Isolation and characterization of conditional lethal amber nonsense mutants of vesicular stomatitis virus.

We describe the isolation and characterization of conditional lethal amber nonsense mutants of vesicular stomatitis virus (VSV), Indiana serotype. The mutants were isolated from a chemically mutagenized stock of wild-type virus by their ability to grow on genetically engineered cells which express a Xenopus laevis amber suppressor tyrosine tRNA gene (su+ cells) but not on the non-suppressor parental cells (su- cells). Five mutations were assigned to complementation group I (the L gene) and one to complementation group V (the G gene) by complementation analysis using temperature-sensitive mutants representing each of the five VSV cistrons. Four of the group I mutants were observed to synthesize a novel polypeptide species in su+ cells. Immunoprecipitation and immunoblotting studies using monospecific antisera directed against the N and C termini of the VSV L protein showed that the novel polypeptide species contain N terminal- but not C terminal-specific sequences and can thus be considered to be truncated versions of the L protein. In addition a protein which again contained N terminal- but not C terminal-specific sequences could be identified for the fifth group I mutant. Revertants of four of the group I mutants were isolated on the su- cells. The revertants all synthesized normal L protein but not the putative truncated version.