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(Received for publication, July 31, 1996)
From the Department of Pediatrics, Washington University School of
Medicine, St. Louis, Missouri 63110
Factor B (Bf) is a constituent of the alternative
pathway of complement activation encoded within the major
histocompatibility complex. Transcription of the murine gene from two
initiation sites generates two Bf mRNA species differing in size
and tissue distribution. Striking genetic, tissue-specific differences
in Bf mRNA levels at extrahepatic sites (kidney and intestine)
among mouse strains correlate with a DNA sequence polymorphism in the 5
Volume 271, Number 47,
Issue of November 22, 1996
pp. 30205-30211
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
-flanking region of the gene and differential nuclear protein binding
at the Bf upstream transcriptional initiation site (UIS). To ascertain
the functional consequences of this polymorphism in the Bf promoter, we
analyzed the effects of strain-specific sequences in the Bf 5
region
on the expression of a chloramphenicol acetyltransferase (CAT) reporter
gene transfected in human and mouse hepatoma cells. The CAT activity
and mRNA level produced when transcription was driven by the
sequence of strains with high extrahepatic expression were reduced to
background levels when the sequence specific to the low expressor
strains was used. Eighty percent of this difference was accounted for
by a point substitution that affects DNA-protein interaction at the
UIS, the sequence of higher affinity conferring higher expression. Hepatocyte nuclear factor 4 (HNF-4), derived from HepG2, mouse liver
and kidney or cell-free translation of HNF-4 RNA, is the nuclear
protein that preferentially binds to the high expressor UIS. Bf-CAT is
not expressed in cells that lack HNF-4 (CV-1). However, co-transfection
of HNF-4 into CV-1 cells drives Bf-CAT expression and reproduces the
differences derived from the substitution that affect HNF-4 binding
in vitro. These data show that interaction of HNF-4 with
polymorphic variants of the upstream Bf promoter is the major
determinant of strain-specific extrahepatic factor B expression.
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