In embryonic chicken liver (ECL) two types of iodothyronine deiodinases are 
expressed: D1 and D3. D1 catalyzes the activation as well as the inactivation of 
thyroid hormone by outer and inner ring deiodination, respectively. D3 only 
catalyzes inner ring deiodination. D1 and D3 have been cloned from mammals and 
amphibians and shown to contain a selenocysteine (Sec) residue. We characterized 
chicken D1 and D3 complementary DNAs (cDNAs) and studied the expression of 
hepatic D1 and D3 messenger RNAs (mRNAs) during embryonic development. 
Oligonucleotides based on two amino acid sequences strongly conserved in the 
different deiodinases (NFGSCTSecP and YIEEAH) were used for reverse 
transcription-PCR of poly(A+) RNA isolated from embryonic day 17 (E17) chicken 
liver, resulting in the amplification of two 117-bp DNA fragments. Screening of 
an E17 chicken liver cDNA library with these probes led to the isolation of two 
cDNA clones, ECL1711 and ECL1715. The ECL1711 clone was 1360 bp long and lacked 
a translation start site. Sequence alignment showed that it shared highest 
sequence identity with D1s from other vertebrates and that the coding sequence 
probably lacked the first five nucleotides. An ATG start codon was engineered by 
site-directed mutagenesis, generating a mutant (ECL1711M) with four additional 
codons (coding for MGTR). The open reading frame of ECL1711M coded for a 
249-amino acid protein showing 58-62% identity with mammalian D1s. An in-frame 
TGA codon was located at position 127, which is translated as Sec in the 
presence ofa Sec insertion sequence (SECIS) identified in the 3'-untranslated 
region. Enzyme activity expressed in COS-1 cells by transfection with ECL1711M 
showed the same catalytic, substrate, and inhibitor specificities as native 
chicken D1. The ECL1715 clone was 1366 bp long and also lacked a translation 
start site. Sequence alignment showed that it was most homologous with D3 from 
other species and that the coding sequence lacked approximately the first 46 
nucleotides. The deduced amino acid sequence showed 62-72% identity with the D3 
sequences from other species, including a putative Sec residue at a 
corresponding position. The 3'-untranslated region of ECL1715 also contained a 
SECIS element. These results indicate that ECL1711 and ECL1715 are 
near-full-length cDNA clones for chicken D1 and D3 selenoproteins, respectively. 
The ontogeny of D1 and D3 expression in chicken liver was studied between E14 
and 1 day after hatching (C1). D1 activity showed a gradual increase from E14 
until C1, whereas D1 mRNA level remained relatively constant. D3 activity and 
mRNA level were highly significantly correlated, showing an increase from E14 to 
E17 and a strong decrease thereafter. These results suggest that the regulation 
of chicken hepatic D3 expression during embryonic development occurs 
predominantly at the pretranslational level.