Crystallization and characterization of PU.1/IRF-4/DNA ternary complex

PU.1 and IRF-4, members of the Ets and interferon regulatory families of transcription factors, respectively, form a cooperative ternary complex that is implicated in the regulation of B-cell-specific gene expression. A portion of the cooperativity involves interaction between the PU.1 and IRF-4 DNA-binding domains. We report here the crystallization and preliminary characterization of PU.1 and IRF-4 DNA-binding domains bound to a 21-mer DNA site from the λB element of immunoglobulin light chain λ_2-4 enhancer. The crystals belong to space group P2₁ with unit cell dimensions of a = 40.7 Å, b = 62:3 Å, c = 67:9 Å, β = 95:6° with one complex molecule per asymmetric unit. Crystals diffract to a resolution of 3 Å using X-rays from a rotating anode generator but improve to 2.3 Å with synchrotron radiation. The crystals are highly mosaic, but the mosaicity can be improved following a series of steps involving the addition of additives, use of peritone oil as a cryoprotectant, slow flash-cooling in the cryostream, and several cycles of crystal annealing.