THE IMMUNOGLOBULIN-MU ENHANCER CORE ESTABLISHES LOCAL FACTOR ACCESS IN NUCLEAR CHROMATIN INDEPENDENT OF TRANSCRIPTIONAL STIMULATION

Factor access in chromatin has been proposed to be facilitated by tran scriptional enhancers. With the aim of uncoupling factor access from t ranscriptional stimulation by protein-protein contacts, we analyzed th e potential of enhancer fragments to confer accessibility upon a linke d promoter for prokaryotic T7 RNA polymerase. Access to the T7 promote r in pre-B cells from transgenic mice was examined by transcribing chr omatin of isolated nuclei with T7 RNA polymerase. A 95-bp immunoglobul in mu enhancer core element was necessary and sufficient to confer acc essibility upon the T7 promoter independent of its chromosomal positio n. This enhancer-dependent factor access could be uncoupled from an ac tive transcriptional state of the transgene and was not accompanied by the formation of pronounced DNase I hypersensitive sites. Additional mu enhancer sequences comprising previously identified matrix attachme nt regions and a cryptic promoter were required to induce DNase I hype rsensitivity. Together, these data provide evidence that the 95.bp mu enhancer core can establish localized factor access in nuclear chromat in independent of detectable transcription by endogenous polymerases a nd suggest that multiple steps are involved in the alteration of chrom atin structure.