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) and mb-1 (Ig
) Promoters1
Department of Microbiology and Immunology and Molecular Biology Institute, University of California School of Medicine, Los Angeles, CA 90095
The B29 (Ig
) and mb-1 (Ig
) gene
products are B cell-specific essential components of the B cell
receptor that are coexpressed at all stages of B cell differentiation,
with the exception of plasma cells, which lack mb-1
expression. Transcription of both genes is governed by a similar
cassette of interactive transcription factor-binding elements,
including octamer motifs, in TATA-less promoters. In this study, we
show the B cell-specific B29 gene promoter is
transactivated in B and non-B cells by cotransfection with the B
cell-specific octamer cofactor gene, Bob1 (OCA-B/OBF-1). The expression
of Bob1 is also sufficient to override the silencing effects of the
B29 silencer. This indicates that Bob1 plays a critical
role in B cell-specific B29 promoter expression. In
contrast, coexpression of Bob1 had no effect on mb-1
promoter activity. Bob1 transactivation only occurs with select octamer
sequences that have an adenosine at position 5 (ATGCAAAT). The
B29 promoter conforms to this consensus octamer motif,
while the mb-1 promoter octamer motif does not. Octamer
motif swapping between B29 and mb-1
promoters renders B29 unresponsive to Bob1
transactivation and makes mb-1 competent for Bob1
transactivation, thereby indicating that the B29 octamer
motif is solely responsible for Bob1 interaction. Additionally, the
mb-1 construct containing the B29 octamer
motif is expressed in a plasmacytoma cell line, while the wild-type
mb-1 promoter is not. Bob1 transactivation of
B29 and the lack of this transactivation of
mb-1 account for the differential expression of
B29 and mb-1 in terminally differentiated
plasma cells.
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