Switch recombination breakpoints occur at nonrandom positions in the Sγ tandem repeat

Ig switch (S) recombination is clearly focused on S regions. It is possible that S-specific DNA binding proteins facilitate the alignment of these regions before recombination. The Sγ3-specific DNA binding proteins, SNAP and SNIP/NF-κB, interact with two discrete regions of the Sγ3 tandem repeat, the A and B sites. Recombination breakpoints in the Sγ3 region were found to significantly correlate with the binding sites of the Sγ3 binding proteins. We now report the conservation of the SNIP and SNAP binding sites in Sγ2b and Sγ1 DNA. SNIP/NF-κB interacts with its cognate sites in Sγ2b and Sγ1 DNA as determined by mobility shift assays, competition binding studies, and supershift analysis using an antiserum specific for the p50 component. SNAP binds specifically to Sγ2b and 5γ1 as measured by mobility shift assays and competition binding studies. SNAP is composed of two closely traveling mobilities that do not separate on partial purification. SNIP and SNAP are expressed in nuclear extracts derived from murine splenic B cell cultures stimulated with mitogen or mitogen and IL-4. No new DNA binding proteins specific for Sγ1 tandem repeats are detectable in nuclear extracts from B cells stimulated with mitogen and IL-4. The sites at which recombination occurs in the Sγ2b and Sγ1 regions have been analyzed statistically and found to correlate with the SNIP/NF-κB and SNAP binding sites. Distinctions have been found regarding the use of DNA substrates within the tandem repeat between primary (μ→γ) and successive (γ→x) S recombination.