The killer virus assay was carried out as described earlier (18) . Briefly, yeast colonies were replica plated to 4.7MB plates newly seeded at an optical density at 595 nm (OD 595 ) of 0.5 of the 5 Â 47 killer indicator strain per plate. After 2-3 days at 20 C, killer activity was scored as a zone of growth inhibition around the Killer + colonies. To monitor programmed À1 frameshifting using the dual luciferase reporter plasmids, glass beads were used to prepare lysates from cells expressing the 0-frame, À1 (L-A derived), or +1 (Ty1 derived) dual luciferase plasmids (20) . After clarification of the lysates by centrifugation, typically 5 ml was used in a total volume of 100 ml of dual luciferase assay reagents (Promega, Madison WI, USA), and Renilla and firefly luciferase activities were quantitated using a TD20/20 luminometer (Turner Designs, Sunnyvale, CA, USA). Frameshifting efficiencies were calculated by dividing the firefly/Renilla luminescence ratios from lysates of cells expressing the PRF test reporters by the same ratio obtained from lysates of cells expressing the zero-frame control reporter. All assays were replicated enough times to achieve >95% confidence levels, and statistical analyses were performed as described earlier (21) .