The dimerization/repression domain of RFX1 is related to a conserved region of its yeast homologues Crt1 and Sak1: A new function for an ancient motif

The RFX protein family includes members from yeast to humans, which functio n in various biological systems, and share a DNA-binding domain and a conse rved C-terminal region. In the human transcription regulator RFX1, the cons erved C terminus is an independent functional domain, which mediates dimeri zation and transcriptional repression. This dimerization domain has a uniqu e ability to mediate the formation of two alternative homodimeric DNA-prote in complexes, the upper of which has been linked to repression. Here, we lo calize the complex formation capacity to several different RFX1 C-terminal subregions, each of which can function independently to generate the upper complex and repress transcription, thus correlating complex formation with repression. To gain an evolutionary perspective, we have examined whether t he different properties of the RFX1 C terminus exist in the two yeast RFX p roteins, which are involved in signaling pathways. Replacement of the RFX1 C terminus with those of Saki and Crt1, its orthologues from Schizosaccharo myces pombe and Saccharomyces cerevisiae, respectively, and analysis of fus ions with the Gal4 DNA-binding domain, revealed that the ability to generat e the two alternative complexes is conserved in the RFX family, from S. cer evisiae to man. While sharing this unique biochemical property, the three C termini differed from each other in their ability to mediate dimerization and transcriptional repression. In both functions, RFX1, Saki, and Crt1 sho wed high capacity, moderate capacity, and no capacity, respectively. This c omparative analysis of the RFX proteins, representing different evolutionar y stages, suggests a gradual development of the conserved C terminus, from the appearance of the ancestral motif (Crt1), to the later acquisition of t he dimerization/repression functions (Sak1), and finally to the enhancement of these functions to generate a domain mediating highly stable protein-pr otein interactions and potent transcriptional repression (RFX1). (C) 1999 A cademic Press.