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 (C(H)) class switch is manifested by DNA deletions which exchange the C(mu) gene of a functional VDJ - C(H) rearrangement for a C(gamma), C(epsilon) or C(alpha) gene. Repetitive sequences (S regions) 5' of each C(H) gene mediate C(H) gene switch recombination by an illegitimate mechanism. S(mu) can be subdivided into S(mu)5' (non-repetitive) and S(mu)3' (repetitive) components with recombination occurring in either part. Here, we describe the properties of ubiquitous and B cell stage specific S(mu) binding factors NFS(mu)-U1 and NFS(mu)-B1 respectively. U1 only bound to S(mu)5' sequences, and B1 to S(mu)5', S(mu)3' sequences and to other S regions with varying affinities. DMS and OP-Cu footprinting revealed the sequence AAAAAGCATGGCTGA in the U1 site while the B1 S(mu)5' site overlapped the 3' end of the U1 binding site and also contained additional 3' flanking S(mu) repeat motifs (GAGCTGAGATGGGTGGGCT). Binding site competition assays reveal that NFS(mu)-B1 is either very related or identical to S(alpha)BP (described by Waters et al., Mol. Cell Biol. 9:5594, 1989) and BSAP (identified by Barberis et al, Genes Devl. 4:849, 1990) which were shown to bind to two sequences upstream of the S(alpha) repeats and within the promoters of sea urchin histone genes respectively. Preincubation of B cell nuclear extracts at 37-degrees-C or treatment with protein dissociating agents such as NP-40, formamide or urea strongly enhanced NFS(mu)B1 binding to switch repeat motifs but abrogated NFS(mu)-U1 binding. The DNA binding properties of NFS(mu)-B1/S(alpha) BP/BSAP invokes provocative roles in B cell specific C(H) switch recombination and transcription.