Isolation and Characterization of the Mouse Gene for the Type 3 Iodothyronine Deiodinase

The type 3 iodothyronine deiodinase (D3) is a selenoenzyme that inactivates thyroid hormones by removing a iodine from the 5-position of the tyrosyl ring. D3 is highly expressed in many tissues during the early stages of development, and its activity is regulated by selected growth factors and various hormones. To gain further insights into the structure, functional role, and regulation of this enzyme, we screened a mouse liver genomic library with a rat D3 complementary DNA probe and isolated a 12-kb clone coding for the Dio3. Restriction analysis followed by Southern blotting and nucleotide sequencing demonstrated that the Dio3 contains a single exon, 1853 bp in length, that encodes the entire length of the messenger RNA expressed in murine placenta and neonatal skin. Primer extension experiments identified two potential transcriptional start sites located 77 and 60 nt upstream of the ATG translational start codon. The region immediately 5' to the start sites contains consensus TATA, CAAT, and GC elements. Furthermore, a 526-nucleotide genomic fragment from this region was demonstrated to efficiently drive a luciferase reporter construct when transfected into COS-7, XTC-2, or XL-2 cells or into primary cultures of rat preadipocytes derived from neonatal brown fat. In conclusion, D3 transcripts in the placenta and skin are encoded by the Dio3 gene from a single exon whose expression is regulated by an upstream region that contains several consensus promoter elements. Further characterization of this gene will provide new insights into the factors regulating the unique pattern of D3 expression during development.