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要旨:
The FTZ-F1-related nuclear orphan receptors xFF1rA and B were identified previously in Xenopus laevis by cDNA cloning. In addition to two cDNAs that encode full-length receptor proteins, a third cDNA encodes a form of xFF1rA truncated at the C terminus. Transcripts encoding the short form of the receptor are present at much lower levels than mRNAs encoding the full-length receptors. Significant activation of reporter genes in xFF1rA-transfected HeLa cells requires two or more copies of a FTZ-F1-responsive element (FRE). However, in vitro, recombinant xFF1rA protein binds FRE monomers and dimers with apparently equal affinity. In cotransfection studies, full-length xFF1rA activates transcription, in contrast to xFF1rAshort. In vitro, xFF1rAshort binds to FRE with a lower efficiency than xFF1rA. A partial truncation of the E domain reduces the DNA-binding activity of domain C, suggesting that parts of the E domain might interact with the DNA-binding domain C. In parallel 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 decreases transcriptional activation by xFF1rA, but not by the active Gal4-xFF1rA fusion protein that contains domain E. This indicates that xFF1rAshort interferes with xFF1A by competition for FRE binding. An excess of xFF1rAshort is required, presumably due to its poor FRE-binding activity. The function of the E domain in regulating DNA-binding and transcriptional activation is discussed.
