The overall goal of our project is to understand transcriptional mechanisms that control differentiation in early B lymphocytes. The differentiation of B lymphocytes from committed precursor cells to antibody-secreting plasma cells proceeds through multiple steps defined by changes in the expression pattern of lineage-specific genes. As a paradigm for a temporally regulated B cell-specific gene, the mb-1 (MB- 1; CD79alpha) gene encodes a trans-membrane protein that is required for cell surface display and signaling functions of membrane-bound immunoglobulin (mIg) on early B cells. We hypothesize that the early B cell-specific transcription factor Pax-5 is an important regulator of mb-1 transcription. Recombinant Pax-5 binds specifically to the mb-1 promoter by itself in vitro, and Pax-5 recruits proteins of the Ets protooncogene family to a site that is essential for promoter function in transfected pre-B cells. These observations represent the first demonstration of protein:protein interactions involving a member of the paired domain (Pax) protein family. Moreover, we describe a novel mechanism governing DNA binding by Ets proteins. Therefore, our experiments will address how Pax-5 and Ets proteins regulate mb-1 gene transcription, and potentially, other genes in early B cells (and potentially neuronal cells, which also express these proteins). We will examine the molecular basis for Pax-5:Ets interactions at the protein:protein and protein:DNA levels and determine how different Ets proteins affect the functional properties of Pax-5:Ets complexes. We will test whether the down-regulation of mb-1 gene expression in terminally differentiated plasma cells is due to the absence of early B cell specific factors in these cells (e.g., Pax-5). We will also address the regulation of functional Pax-5 activity in resting vs. activated B cells. In summary, our goal is to understand how these proteins interact to regulate target genes in B lymphocytes and thus, control differentiation. Regulation of the developmentally important mb-1 gene by Pax-5 suggests a mechanism for the absence of mature B cells in genetically altered mice lacking functional pax-5 genes. Ultimately, our study may define a novel mechanism for control of normal and abnormal cell proliferation (including B lymphomas and leukemias), because both Pax-5 and Ets proteins have been implicated as regulators of proliferation in B cells.