The two-hybrid system was used to isolate cDNA clones encoding polypeptides that interact with the N-terminal region (activation domains A, B and C) of the Sp1 transcription factor. Among the 65 collected clones, 43 contained cDNA fragments with open reading frames. They corresponded to 13 genes encoding proteins of known function and to 15 genes, the proteins of which have no known function. Six overlapping cDNA clones corresponded to the Hsc70 protein. Host cell factor (HCF-1) and the KIAA0461 gene (encoding a putative Zn-finger protein of unknown function) were both identified through the isolation of three overlapping cDNA clones. Two cDNA fragments encoding the same region of the SREBP-2 transcription factor were independently selected and two overlapping cDNA clones corresponded to the splicing factor SF3A120. Two different cDNA clones encoded the N- and C-terminal region of the Oct-1 transcription factor. Transcription factors Elf-1 and TIEG, as well as HSph2, the putative human homologue of a murine polyhomeotic gene, were each represented by a single clone. Noticeably, for the four identified transcription factors, the DNA-binding domain was excluded from the selected polypeptides. In vitro binding of the selected polypeptides to the Sp1 protein was demonstrated for the four transcription factors and for the SF3A120, Hsc70, HCF-1, HSph2 and pKIAA0461(245) proteins. Four other cDNA clones encoding polypeptides of unknown function were tested in the in vitro binding assay. All four polypeptides were found to interact with Sp1 in this assay.
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