MEF2 PROTEINS, REQUIRED FOR MUSCLE DIFFERENTIATION, BIND AN ESSENTIALSITE IN THE IG LAMBDA-ENHANCER

The Ig lambda light chain gene enhancer has two unique essential motif s, lambda A and lambda B, The transcription factors that bind the lamb da B motif have been identified as Pu.1 and Pu.1-interacting partner ( Pip). We report here that the lambda A site includes a binding site fo r the myocyte-specific enhancer factor 2 (Mef2) family of transcriptio n factors. Mef2 proteins were first described in muscle cells and, in vertebrates, include four known members designated A to D, Using a lam bda A electrophoretic-mobility shift assay (EMSA), in conjunction with a high affinity Mef2 binding site and anti-Mef2 Abs, we show that mem bers of the Mef2 family are present in nuclear extracts of lambda-prod ucing B cells and bind the lambda A site. Functional assays using the chloramphenicol acetyltransferase (CAT) reporter construct containing three copies of the lambda A motif demonstrate that the lambda A seque nce can function as an enhancer in conjunction with the thymidine kina se (TK) promoter and is regulated by Mef2 proteins. Extrapolating from other systems where transcriptional regulation by Mef2 has been studi ed, other transcription factors may be involved along with Mef2 in tra nscriptional regulation at the lambda A site.