Transcriptional factor that can act as an activator or repressor depending on isoform and/or post-translational modifications. Binds to GT and GC boxes promoter elements. Competes with SP1 for the GC-box promoters. Weak activator of transcription but can activate a number of genes involved in different processes such as cell-cycle regulation, hormone-induction and house-keeping (By similarity). Interacts with HLTF; the interaction may be required for basal transcriptional activity of HLTF. Interacts with HDAC1; the interaction deacetylates SP3 and regulates its transcriptional activity. Interacts with HDAC2 (preferably the CK2-phosphorylated form); the interaction deacetylates SP3 and regulates its transcriptional activity. Ceramides can also regulate acetylation/deacetylation events through altering the interaction of HDAC with SP3. Interacts with MEIS2 isoform Meis2D and PBX1 isoform PBX1a (By similarity). Localizes to the nuclear periphery and in nuclear dots when sumoylated. Some localization in PML nuclear bodies (By similarity). The 9aaTAD motif is a transactivation domain present in a large number of yeast and animal transcription factors. Acetylated by histone acetyltransferase p300, deacetylated by HDACs. Acetylation/deacetylation states regulate transcriptional activity. Acetylation appears to activate transcription. Alternate sumoylation and acetylation at Lys-553 also control transcriptional activity (By similarity). Sumoylated on all isoforms. Sumoylated on 2 sites in longer isoforms with Lys-553 being the major site. Sumoylation at this site promotes nuclear localization to the nuclear periphery, nuclear dots and PML nuclear bodies. Sumoylation on Lys-553 represses the transactivation activity, except for the largest isoform which has little effect on transactivation. Alternate sumoylation and acetylation at Lys-553 also control transcriptional activity (By similarity). Belongs to the Sp1 C2H2-type zinc-finger protein family.