FIGURE 3.

Chemical biology toolbox for chromatin studies. (A) Lysine trimethylation (me³) can be installed enzymatically or by cysteine alkylation to yield a methylated lysine analog. (B) In native chemical ligation, a synthetic peptide containing a C-terminal thioester (1) is linked to a second polypeptide bearing an N-terminal cysteine (2). (C) In unnatural amino acid incorporation, the UAA is loaded onto the corresponding tRNA by an engineered tRNA synthetase. The tRNA recognizes the amber stop codon UAG, allowing the ribosome to install the UAA at the desired position in the protein sequence. (D) Segmental isotopic labeling is mediated by intein splicing of an isotopically labeled protein segment with a segment at natural abundance, producing the full-length protein.