A REGULATORY ELEMENT IN THE BETA-2-MICROGLOBULIN PROMOTER IDENTIFIED BY IN-VIVO FOOTPRINTING

Expression of the beta2-microglobulin (beta2-m) and major histocompatibility complex (MHC) class I genes is coordinately regulated. By ligation-mediated polymerase chain reaction, we have analyzed in vivo factor binding to the promoter region of the murine beta2-m gene. In adult spleen, in which beta2-m is expressed, strong protection was found in three elements. Two of these elements, the beta2-m NF-kappaB binding site and the interferon consensus sequence, are homologous to the regulatory elements of the MHC class I genes and were also found to be protected in spleen. A third protected element, PAM, identified in this work, is unique to the beta2-m gene. None of the elements showed protection in brain tissue, in which neither the beta2-m nor the MHC class I gene is expressed. In vivo footprinting was also performed with F9 embryonal carcinoma cells, in which expression of the beta2-m and MHC class I genes is induced at a low level only upon stimulation with retinoic acid (RA). No in vivo protection was detected before and after RA treatment of F9 cells, indicating that RA induction of beta2-m (and MHC class I) expression occurs without detectable in vivo factor occupancy, whereas EL4 T lymphocytes expressing beta2-m at a high level exhibited strong protection similar to that in spleen. Despite the lack of in vivo occupancy, the nuclear factors specific for each of the three elements were present in brain tissue and F9 cells as well as in spleen tissue and EL4 cells. We show that PAM, an element identified by its in vivo protection, binds nuclear factors ranging from 40 to 50 kDa in size and is capable of enhancing transcription of a reporter in F9 and other cells. Taken together, these results indicate that in vivo factor occupancy for the beta2-m and MHC class I promoters is coordinated and occurs through a mechanism other than mere expression of relevant factors.