AbstractPlant ribosome‐inhibiting proteins are shown to be homologous at the domain level to RNase H formEscherichia coliand to two regions of thepolgene product of retroviral reverse transcriptases. One of these regions carries the viral integrase orintfunction, while the other has previously been suggested to contain the viral RNase H exo activity. Several residues conserved among the ribosome inhibitors,E. coliRNase H, and the integrase proteins are seen to occupy a prominent cleft in the tertiary structure of the ribosome inhibitor ricin, suggesting roles in binding or catalysis. It is likely that these homologous sequences represent modern derivatives of an ancient protein‐folding unit capable of nucleic acid binding and modification which has been incorporated into a variety of enzyme functi