Repression of enhancer activity by the adenovirus E1A tumor protein(s) is mediated through a novel HeLa cell nuclear factor.

We have examined the mechanism for the host cell-dependent repression of enhancer activity by the adenovirus early region 1A (E1A) proteins. The enhancer used in this study, from the human BK virus P2, functions efficiently in cis to activate expression from the adenovirus major late promoter in the human kidney cell line, 293, and in a monkey kidney cell line, MK2. In addition, enhancer activity can be stimulated by the E1A gene products in these cells. However, cis-enhancer activity is repressed in the HeLa cell line, and we demonstrate here that further repression can be induced by the E1A proteins. We show that the binding site for the negative regulatory factor involved in cis-repression, designated BK virus enhancer factor 1 (BEF-1), is also required for E1A-induced repression. Using gel mobility retardation assays, we demonstrated a 4-fold increase in active BEF-1 in nuclear extracts containing the E1A proteins. However, the E1A proteins did not change the binding pattern or the strength of binding of BEF-1 to its target sequence. BEF-1 was identified as a 98-kDa nuclear factor, and phosphorylation was shown to be important for DNA binding. Three potential nuclear factor 1 (NF-1) sites are present in the BEF-1-binding site. Using a known NF-1 site as competitor DNA in a gel mobility retardation assay, we provide evidence that BEF-1 may be a newly identified NF-1 family member. In addition, the sequence TGA present in the repressor-binding site was shown to be essential for high affinity binding of BEF-1. Overall, our data demonstrate that an enhancer can be repressed by the trans-activation of a negative regulatory factor.     

A single point mutation activates the Moloney murine leukemia virus long terminal repeat in embryonal stem cells.

The expression of Moloney murine leukemia virus (Mo-MuLV) and Mo-MuLV-derived vectors is restricted in undifferentiated mouse embryonal carcinoma and embryonal stem (ES) cells. We have previously described the isolation of retroviral mutants with host range properties expanded to embryonal cell lines. One of these mutants, the murine embryonic stem cell virus (MESV), is expressed in ES cell lines. Expression of MESV in these cells relies on DNA sequence motifs within the enhancer region of the viral long terminal repeat (LTR). Here we show that replacement of the Mo-MuLV enhancer region by sequences derived from the MESV LTR results in the activation of the Mo-MuLV LTR in ES cells. The enhancer regions of MESV and Mo-MuLV differ by seven point mutations. Of these, a single point mutation at position -166 is sufficient to activate the Mo-MuLV LTR and to confer enhancer-dependent expression to Mo-MuLV-derived retroviral vectors in ES cells. This point mutation creates a recognition site for a sequence-specific DNA-binding factor present in nuclear extracts of ES cells. This factor was found by functional assays to be the murine equivalent to human Sp1.