SpRunt-1, a New Member of the Runt Domain Family of Transcription Factors, Is a Positive Regulator of the Aboral Ectoderm-SpecificCyIIIAGene in Sea Urchin Embryos

In this paper we present a structural and functional characterization of a new sea urchin embryo transcription factor, SpRunt-1. This factor was isolated by means of its specific interaction with acis-regulatory target site of theCyIIIagene. Here we show that this target site, the P7I site, is required for normal embryonic activation ofCyIIIa·CATreporter gene constructs. An oligonucleotide affinity column bearing the P7I target site purifies a 21-kDa polypeptide from blastula-stage nuclear extracts, and the amino acid sequence obtained from this polypeptide was used to generate a nucleic acid probe with which the corresponding cDNA was cloned. The cDNA encodes an approximately 60-kDa protein, SpRunt-1, which includes a “runt domain” that is closely homologous to those ofDrosophilaand mammalian runt domain transcription factors. RNA and genomic blots show that SpRunt-1 is represented by a single embryonic transcript, encoded by one of possibly two runt-domain-containing genes. By RNA probe protection we found that transcripts of SpRunt-1 increase in concentration dramatically after the blastula stage of development, suggesting that the up-regulation ofCyIIIathat occurs after blastula stage is a function of zygotically transcribed SpRunt-1. These results are discussed with reference to known features of the runt domain family of transcription factors.