<p>DNA sequences are transcripted to mRNA and then translated to amino acids. During the translation, each three mRNA sequence is assigned to specific amino acid. In 2001 science, one interesting paper was published; Expanding genetic code of escherichia coli. In this paper, they generated&nbsp;unique tRNA/aminoacyl-tRNA synthetase pair to express non-canical amino acid at the site corresponed to&nbsp;amber codon in mRNA. In this way, they conducted in vivo incorporation&nbsp;O-methyl-tyrosine into protein in E-coli.</p>

<p>Then why don&#39;t we improve this technique to express some unstable non-canonical amino acids?</p>

<p>Let&#39;s focus on N-phosphorylated amino acids as a subject for expanding genetic code. There were some previous researches about pHis and it is known that pHis conduct stress regulation system, so called two-component system, in E-coli. But still we don&#39;t know much about function of pHis in eukaryotic cell. Compared to other phosphorylated amino acids&nbsp;such as pTyr, pSer and pThr, N-phosphorylated amino acid were less understood because of acid-labile property.</p>

<p>N-phosphorylated amino acid are fragile because it undergoes hydrolysis in acidic or even neutral pH. Therefore most of in vitro phosphorylation to get pHis, pAsp&nbsp;and pArg in concucted at basic condition using potassium phosphoramidate.</p>