Pre-B cell-specific λ5 gene expression due to suppression in non pre-B cells

The λ5 gene is expressed specifically in pre-B cells; during B lymphocyte differentiation the expression of the λ5 gene is turned on in pre-B cells and turned off again at the mature B cell stage. No other cell type has yet been found to express the λ5 gene. The pre-B cell-specific expression of the λ5 gene is regulated at the level of transcription. We asked whether the region 5′ of the λ5 gene (5′λ5) could explain the stage- and tissue-specific expression. The λ5 promoter lacks a TATA box and transcription is initiated at multiple sites. In the presence of a heterologous enhancer, 5′λ5confers pre-B cell-specific expression on the reporter gene chloramphenlcol acetyl-transferase. Deletion analysis of 5′λ5defines two separaste regions, referred to as Aλ5 and Bλ5. Region Bλ5 suppresses the expression in non pre-B cells since deletion of Bλ5 allows Aλ5 to promote transcription of the reporter gene in all cell types tested. In addition, Bλ5 acts as an enhancer on the heterologous χ light chain promoter in pre-B cells but not in B cells. Thus region Aλ5 functions as a basal promoter in all cell types tested. Region Bλ5, In concert with a heterologous enhancer, acts as a suppressing region in non-pre-B cells and therefore confers pre-B cell specificity on the expression of the λ5 gen