CHROMATIN STRUCTURE OF HORMONO-DEPENDENT PROMOTERS

Transient transfections of mutated MMTV LTRs, driving the luciferase reporter gene, have shown the presence of at least one cis-acting element cooperating with the GREs. Studies of the chromatin structure of two glucocorticoid-regulated promoters, the mouse mammary tumor virus (MMTV) long terminal repeat (LTR), a retroviral promoter, and the rat tyrosine aminotransferase (TAT) promoter, demonstrate that both DNAs are organized into precisely positioned nucleosomes. Hormonal activation of transcription is accompanied by structural changes of one (MMTV LTR) or two (TAT promoter) nucleosomes associated with the hormone-response elements (HREs). These changes can be visualized by the appearance of DNasel hypersensitive sites. Association of the hormone-receptor complex with the nucleus is necessary to induce the DNasel hypersensitive site and to maintain transcription, but is not necessary to maintain DNasel hypersensitivity. Anti-hormones, even when able to promote a strong binding of the receptor to the nucleus, are unable to induce the chromatin structural change. Using cell lines containing approx. 200 copies of a MMTV LTR/(H)v-ras chimeric construct, we have demonstrated a strong, hormono-independent nuclear matrix interaction of sequences located just upstream and downstream of the ras coding sequences.