Nonsense suppression reflects competition between a nonsense suppressor tRNA and a translational release factor. This provides a simple way to screen for release factor genes cloned into a multicopy plasmid. We have confirmed that the expected competition occurs with the gene for release factor 2, cloned by C.T. Caskey et al. (C.T. Caskey, W.C. Forrester, W. Tate, and C.D. Ward, J. Bacteriol. 158:365-368, 1984), and used it to clone the gene for release factor 1.