Extensive digestion of the covalent intermediate between DNA and Saccharomyces cerevisiae DNA topoisomerase I with trypsin yields a 7-amino acid peptide covalently linked to DNA. Direct sequencing of the DNA-linked peptide identifies Tyr-727 as the active site tyrosine that forms an O4-phosphotyrosine bond with DNA when the enzyme cleaves a DNA phosphodiester bond. Site-directed mutagenesis of the cloned yeast TOP1 gene encoding the enzyme confirms the essentiality of Tyr-727 for the relaxation of supercoiled DNA by the enzyme. From amino acid sequence homology, Tyr-771 and -773 are readily identified as the active site tyrosines of Schizosaccharomyces pombe and human DNA topoisomerase I, respectively. Sequence comparison and site-directed mutagenesis also implicate Tyr-274 of vaccinia virus DNA topoisomerase as the active site residue. There appears to be a 70-amino acid domain near the carboxyl terminus of eukaryotic DNA topoisomerase I and vaccinia topoisomerase, within which the active site tyrosine resides.