Nuclear factors binding specific sequences within the immunoglobulin enhancer interact differentially with other enhancer elements.

The mouse immunoglobulin heavy chain (IgH) enhancer represents a cis essential control element that confers lymphoid-specific expression. Based on in vivo and in vitro competition experiments, as well as on in vivo dimethylsulfate (DMS) protection experiments, it has been inferred that cellular factors interact in trans with IgH enhancer sequences. In addition, transcription is stimulated in vitro by up to one order of magnitude in the presence of IgH enhancer sequences on an appropriate template. Thus, at least some of these factors have to be present in nuclear extracts. To examine the factors interacting with this lymphoid-specific enhancer in more detail we compared the binding pattern of nuclear factors present in B-cell, T-cell and HeLa cell extracts. We demonstrate here, using the DNase I and DMS protection methods, the specific interaction of three different nuclear factors with the central PstI--EcoRI fragment of the IgH enhancer. This fragment has previously been suggested to retain the major enhancing activity. Surprisingly, no or only minor differences were discovered when the footprints obtained with B-cell extracts were compared with those obtained with HeLa cell and T-cell extracts. Intriguingly, two factors binding specifically to different sequences of the IgH enhancer are shared by polyoma as well as Moloney sarcoma virus (MSV) and lymphotropic papova virus (LPV) enhancer, respectively. All three of these enhancer elements exhibit altered cell type specificities. This indicates the utilization of similar or identical factors for transcriptional enhancement in different cell types. A cassette model consisting of different factor binding sites will be discussed.