Nucleic Acids Research, Vol 24, Issue 19 3661-3669, Copyright © 1996 by Oxford University Press
M Collins and P Bornstein
Metaxin (Mtx) is an essential nuclear gene which is expressed ubiquitously
in mice and encodes a mitochondrial protein. The gene is located upstream
and is transcribed divergently from the thrombospondin 3 (Thbs3) gene; 1352
nucleotides separate the putative translation start sites. Although the Mtx
and Thbs3 genes share a common intergenic region, transient transfection
experiments in rat chondro-sarcoma cells and in NIH-3T3 fibroblasts
demonstrated that the elements required for expression of the Mtx gene are
situated within a short proximal promoter and have no major effect on the
transcription of Thbs3. The metaxin --377 bp promoter contains four
clustered GC boxes between nucleotides --146 and --58 and an inverted GT
box between nucleotides -- 152 and --161, but does not contain TATA or
CCAAT boxes. Like many genes regulated by a TATA-less promoter, the
transcription start site of metaxin is heterogeneous. The major start site
is only 13 bp upstream from the putative translation start site.
Electrophoretic mobility shift, competition and supershift assays showed
that the ubiquitous transcription factor, Sp1, and, to a lesser extent, the
Sp1- related protein, Sp3, bind to four of these Sp1-binding motifs. Co-
transfection of metaxin promoter-luciferase constructs and an Sp1
expression vector into Schneider Drosophila cells, which do not synthesize
Sp1, demonstrated that the metaxin gene is activated by Sp1. Deletion of
the four upstream Sp1-binding elements, on the other hand, demonstrated
that these motifs are superfluous in context of the larger Mtx promoter.
Thus, despite the potential for common regulatory mechanisms, the available
evidence indicates that the Mtx minimal promoter does not significantly
affect Thbs3 gene expression.
ARTICLES
SP1-binding elements, within the common metaxin-thrombospondin 3 intergenic region, participate in the regulation of the metaxin gene
Department of Biochemistry, University of Washington, Seattle 98195, USA.
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