A survey of protein‐DNA interaction sites within the murine immunoglobulin heavy chain locus reveals a particularly complex pattern around the DQ 52 element

The expression of immunoglobulin (Ig) genes is regulated at two levels: rearrangement of individual gene segments and transcription of continuous genes. To find transacting factors involved in mediating locus‐ and segment specific gene activation and expression, we surveyed a 3600‐bp genomic region of the murine Ig heavy chain locus, spanning from the DQ 52 element to the Ig heavy chain intron enhancer. We discovered nine, previously undescribed, protein‐DNA complexes and estimated their individual binding affinity preferences (K r ) by quantitative gel shift measurements. We observed one novel protein DNA interaction at the enhancer, two in the J H 1 region and six within a 300‐bp region immediately 5′ to the DQ 52 locus. The latter show a complex and specific binding pattern when comparing nuclear extracts derived from pre‐B cells and fibroblasts. Further characterization of the interactions at the DQ 52 locus by electron microscopy revealed the preferential formation of a protein complex binding to the DQ 52 locus with pre B cell extracts. This behavior and the clustering of interaction sites 5′ to the DQ 52 element suggest that this region is involved in the regulation of heavy chain gene assembly.