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. 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. Interacts with MEIS2 isoform 4 and PBX1 isoform PBX1a. Localizes to the nuclear periphery and in nuclear dots when sumoylated. Some localization in PML nuclear bodies. Ubiquitously expressed. The 9aaTAD motif is a transactivation domain present in a large number of yeast and animal transcription factors. Not glycosylated. 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-551 also control transcriptional activity. Ceramides can also regulate acetylation/deacetylation events through altering the interaction of HDAC with SP3. In vitro, C(18)-ceramides, but not C(16)-ceramides, increase the interaction of HDAC1 with SP3 and enhance the deacetylation of SP3 and the subsequent repression of the TERT promoter. Sumoylated on all isoforms. Sumoylated on 2 sites in longer isoforms with Lys-551 being the major site. Sumoylation at this site promotes nuclear localization to the nuclear periphery, nuclear dots and PML nuclear bodies. Sumoylation on Lys-551 represses the transactivation activity, except for the largest isoform, L-Sp3, which has little effect on transactivation. Alternate sumoylation and acetylation at Lys-551 also control transcriptional activity. Produced by alternative initiation at Met-13 of isoform 1. Produced by alternative initiation at Met-286 of isoform 1. Produced by alternative initiation at Met-303 of isoform 1. Produced by alternative splicing. An AUA codon is translated into Met and used as a translation initiation site (in vitro). Produced by alternative splicing. Belongs to the Sp1 C2H2-type zinc-finger protein family.