UAS regulates expression of smt3 with an N-terminal Tag:Bio(Unk) tag, followed by a '2A' sequence, and then a human codon-optimized version of Ecol\birA. The resulting product is self-processed via 2A sequences to release the two separate proteins: the Ecol\birA enzyme recognizes the Tag:Bio(Unk) sequence, and transfers biotin to the lysine within the Tag:Bio(Unk) tag on smt3 .