ACTIVATION OF TRANSCRIPTION OF THE MELANOMA INDUCING XMRK ONCOGENE BYA GC BOX ELEMENT

Melanoma formation in Xiphophorus is caused by overexpression of the X mrk gene. The promoter region of the Xmrk oncogene differs strikingly from the corresponding proto-oncogenic sequences and was acquired in t he course of a nonhomologous recombination with another gene locus, D. In order to identify regulatory elements leading to the strong transc riptional activation of Xmrk in melanoma tissue and to contribute to a n understanding of the role the regulatory locus R might play in suppr essing the tumor phenotype in wild-type Xiphophorus, we performed func tional analysis of the Xmrk oncogene promoter. Transient transfections in melanoma and nonmelanoma cells revealed the existence of a potent positive regulatory element positioned close to the transcriptional st art site, Contained within this promoter segment is a GC-rich sequence identical to the binding site described for human Sp1. In vitro bindi ng studies and biochemical characterizations demonstrated the existenc e of GC-binding proteins in fish that share immunological properties w ith members of the human Sp family of transcription factors and appear to be involved in the high transcriptional activation of the Xmrk onc ogene. Since the identified cis element is functional in both melanoma and nonmelanoma cells, additional silencer elements suppressing Xmrk expression in nonpigment cells must exist, thereby suggesting a negati ve regulatory function for the genetically defined R locus.