Ubiquitin (Ub) plays a crucial role in almost every aspect of cellular functions. It is encoded by four genes, of which UbC is known to meet cell demand for ubiquitin in both basal and stressful conditions. To understand the molecular mechanisms regulating UbC gene expression, we performed a functional characterization of the UbC promoter. Deletion analyses on the 5' end of the -916/+878 promoter region, excluded the functional importance of nt -916/-371 in the transcriptional regulation of the gene, while 3' deletions revealed that intron removal (nt+65/+876) resulted in a marked reduction of promoter activity in all the reporter constructs, regardless of the cell types. Intron substitution with a heterologous chimeric intron failed to restore promoter activity, thus allowing to exclude that the splicing event, per se, can be responsible for the intron-mediated burst of transcription. Gel shift assays demonstrated nuclear factor binding with the +137/+766 intron region. Reporter constructs carrying partial intron deletions confirmed that this sequence is, indeed, required for maximal transcriptional activity. Computer-based analysis found potential Sp1 binding motifs within the intron sequence and electrophoretic mobility shift and supershift assays demonstrated that both Sp1 and Sp3 transcription factors interact, in vitro, with the UbC intron, at multiple binding sites. Moreover, ectopic expression of Sp1 and Sp3 revealed that both transcription factors positively regulate UbC promoter activity. Collectively, our data highlight the very new evidence that the 5'-UTR intron is crucial in regulating UbC gene expression and provide insights into the pivotal role of Sp1/Sp3 binding to the intronic enhancer in the regulation of UbC transcription.