The human lα1 and lα2 germline promoter elements: proximal positive and distal negative elements may regulate the tissue specific expression of Cα1 and Cα2 germline transcripts

Abstract

Treatment of human splenic B lymphocytes with the mitogen Branhamella catarrhalis (BC) and transforming growth factor-β (TGF-β) induces expression of germline Ig C_α transcripts and class switching to this Isotype. To further characterize the molecular mechanism by which TGF-β and mltogenic signals regulate the expression of unrearranged C_α1 and C_α2 genes, we have characterized the promoter elements that are responsible for the transcrlptlonal activation of their corresponding germline genes using transient expression assays. We report here that both in the l_α1 and the l_α2 regions, maximal phorbol myrlstate acetate (PMA) and TGF-β responsiveness of the promoters can be conferred by 327 bp spanning the transcription initiation sites and a previously identified phylogenetlcally conserved region. The expression of these 327 bp segments is not restricted to the B cell lineage since they are also active in the erythroleukemla cell line K562 as well as the B cell lines Raji and DG75. Mutatlonal analyses have demonstrated the Importance of sequences within the 327 bp segment that contain a putative cyclic AMP responsive element binding protein (CREB) binding site for TGF-β and PMA responsiveness and putative PU-1 and Sp1 binding sites for basal promoter activity. Upstream distal elements that could negatively modulate the expression of the l_α1 and l_α2 promoters, particularly in non-B cells, have been identified. Three such elements were mapped between positions −352 to −243, −627 to −516, and upstream of position −731 respectively. The Influence of these elements presumably contributes to the B cell specific expression of the l_α1 and 1_α2 promoters. The l_α1 and l_α2 promoters were found to be functionally indistinguishable from each other with respect to their basal level of expression, and their responsiveness to TGF-β₁.