THE HUMAN I(ALPHA)1 REGION CONTAINS A TGF-BETA-1 RESPONSIVE ENHANCER AND A PUTATIVE RECOMBINATION HOTSPOT

It appears that the switch recombination machinery of a B lymphocyte t argets preferentially unrearranged heavy chain genes that have been re ndered transcriptionally active, Transcriptional activation of the 'ge rmline' human C(alpha)1 and C(alpha)2 genes is triggered by TGF-beta 1 and is controlled by proximal positive and distal negative regulatory elements residing upstream of the alpha 1 and alpha 2 switch regions respectively. In this report we characterize the positive proximal reg ulatory elements and analyse their interaction with DNA binding protei ns, Our data demonstrate that a 100 bp fragment that contains a cAMP r esponsive element (CRE)/activating transcription factor (ATF) motif, a putative Ets binding site and an element that is created by two previ ously described neighbouring direct repeats (DRE), can increase the ba sal level of transcription and confer TGF-beta 1 inducibility to a het erologous promoter in an orientation- and position-independent manner. Ubiquitously expressed DNA binding proteins interact specifically wit h the CRE/ATF, the Ets site and the DRE element. Additionally, nuclear proteins interact with sequences which are located downstream of this enhancer are not essential for transcription in the transient express ion assays utilized; however, they contain motifs that have been previ ously implicated in regulating DNA recombination events. These motifs include a Chi motif and a Chi-like element previously found in the rec ombination hotspot region of the Bcl-2 proto-oncogene and close to chr omosomal breakpoints in T-ALL lines. Our findings raise the possibilit y that the intervening region associated regulatory elements in additi on to regulating the transcriptional activation of the Ig heavy chain genes could also facilitate the physical interaction of transcription and recombination controlling molecular mechanisms.