The movement of urea across plasma membranes is modulated by facilitated urea transporter proteins. These proteins are the products of two closely related genes, termed UT-A (Slc14a2) and UT-B (Slc14a1). By genomic library screening and P1 artificial chromosome (PAC) clone ‘shotgun’ sequencing, we have determined the structure of the mouse UT-A gene. The gene is greater than 300 kb in length, contains 24 exons and has two distinct promoters. Flanking the 5Ì-region of the gene is the UT-Aα promoter that regulates transcription of UT-A1, UT-A3 and UT-A5. The second promoter, termed UT-Aβ, is present in intron 13 and regulates transcription of UT-A2. cAMP agonists (100 mM dibutryl cAMP, 25 mM forskolin, 0.5 mM 3-isobutyl-1-methylxanthine) significantly increased the activity of a 2.2 kb UT-Aα promoter construct 6.2-fold (0.026 ± 0.003 to 0.160 ± 0.004 RLU (µg protein)^-1, means ± S.D., n = 3, P < 0.05, t test) and a 2.4 kb UT-Aβ promoter construct 9.5-fold (0.020 ± 0.002 to 0.190 ± 0.043 RLU (µg protein)^-1, n = 3, P < 0.05, t test) above untreated controls. Interestingly, only the UT-Aβ promoter contained consensus sequences for cyclic AMP responsive elements and deletion of these elements abolished cAMP sensitivity. Increasing the tonicity of culture medium from 300 to 600 mosmol with NaCl caused a significant increase (0.06 ± 0.004 to 0.095 ± 0.01 RLU (µg protein)^-1, n = 3, P < 0.05, t test) in UT-Aα promoter activity, but had no effect on the UT-Aβ promoter. A TonE response element was identified in UT-Aα and is suggested to be responsible for mediating this effect. Levels of UT-A2 and UT-A3 mRNA were increased in dehydrated mice compared with control animals, indicating that the activities of both promoters are likely to be elevated during prolonged antidiuresis.

In conclusion, we have determined the genomic structure of the mouse UT-A gene, thus providing data to enable gene deletion/disruption studies. In addition we have shown that the mouse UT-A gene contains two promoters that are differentially regulated by cAMP or tonicity.