We have isolated a cDNA that encodes the murine CCAAT-binding protein mYB-1. The deduced amino acid sequence shows 95% identity with its presumed human homologue (hYB-1A) which was originally isolated as a protein that bonds to the Y box of MHC class ll genes. In vitro translated mYB-1 binds to CCAAT boxes of the MHCIlEα, HSVTK and mouse PCNA promoters but not to αglobin or human thymidine kinase CCAAT boxes. Interestingly, complexes formed between the in vitro translated protein and the various CCAAT boxes display the property of being competed more efficiently with self competitor ONA, regardSess of the CCAAT box initially used as a probe. A similar phenomenon was observed in a cell extract of Con-A stimulated murine splenocytes when the same competition assays were performed. These results may reflect the generation of multiple forms of a particular CCAAT-binding protein, such as mYB-1, that display distinct, yet overlapping, DNA binding specificities.
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