Transcription factor HUB1 represses Sp1-mediated gene expression through the CACCC box of HTLV-I U5RE but not the GC box

Human T-cell leukemia virus type I U5 repressive element binding protein 1 (HTLV-I U5RE Binding protein 1; HUB1 and renamed ZNF282) - a member of the Krüppel type zinc finger family - represses HTLV-I long terminal repeat (LTR)-mediated transcription by binding to the TCCACCC motif of the U5RE. Sp1 and other Sp1 family proteins also recognize the CACCC box of the U5RE, as well as the GC box (GGGCGG). We would therefore expect HUB1 to compete with Sp1 for binding to the former. In the present study, we demonstrated that Sp1 activates transcription via the U5RE, and that HUB1 represses these Sp1-mediated effects. Electrophoretic mobility shift assays (EMSA) confirmed that HUB1 was bound to the CACCC box, but not the GC box. Consistent with that finding, overexpression of HUB1 repressed Sp1-mediated transcription reporter genes controlled via the CACCC box, but not via the GC box. These results suggest that, by binding to the CACCC box, HUB1 represses the Sp1-dependent transcriptional activation.