Which amino acid is critical for crosslinking of keratin fibers?

In order for a eukaryotic gene to be engineered into a bacterial colony to be expressed, what must beincluded in addition to the coding exons of the gene? A) the intronsB) eukaryotic polymerasesC) a bacterial promoter sequenceD) eukaryotic ribosomal subunitsE) eukaryotic tRNAs