Genetic code expansion allows the site-specific incorporation of tailor-made amino acids into recombinant proteins. Equipped with a unique biophysical or chemical property, such amino acids may aid in studying the structure and cellular function of proteins. For example, the incorporation of modified amino acids carrying a naturally occurring post-translational modification or a photo-protecting group. Using genetic code engineering and directed evolution, we aim to develop and apply novel methods for in vivo and in vitro studies of post-translationally modified proteins. Specifically, we are interested in studying the effect of acetylation on the structure, subcellular localization and DNA binding affinity of transcription factors such as NF-kB and p53. In recent years a direct link was found between acetylation and cellular metabolism. In light of the high frequency of metabolic disorders associated with diseases ranging from cancer to obesity, we aim at understanding how cellular metabolism and acetylation level are correlated with acetylation and transcription activity of key transcription factors.