EMBRYONIC ENHANCER-BINDING PROTEIN SSAP CONTAINS A NOVEL DNA-BINDING DOMAIN WHICH HAS HOMOLOGY TO SEVERAL RNA-BINDING PROTEINS

Stage-specific activator protein (SSAP) is a 43-kDa poly-peptide that binds to an enhancer element of the sea urchin late histone H1 gene. T his enhancer element mediates the transcriptional activation of the la te histone H1 gene in a temporally specific manner at the mid-blastula stage of embryogenesis. We have cloned cDNAs encoding SSAP by using p olyclonal antibodies raised against purified SSAP to screen expression libraries, SSAP is unrelated to previously characterized transcriptio n factors; however, it exhibits striking homology to a large family of proteins involved in RNA processing, The protein is a sequence-specif ic DNA-binding protein that recognizes both single- and double-strande d DNA. The DNA-binding domain of the protein was localized to the cons erved RNA recognition motif (RRM), In addition to tandem copies of thi s conserved domain, SSAP contains a central domain that is rich in glu tamine and glycine and a C-terminal domain that is enriched in serine, threonine, and basic amino acids. Overexpression of SSAP in sea urchi n embryos by microinjection of either synthetic mRNA or an SSAP expres sion vector results in four- to eightfold transactivation of target re porter genes that contain the enhancer sequence, Transactivation occur s beginning only at the mid-blastula stage of development, suggesting that SSAP must be modified in a stage-specific manner in order to acti vate transcription, In addition, there are a number of other RRM-conta ining proteins that contain glutamine-rich regions which are postulate d to function in the regulation of RNA processing, Instead, we suggest that SSAP is a member of a family of glutamine-rich RRM proteins whic h constitute a novel class of transcription factors.