Regulatory elements of the mb-1 gene encoding the Ig-alpha component of the human B-cell antigen receptor.

The mb-1 gene encodes the Ig-alpha component of the B-cell antigen receptor. It is specifically expressed in pre-B and mature B cells but not in plasma cells losing membrane Ig (mIg) expression. We looked for transcriptional regulatory elements within a 12 kb genomic fragment. A strong promoter activity was found in a 591 bp fragment harboring consensus binding sites for known transcription factors including Ets, EBF/BlyF, LyF1/micro B and Spl. It was able to drive transcription of a reporter gene in the absence of any additional enhancer and was mostly active in B lymphocytes not in plasma cells or T cells. Although no fragment from the mb-1 gene displayed enhancer activity in combination with either the SV40, a Ig VH or a Ig VL promoter, a 1078 bp fragment corresponding to the 5' part of the gene behaved as a strong enhancer in either orientation in constructs driven by the mb-1 promoter itself. Deletions within this fragment allowed to delineate shorter sequences with enhancer activity upstream the first exon. The tissue-restricted, promoter-restricted and stage-specific activity of this 5' flanking region suggests that it is the main regulatory element of the mb-1 gene.