Binding of multiple nuclear factors to the 5' upstream regulatory element of the murine major histocompatibility class I gene.

Transcription of mouse major histocompatibility complex class I genes is controlled by the conserved class I regulatory element (CRE) in the 5' flanking region. The CRE, approximately 40 base pairs long, acts as a negative control element in undifferentiated F9 embryonal carcinoma cells which do not express the major histocompatibility complex genes. The same element, however, acts as a positive control element in cells expressing the genes at high levels. To investigate the molecular basis of the regulatory role of the CRE, we studied the binding of nuclear proteins to the CRE of the H-2Ld gene by gel mobility shift and methylation interference experiments. Nuclear extracts from L fibroblasts and LH8 T lymphocytes revealed three distinct factors that bind discrete sequences within the CRE. The three sequences correspond to the inverted and direct repeats within the CRE. In contrast, F9 extracts exhibited factor binding to only two of the three sequences and lack a major factor detected in the above two cell types. Protein-binding sites within each of the three sequences were identified by methylation interference experiments. These data were in full agreement with results obtained by a competition assay performed with a series of mutant oligonucleotides containing a few nucleotide substitutions in each of the three regions. The results illustrate complex DNA-protein interactions in which several independent proteins bind to overlapping sequences in the CRE in a cell type-specific fashion.