of the transcriptionally competent nature of these templates, they provide chromatin with characteristics distinct from, but complementary to, most in vitro reconstituted chromatin. SV40 MCs are especially useful in approaching questions regarding stages in transcriptional activation from a potentially competent to a fully active state. Furthermore, because of the utility of SV40 as a viral vector for exogenous promoters (5 ,6 ), fully functional cellular promoters in a native chromatin context can also be isolated and studied in this manner.