Properties of B cell stage specific and ubiquitous nuclear factors binding to immunoglobulin heavy chain gene switch regions

The Ig heavy chain (IgH) constant region (CH) class switch is manifested by DNA deletions which exchange the Cμ gene of a functional VDJ-CH rearrangement for a Cγ, Cε or Cα gene. Repetitive sequences (S regions) 5' of each CH gene mediate CH gene switch recombination by an illegitimate mechanism. Sμ can be subdivided Into Sμ 5' (non-repetitive) and Sμ3' (repetitive) components with recombination occurring in either part. Here, we describe the properties of ubiquitous and B cell stage specific Sμ binding factors NFSμ-U1 and NFSμ-B1 respectively. U1 only bound to Sμ5' sequences, and B1 to Sμ5', Sμ3' sequences and to other S regions with varying affinities. DMS and OP-Cu footprintlng revealed the sequence AAAAAGCATGGCTGA In the U1 site while the B1 Sμ5' site overlapped the 3' end of the U1 binding site and also contained additional 3' flanking Sμ repeat motifs (GAGCTGAGATGGGTGGGCT). Binding site competition assays reveal that NFSμ-B1 Is either very related or Identical to SαBP (described by Waters et al., Mol. Cell Blol. 9: 5594, 1989) and BSAP (identified by Barberis et al., Genes Devi. 4: 849, 1990) which were shown to bind to two sequences upstream of the Sα repeats and within the promoters of sea urchin histone genes respectively. Preincubatlon of B cell nuclear extracts at 37°C or treatment with protein dissociating agents such as NP-40, formamide or urea strongly enhanced NFSμ-B1 binding to switch repeat motifs but abrogated NFSμ-U1 binding. The DNA binding properties of NFSμB1/Sα BP/BSAP Invokes provocative roles In B cell specific CH switch recombination and transcription.