The ribosomal peptidyl transferase center (PTC) resides in the large ribosomal subunit (50S), where two fundamental biological reactions are processed and catalyzed: peptidyl transfer, the formation of a peptide bond during protein synthesis, and peptidyl hydrolysis, the release of the complete protein from the peptidyl tRNA upon completion of translation. Prokaryotic ribosomes consist of two subunits, the large 50S subunit and the smaller 30S subunit; together they form the 70S ribosome, a molecular machine that selects its substrates, aminoacyl-tRNAs (aa-tRNAs), rapidly and accurately and catalyzes the synthesis of peptides from amino acids. The 30S subunit contains the decoding site, where base-pairing interactions between the mRNA codon and the tRNA anticodon determine the selection of the cognate aa-tRNA. The large ribosomal subunit contains the site of catalysis: the peptidyl transferase (PT; EC 2.3.2.12) center, which is responsible for making peptide bonds during protein elongation and for the hydrolysis of peptidyl-tRNA (pept-tRNA) during the termination of protein synthesis. The peptidyl transferase center is a major target of many natural and synthetic antibiotics.