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

Abstract

The Ig heavy chain (IgH) constant region (C_H) class switch is manifested by DNA deletions which exchange the C_μ gene of a functional VDJ-C_H rearrangement for a C_γ, C_ε or C_α gene. Repetitive sequences (S regions) 5' of each C_H gene mediate C_H gene switch recombination by an illegitimate mechanism. S_μ can be subdivided Into S_μ 5' (non-repetitive) and S_μ3' (repetitive) components with recombination occurring in either part. Here, we describe the properties of ubiquitous and B cell stage specific S_μ binding factors NFS_μ-U₁ and NFS_μ-B₁ respectively. U₁ only bound to S_μ5' sequences, and B₁ to S_μ5', S_μ3' 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 S_μ5' site overlapped the 3' end of the U₁ binding site and also contained additional 3' flanking S_μ repeat motifs (GAGCTGAGATGGGTGGGCT). Binding site competition assays reveal that NFS_μ-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 S_α 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 NFS_μ-B₁ binding to switch repeat motifs but abrogated NFS_μ-U₁ binding. The DNA binding properties of NFS_μB₁/S_α BP/BSAP Invokes provocative roles In B cell specific C_H switch recombination and transcription.