Nascent proteins are extensively modified. Over 50% of proteins undergo N-terminal methionine excision (NME), an essential and ubiquitous process conserved across all kingdoms of life. 50-90% of eukaryotic proteins are also acetylated at their N-terminus. These modifications mediate new protein-protein interactions that regulate the maturation and quality control of the protein via diverse mechanisms, including folding, assembly, degradation, subcellular localization, and/or aggregation propensity. However, little is known about how these enzymes are recruited to and process substrate proteins on the ribosome during protein synthesis, nor how these processes are regulated. Using fluorescence spectroscopy and mechanistic enzymology, we aim to understand how these essential enzymatic reactions are regulated in their physiological context: as a cotranslational process during which these enzymes need to coordinate in space and time with other ribosome-bound biogenesis factors and a host of other regulatory proteins. Reciprocally, these enzymes allow us to probe how timely and selective protein biogenesis occurs in the crowded environment of the ribosome exit site.