Gpx1 fw CTAGCGTTAATTAAAGTGCTGCTGTCTCGGGGGGGTTT

1. Gpx1 fw CTAGCGTTAATTAAAGTGCTGCTGTCTCGGGGGGGTTT Gpx1 rev CTAGCGGCGGCCGCACCCATCTCGAGGTGGTATTTTCTGT Gpx2 fw CTAGCGTTAATTAAAGACTTGGGTAAGCTCTGGGCC Gpx2 rev CTAGCGGCGGCCGCTTCTAGCAGAGTGGCTCCAGGC Gpx3 fw CTAGCGTTAATTAACACGGACCCCATGGCAGGGGTGGC Gpx3 rev CTAGCGGCGGCCGCATGCACTAAGGGCTGGCCCCTCAG Gpx4 fw CTAGCGTTAATTAACCACGCCCTCGGAGCCTTCCACCG Gpx4 rev CTAGCGGCGGCCGCGACCTTCCTCCGGCGGGGTGCACG Gpx6 fw CTAGCGTTAATTAACCCACCTCACATGAAGGGAAGG Gpx6 rev CTAGCGGCGGCCGCTGCACTAAGGGACAGGCACCAAGG Dio1 fw CTAGCGTTAATTAATTTTAACTCTGTGTCTTTACATATTTG Dio1 rev CTAGCGGCGGCCGCAGTTTACTGCATCTTATAACATTTTCT Dio2 fw CTAGCGTTAATTAACAGAGATGTGCAGAGTTGACCAGT Dio2 rev CTAGCGGCGGCCGCGAGAGGCAAACTAATGTGACTACATCC Dio3 fw CTAGCGTTAATTAATGTTTTGGGTGCACAGGAGCCCCA Dio3 rev CTAGCGGCGGCCGCTTTAGGTGCTGCTTTGAGGCCCCT Trxr1 fw CTAGCGTTAATTAACATTTGGCAGGGCATCGAAGGGAT Trxr1 rev CTAGCGGCGGCCGCAGGTCATGCTAAAACTGATTTGACAG Trxr2 fw CTAGCGTTAATTAACACCCCCCCCCAGGCTCCTGGTGC Trxr2 rev CTAGCGGCGGCCGCTATTTGCATTGCAGAAATGCCAGGGG Trxr3 fw CTAGCGTTAATTAACTGACAGCGAGAAGCAGTGGGACT Trxr3 rev CTAGCGGCGGCCGCGGGAAGCTCAGTCCTGGCTTGCGA SelH fw CTAGCGTTAATTAACTTTGTGTCCCTGGTGATGTTGGA SelH rev CTAGCGGCGGCCGCTGAACCTTCATTTCTGGCAGGGAA SelI fw CTAGCGTTAATTAATATTTTTCACTGAATGAAGTTTGTGC SelI rev CTAGCGGCGGCCGCATCAACACAGTGGTGCCTATTTTAT SelK fw CTAGCGTTAATTAAACAAGGACTGCTCTGTGTCCTCAC SelK rev CTAGCGGCGGCCGCAACATCTGCATTTATGGAACTGCAT SelM fw CTAGCGTTAATTAAGACCTACCTGCCTGAGTCCTGGAGACAGAA SelM rev CTAGCGGCGGCCGCCCCATCCCTGCTGGCCCGGGG SelN fw CTAGCGTTAATTAAAGGGGCAGTGGCTTCCCCGGCAGC SelN rev CTAGCGGCGGCCGCCTACTTCTTCCCATAGGTGCAGCTGC SelO fw CTAGCGTTAATTAATGCCCTGGCCCATGCACACCCGTC SelO rev CTAGCGGCGGCCGCGGGACCAGGCTGCACTGCTGAGCC SelP1 fw CTAGCGTTAATTAATTTTCTTTTTCCAGTGTTCTATTTGC SelP1 rev CTAGCGGCGGCCGCGTTCTTCCTCCATTCTAAACTGCTAA SelP2 fw CTAGCGTTAATTAATATATTGCTTAGTAAGTATTTCCATAGTC SelP2 rev CTAGCGGCGGCCGCCATTCTTGCTTAATAGTATTAACCATAAA SelS fw CTAGCGTTAATTAATAGGACAGTCTCTGTGACAGGTTGCG SelS rev CTAGCGGCGGCCGCGAAACAAACCCCATCAACTGTCCAC SelT fw CTAGCGTTAATTAAGATCATTGCAAGAGCAGCGTGACT SelT rev CTAGCGGCGGCCGCGAGGTACAACGAGCCTGCCAAGAAA SelV fw CTAGCGTTAATTAAGCAAGGGGTGGAGCTGGAGGAGTC SelV rev CTAGCGGCGGCCGCTGAGCTCACCCCAGCCCCAGCCAC SelW fw CTAGCGTTAATTAACTTGACCCAGCCCCTCTCAGCAGA SelW rev CTAGCGGCGGCCGCGCCCCCAACAGCAGCCACGAGAACA SelX fw CTAGCGTTAATTAACCTGCCAGCCGCCCTGGCCCT SelX rev CTAGCGGCGGCCGCGCCTGCCTTCCCAGCAGGTCACAGA Sel15 fw CTAGCGTTAATTAAAGAGTGAAACATTCACAAAGATTTGCG Sel15 rev CTAGCGGCGGCCGCAAAACTCAAAAATTTGCCAAGTATGT SPS2 fw CTAGCGTTAATTAAACCTGCAACCATCTGACTTGGTCTCTGT SPS2 rev CTAGCGGCGGCCGCCACTGAGGCCTGGGCTCTGAGGCT Supplementary Figure 1. List of primers used for PCR amplification of human SECIS elements and cloning downstream of luciferase UGA258 coding sequence.

2. SelX AUGA fw GGCCCTGGTCACTGCTCCGCTCTGGTCAAA SelX AUGA rev TTTGACCAGAGCGGAGCAGTGACCAGGGCC SelX AAAC fw TCCGCTCTGGTCCCTTCCAGGCCAGCCAGAG SelX AAAC rev CTCTGGCTGGCCTGGAAGGGACCAGAGCGGA Gpx3 AUGA fw AGGGGTGGCGTCTTCGGGAGGGGCCCAAAG Gpx3AUGArev CTTTGGGCCCCTCCCGAAGACGCCACCCCT Gpx3 AAAG fw GGGAGGGGCCCCCCTTGTGGGCGGACCTCC Gpx3 AAAG rev GGAGGTCCGCCCACAAGGGGGGCCCCTCCC LucUAA258 fw GACTCGAAATCCTTATATCAAATATC LucUAA258 rev GATATTTGATATAAGGATTTCGAGTC LucUGU258 fw GACTCGAAATCCACATATCAAATATC LucUGU258 rev GATATTTGATATGTGGATTTCGAGTC Supplementary Figure 2. List of primers used to generate mutations in luciferase UGA258-SelX and luciferase UGA258-Gpx3 constructs.

3. A SelX Gpx3 Ch1 Ch2 Ch3 Ch4 Ch5 Ch6 Ch7 Ch8 Ch9 Ch10 Ch11 Ch12 B Ch1 CACGGACCCCATGGCAGGGGTGGCGTCTTCTGCATGATCCGCTCTGGTCAAACCCTTCCAGGCCAGCCAGAGTGGGGATGGTCTGAGGGGCCAGCCCTTAGTGCAT Ch2 CACGGACCCCATGGCAGGGGTGGCGTCTTCATGATCCGCTCTGGTCAAACCCTTCCAGGCCAGCCAGAGTGGGGAGCCTGTCTGAGGGGCCAGCCCTTAGTGCAT Ch3 CACGGACCCCATGGCAGGGGTGGCGTCTTCATGAGGGAGGGGCCCCAAACCCTTCCAGGCCAGGACCTCCCCTGAGCCTGTCTGAGGGGCCAGCCCTTAGTGCAT Ch4 CCTGCCAGCCGCCCTGGCCCTGGTCACTGCATGATCCGCTCTGGTAAAGCCCTTGTGGGCGCCAGAGTGGGGATGGTCTGTGACCTGCTGGGAAGGCAGGC Ch5 CCTGCCAGCCGCCCTGGCCCTGGTCACTGCATGAGGGAGGGGCCCAAAGCCCTTGTGGGCGGACCTCCCCTGATGGTCTGTGACCTGCTGGGAAGGCAGGC Ch6 CCTGCCAGCCGCCCTGGCCCTGGTCACATGAGGGAGGGGCCCAAAGCCCTTGTGGGCGGACCTCCCCTGAGCCTGTCTGTGACCTGCTGGGAAGGCAGGC Ch7 CCTGCCAGCCGCCCTGGCCCTGGTCACATGATCCGCTCTGGTCAAACCCTTCCAGGCCAGCCAGAGTGGGGAGCCTGTCTGTGACCTGCTGGGAAGGCAGGC Ch8 CACGGACCCCATGGCAGGGGTGGCGTCTTCTGCATGAGGGAGGGGCCCAAAGCCCTTGTGGGCGGACCTCCCCTGATGGTCTGAGGGGCCAGCCCTTAGTGCAT Ch9 CACGGACCCCATGGCAGGGGTGGCGTCTTCTGCATGATCCGCTCTGGTCAAACCCTTCCAGGCCAGCCAGAGTGGGGATGGTCTGAGGGGCCAGCCCTTAGTGCAT Ch10 CCTGCCAGCCGCCCTGGCCCTGGTCACATGAGGGAGGGGCCCAAAGCCCTTGTGGGCGGACCTCCCCTGAGCCTGTCTGTGACCTGCTGGGAAGGCAGGC Ch11 CACGGACCCCATGGCAGGGGTGGCGTCTTCATGATCCGCTCTGGTAAAGCCCTTGTGGGCGCCAGAGTGGGGAGCCTGTCTGAGGGGCCAGCCCTTAGTGCAT Ch12 CCTGCCAGCCGCCCTGGCCCTGGTCACTGCATGAGGGAGGGGCCCCAAACCCTTCCAGGCCAGGACCTCCCCTGATGGTCTGTGACCTGCTGGGAAGGCAGGC C Ch1 fw: CACGGACCCCATGGCAGGGGTGGCGTCTTCTGCATGATCCGCTCTGGTCAAACCCTTCCAGGCC Ch1 rev: ATGCACTAAGGGCTGGCCCCTCAGACCATCCCCACTCTGGCTGGCCTGGAAGGGTTTGACCAGA Ch2 fw: CACGGACCCCATGGCAGGGGTGGCGTCTTCATGATCCGCTCTGGTCAAACCCTTCCAGGCCAG Ch2 rev: ATGCACTAAGGGCTGGCCCCTCAGACAGGCTCCCCACTCTGGCTGGCCTGGAAGGGTTTGACCA Ch3 fw: CACGGACCCCATGGCAGGGGTGGCGTCTTCATGAGGGAGGGGCCCCAAACCCTTCCAGGCCAG Ch3 rev: ATGCACTAAGGGCTGGCCCCTCAGACAGGCTCAGGGGAGGTCCTGGCCTGGAAGGGTTTGGGGC Ch4 fw: CCTGCCAGCCGCCCTGGCCCTGGTCACTGCATGATCCGCTCTGGTAAAGCCCTTGTGGGCG Ch4 rev: GCCTGCCTTCCCAGCAGGTCACAGACCATCCCCACTCTGGCGCCCACAAGGGCTTTACCAGA Ch5 fw: CCTGCCAGCCGCCCTGGCCCTGGTCACTGCATGAGGGAGGGGCCCAAAGCCCTTGTGGGCG Ch5 rev: GCCTGCCTTCCCAGCAGGTCACAGACCATCAGGGGAGGTCCGCCCACAAGGGCTTTGGGCCC Ch6 fw: CCTGCCAGCCGCCCTGGCCCTGGTCACATGAGGGAGGGGCCCAAAGCCCTTGTGGGCGGAC Ch6 rev: GCCTGCCTTCCCAGCAGGTCACAGACAGGCTCAGGGGAGGTCCGCCCACAAGGGCTTTGGG Ch7 fw: CCTGCCAGCCGCCCTGGCCCTGGTCACATGATCCGCTCTGGTCAAACCCTTCCAGGCCAGCC Ch7 rev: GCCTGCCTTCCCAGCAGGTCACAGACAGGCTCCCCACTCTGGCTGGCCTGGAAGGGTTTGAC Ch8 fw: CACGGACCCCATGGCAGGGGTGGCGTCTTCTGCATGAGGGAGGGGCCCAAAGCCCTTGTGGGC Ch8 rev: ATGCACTAAGGGCTGGCCCCTCAGACCATCAGGGGAGGTCCGCCCACAAGGGCTTTGGGCCCC Ch9 fw: CACGGACCCCATGGCAGGGGTGGCGTCTTCTGCATGATCCGCTCTGGTCAAACCCTTCCAGGCC Ch9 rev: ATGCACTAAGGGCTGGCCCCTCAGACCATCCCCACTCTGGCTGGCCTGGAAGGGTTTGACCAGA Ch10 fw: CCTGCCAGCCGCCCTGGCCCTGGTCACATGAGGGAGGGGCCCAAAGCCCTTGTGGGCGGAC Ch10 rev:GCCTGCCTTCCCAGCAGGTCACAGACAGGCTCAGGGGAGGTCCGCCCACAAGGGCTTTGGG Ch11 fw: CACGGACCCCATGGCAGGGGTGGCGTCTTCATGATCCGCTCTGGTAAAGCCCTTGTGGGCGC Ch11 rev:ATGCACTAAGGGCTGGCCCCTCAGACAGGCTCCCCACTCTGGCGCCCACAAGGGCTTTACCAG Ch12 fw: CCTGCCAGCCGCCCTGGCCCTGGTCACTGCATGAGGGAGGGGCCCCAAACCCTTCCAGGCCA Ch12 rev:GCCTGCCTTCCCAGCAGGTCACAGACCATCAGGGGAGGTCCTGGCCTGGAAGGGTTTGGGGCC Supplementary Figure 3. Structures, sequences and nomenclature of chimeric SECIS between SelX and Gpx3 cloned downstream of luciferase UGA258 coding sequence. (A) Schematic representation of the chimeric SECIS elements together with wild-type SelX and Gpx3. Structural domains from SelX and Gpx3 are represented in black and grey respectively. Ch: Chimera (B) Sequences of the chimeric SECIS elements. (C) List of overlapping primers used to generate chimerical SECIS elements