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Mol. Cell. Biol., Mar 1995, 1254-1264, Vol 15, No. 3
DJ DeAngelo, J DeFalco, L Rybacki and G Childs
Stage-specific activator protein (SSAP) is a 43-kDa polypeptide that binds
to an enhancer element of the sea urchin late histone H1 gene. This
enhancer element mediates the transcriptional activation of the late
histone H1 gene in a temporally specific manner at the mid- blastula stage
of embryogenesis. We have cloned cDNAs encoding SSAP by using polyclonal
antibodies raised against purified SSAP to screen expression libraries.
SSAP is unrelated to previously characterized transcription factors;
however, it exhibits striking homology to a large family of proteins
involved in RNA processing. The protein is a sequence-specific DNA-binding
protein that recognizes both single- and double-stranded DNA. The
DNA-binding domain of the protein was localized to the conserved RNA
recognition motif (RRM). In addition to tandem copies of this conserved
domain, SSAP contains a central domain that is rich in glutamine and
glycine and a C-terminal domain that is enriched in serine, threonine, and
basic amino acids. Overexpression of SSAP in sea urchin embryos by
microinjection of either synthetic mRNA or an SSAP expression vector
results in four- to eightfold transactivation of target reporter genes that
contain the enhancer sequence. Transactivation occurs beginning only at the
mid-blastula stage of development, suggesting that SSAP must be modified in
a stage- specific manner in order to activate transcription. In addition,
there are a number of other RRM-containing proteins that contain glutamine-
rich regions which are postulated to function in the regulation of RNA
processing. Instead, we suggest that SSAP is a member of a family of
glutamine-rich RRM proteins which constitute a novel class of transcription
factors.
Copyright © 1995, American Society for Microbiology
The embryonic enhancer-binding protein SSAP contains a novel DNA- binding domain which has homology to several RNA-binding proteins
Department of Molecular Genetics, Albert Einstein College of Medicine, Bronx, New York 10461.
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