Disclosed are transgenic plants containing an exogenous nucleic acid encoding an L3 protein. The plant exhibits increased resistance to viruses and/or fungi that infect plants. The L3 proteins include wild-type proteins, spontaneously occurring mutants and non-naturally occurring L3 mutants. Also disclosed are methods of reducing the toxicity of single-chain ribosome inhibitory proteins in cells, e.g., yeast, plant and animal cells, by co-administering the L3 protein with the RIP. Further disclosed are non-naturally occurring L3 mutants that (a) substantially fail to bind single-chain RIPs that bind endogenous L3 proteins, (b) are unable to maintain M1 killer virus, (c) promote altered programmed ribosomal frameshift efficiency, (d) exhibit resistance to peptidyltransferase inhibitors, and combinations of any of (a) (d).