Transcriptional regulation of expression of the human thiamin transporter-2 (the product of the SLC19A3 gene) is unknown. In this study, we cloned the 5'-regulatory region of the human SLC19A3 gene (2,016 bp), identified the minimal promoter region required for basal activity, demonstrated a critical role for specific cis- regulatory elements in determining the promoter activity, and confirmed activity and physiological relevance of the cloned SLC19A3 promoter in vivo. Using transiently transfected human intestinal epithelial Caco-2 cells and 5'-deletion analysis, the minimal promoter region required for basal activity of the SLC19A3 promoter was found to be encoded in a sequence between -77 and +59, using the start of transcription initiation as position 1. This minimal region was found to contain a number of putative cis- regulatory elements, with a critical role for an SP1/GC-box binding site (at position -48/-45 bp) established by means of mutational analysis. Using electromobility shift and supershift assays, the binding of SP1 (and SP3) to the minimal promoter region was also demonstrated. In transiently transfected Drosophila SL2 cells, both SP1 and SP3 transactivated the SLC19A3 minimal promoter in a dose-dependent manner, and in combination demonstrated an additive stimulatory effect. Functionality of the full-length SLC19A3 promoter was confirmed in vivo in transgenic mice expressing the promoter-luciferase reporter gene. These studies report the first characterization of the SLC19A3 promoter in vitro and in vivo, and demonstrate the importance of an SP1 cis-regulatory element in regulating promoter activity of this important human gene.