A NATURALLY-OCCURRING SHORT VARIANT OF THE FTZ-F1-RELATED NUCLEAR ORPHAN RECEPTOR XFF1RA AND INTERACTIONS BETWEEN DOMAINS OF XFF1RA

The FTZ-F1-related nuclear orphan receptors xFF1rA and B were identifi ed previously in Xenopus laevis by cDNA cloning (1). In addition to tw o cDNAs that encode full-length receptor proteins, a third cDNA encode s a form of xFF1rA truncated at the C terminus. Transcripts encoding t he short form of the receptor are present at much lower levels than mR NAs encoding the full-length receptors. Significant activation of repo rter genes in xFF1rA-transfected HeLa cells requires two or more copie s of a FTZ-F1-responsive element (FRE). However, in vitro, recombinant xFF1rA protein binds FRE monomers and dimers with apparently equal af finity. In cotransfection studies, full-length xFF1rA activates transc ription, in contrast to xFF1rAshort. In vitro, xFF1rAshort binds to FR E with a lower efficiency than xFF1rA. A partial truncation of the E d omain reduces the DNA-binding activity of domain C, suggesting that pa rts of the E domain might interact with the DNA-binding domain C. In p arallel with the loss of DNA-binding efficiency, such truncations lead to loss of transcriptional activation. For transcriptional activation , either the A/B domain or the complete E domain is required, as shown by recombination of different domains of xFF1rA with the DNA-binding domain of Gal4. Coexpression of the truncated form xFF1rAshort decreas es transcriptional activation by xFF1rA, but not by the active Gal4-xF F1rA fusion protein that contains domain E. This indicates that xFF1rA short interferes with xFF1A by competition for FRE binding. An excess of xFF1rAshort is required, presumably due to its poor FRE-binding act ivity. The function of the E domain in regulating DNA-binding and tran scriptional activation is discussed.