PU-1 IS A COMPONENT OF A MULTIPROTEIN COMPLEX WHICH BINDS AN ESSENTIAL SITE IN THE MURINE IMMUNOGLOBULIN LAMBDA-2-4 ENHANCER

B-cell-specific enhancers have been identified in the immunoglobulin l ambda locus 3' of each constant-region cluster. These enhancers contai n two distinct domains, lambdaA and lambdaB, which are essential for e nhancer function. lambdaB contains a near-consensus binding site for t he Ets family of transcription factors. In this study, we have identif ied a B-cell-specific protein complex which binds the lambdaB motif of the lambda2-4 enhancer in vitro and appears necessary for the activit y of the enhancer in vivo, since mutations in lambdaB which prevent th is interaction also eliminate enhancer function. This complex contains PU.1, a member of the Ets family, and a transcriptional activator who se expression is restricted to cells of the hematopoietic system with the exception of T lymphocytes. In addition, it contains a factor whic h binds specifically to a region adjacent to the PU.1 binding site. Th is factor cannot bind lambdaB autonomously but appears to require inte raction with the PU.1 protein to stabilize its association with the DN A. This complex may be identical or related to the PU.1/NF-EM5 complex which interacts with a homologous DNA element in the immunoglobulin k appa 3' enhancer.