The promoter of the protoplast auxin-regulated (parAt) gene of tobacco, which is expressed throughout the tissues of hairy roots, can be useful for developing a bioconversion system with hairy roots. The parAt gene is shown to be expressed in roots of seedlings and in those of mature tobacco plants. The 5’-upstream region of parAt was fused to the coding sequence of the beta-D-glucuronidase (GUS) gene to generate the parAt-GUS fusion gene, which was introduced into the binary vector for Agrobacterium. Hairy roots that carried the fusion gene were obtained (parAt-GUS/hairy root) by infecting tobacco plants with A. rhizogenes carrying the fusion gene in the binary vector. Biochemical analysis with 4-methylumbelliferyl beta-D-glucuronide (MUG), a substrate for GUS, showed that the level of GUS activity was tenfold higher than that of hairy roots carrying the reporter GUS gene, which is linked to the cauliflower mosaic virus 35S RNA promoter.(35S-GUS/hairy root). We also examined the rate of conversion of MUG to 4-methylumbelliferone (MU) by hairy roots when MUG was added to the culture medium of the parAt-GUS/hairy roots. The hairy roots converted MUG to MU at more than ten times as high efficiency as the 35S-GUS/hairy roots. In addition to tobacco, the parAt-GUS gene was similarly expressed in hairy roots from Atropa and Arabidopsis. These results suggest that the promoter of the parAt gene is a useful tool for conversion of various metabolites by hairy root cultures.