Covalent addition of acetyl groups to lysine's side-chain nitrogen is a reversible process that regulates essential events such as DNA replication and repair. Deciphering the roles of acetylation in biology has been challenging because it is difficult to make pure protein that is acetylated at a single site. Now, Jason W. Chin and coworkers at the Medical Research Council Laboratory of Molecular Biology, in Cambridge, England, have genetically encoded side-chain-acetylated lysine (Nε-acetyllysine, shown) into proteins (Nat. Chem. Biol., DOI: 10.1038/nchembio.73). The team supplied Escherichia coli with tRNA machinery that methane-producing bacteria use to incorporate pyrrolysine, a lysine derivative, into proteins. They then made a few mutations to alter the machinery so that it would introduce Nε-acetyllysine instead. With the help of an inhibitor that suppresses E. coli's endogenous deacetylating enzyme, Chin's team used the modified E. coli to make a protein that naturally contains a single Nε-acetyllysine. Now they hope to examine how acetylation affects DNA packing into chromatin.