Abstract
The Abelson-virus-transformed mouse pre-B-cell line 18-81 switches almost exclusively from μ to γ2b. From nuclear extracts of this cell line, we have isolated a factor that specifically binds to Sγ2b. After an eight-step purification scheme, in which different types of DNA-affinity chromatography were used as key elements, we obtained a preparation with two narrowly spaced bands at approximately 69 kD on a silver-stained SDS gel. Binding specificity of main-peak fractions of affinity-purified proteins was analyzed by gel shift assays in which Sγ2b, but not Sμ, competes. The results are consistent with this factor being part of the switch recombinase.