The purine-rich DNA-binding protein OsPurα participates in the regulation of the rice sucrose synthase 1 gene expression.

The rice sucrose synthase 1 (RSus1) gene is transcriptionally induced by sucrose, and a region within its promoter, at -1117 to -958 upstream of the transcription initiation site, was found to be essential for enhancing the sucrose-induced expression. Further dissection of this region revealed that a group of nuclear proteins interact with a 39-bp fragment named A-3-2 (-1045 to -1007). A protein that specifically and directly interacted with A-3-2 was isolated from the suspension-cultured cells of rice and was subsequently identified as a purine-rich DNA-binding protein. The amino acid sequence of this protein, OsPurα, exhibited 73% identity with the Arabidopsis Purα-1 protein, and its modeled structure resembled the structure of Pur-α in Drosophila. Recombinant OsPurα expressed and purified from Escherichia coli was demonstrated to have DNA-binding activity and to interact with A-3-2 specifically. Moreover, OsPurα was able to enhance sucrose-induced expression of the β-glucuronidase (GUS) reporter gene, which was transcriptionally fused to two copies of a DNA fragment containing A-3-2 and the cauliflower mosaic virus 35S minimal promoter, in vivo. The level of OsPurα bound to A-3-2 was higher in cells cultured in the presence of sucrose; however, the level of OsPurα mRNA in cells was not affected by sucrose. The results of this study demonstrate that OsPurα participates in the regulation of RSus1 expression in response to sucrose; nevertheless, it may require other partner proteins for full function.