Mef2 proteins, required for muscle differentiation, bind an essential site in the Ig lambda enhancer.

The Ig lambda light chain gene enhancer has two unique essential motifs, lambdaA and lambdaB. The transcription factors that bind the AB motif have been identified as Pu.1 and Pu.1-interacting partner (Pip). We report here that the lambdaA site includes a binding site for the myocyte-specific enhancer factor 2 (Mef2) family of transcription factors. Mef2 proteins were first described in muscle cells and, in vertebrates, include four known members designated A to D. Using a AA electrophoretic-mobility shift assay (EMSA), in conjunction with a high affinity Mef2 binding site and anti-Mef2 Abs, we show that members of the Mef2 family are present in nuclear extracts of lambda-producing B cells and bind the AA site. Functional assays using the chloramphenicol acetyltransferase (CAT) reporter construct containing three copies of the lambdaA motif demonstrate that the AA sequence can function as an enhancer in conjunction with the thymidine kinase (TK) promoter and is regulated by Mef2 proteins. Extrapolating from other systems where transcriptional regulation by Mef2 has been studied, other transcription factors may be involved along with Mef2 in transcriptional regulation at the lambdaA site.