The translation of an mRNA begins with the codon _______, for which a special charged tRNA is required. This initiator tRNA always carries the amino acid methionine (or a modified form of methionine, formylmethionine, in bacteria). Thus newly made proteins all have methionine as the first amino acid at their N-terminal end, the end of a protein that is synthesized first. This methionine is usually removed later by a specific protease.