In vitro analysis of transcriptional repression of the mouse mammary tumor virus promoter

The HSS3/4 enhancer of Crlz1-IgJ locus is another target of EBF in the pre-B cell stage of B cell development

The HSS3/4 enhancer of Crlz1-IgJ locus was first characterized with regard to the activity of HSS1 IgJ promoter in the plasma cells, where both of HSS3/4 enhancer and HSS1 IgJ promoter were found to be opened simultaneously to drive the IgJ gene expression. Unexpectedly, the HSS3/4 enhancer was also found to be opened in the pre-B cells. However, this opening of HSS3/4 enhancer in the pre-B cells could not be related to the IgJ gene expression, because neither the IgJ promoter was opened nor its gene was expressed at the pre-B cell stage of B cell development. Instead, it was postulated that the opened HSS3/4 enhancer might act on some other nearby promoter in pre-B cells, which is now guessed to be the Crlz1 promoter located at 22.5 kb from it. In consistence with this pre-B cell-specific opening of the HSS3/4 enhancer, a pre-B cell-specific in vivo footprint on a sequence similar to the EBF-binding consensus was detected within the enhancer. In this paper, we show that the protein causing the pre-B cell-specific in vivo footprint on a sequence similar to the EBF-binding consensus is truly EBF as judged by EMSA using various oligo-DNA competitors and anti-EBF antibodies. Also, as expected from other previous reports, EBF was shown to be expressed highly in pre-B cells, but very little or not in immature B, mature B and plasma cells using both the cell lines and FACS-sorted normal primary cells. Convincingly, mutations within the EBF site of HSS3/4 enhancer were shown to significantly impair the HSS3/4 enhancer activity in the pre-B cells, but not in the plasma cells.

A Stat5-overlapping site is critical for the IgJ enhancer activity in the plasma cells and bound by a ubiquitous protein

Although the IgJ enhancer chromatin is induced open by an IL-2/Stat5 signaling during terminal B cell differentiation, the opened chromatin of IgJ enhancer is then maintained in the absence of IL-2/Stat5 signaling. Nevertheless, the sequence overlapping the Stat5 site was shown still to be essential for the function of IgJ enhancer in the plasma cells. An in vivo footprint was identified over the Stat5-overlapping site, indicating that the site should be bound by a certain other protein than Stat5. In EMSA using the Stat5-overlapping sequence as a probe, its specific binding protein was identified. The specific binding protein corresponded neither to any of other Stat family proteins, nor to any of potential candidate proteins as tested in EMSA using their corresponding oligo DNA competitors and antibodies. Although its identity remains to be found by its purification, the protein binding specifically to the Stat5-overlapping site was shown to be expressed rather ubiquitously in B and non-B cells, and its molecular weight appeared to be below 52 kDa as determined in the UV-crosslinking-coupled SDS-PAGE.