The immunoglobulin mu enhancer core establishes local factor access in nuclear chromatin independent of transcriptional stimulation.

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Abstract
Factor access in chromatin has been proposed to be facilitated by transcriptional enhancers. With the aim of uncoupling factor access from transcriptional stimulation by protein-protein contacts, we analyzed the potential of enhancer fragments to confer accessibility upon a linked promoter for prokaryotic T7 RNA polymerase. Access to the T7 promoter in pre-B cells from transgenic mice was examined by transcribing chromatin of isolated nuclei with T7 RNA polymerase. A 95-bp immunoglobulin mu enhancer core element was necessary and sufficient to confer accessibility upon the T7 promoter independent of its chromosomal position. This enhancer-dependent factor access could be uncoupled from an active 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 attachment regions and a cryptic promoter were required to induce DNase I hypersensitivity. Together, these data provide evidence that the 95-bp mu enhancer core can establish localized factor access in nuclear chromatin independent of detectable transcription by endogenous polymerases and suggest that multiple steps are involved in the alteration of chromatin structure.