sentence: We generated two sets of mutants , the first one disabling essential motifs from the SECIS , either the tandem GA pairs ( Δ AUGA mutant of luciferase UGA - SECIS constructs ) or the AAA / G motif ( Δ AAAC and Δ AAAG for SelX and Gpx3 , respectively ) , the other , replacing the UGA at position 258 of the luciferase coding sequence by the UAA stop codon , which does not support selenocysteine incorporation .