CONDITIONAL-LETHAL MUTANTS OF AN ANIMAL VIRUS: IDENTIFICATION OF TWO CISTRONS.

Two different temperature-sensitive conditional-lethal mutants of Sindbis virus, an animal virus that contains RNA, have been isolated. When cultured in chick fibroblast monolayers at 42 degrees C, these mutants yield less than 0.05 percent as much virus as does the wild type, whereas at 27 degrees C they grow normally. One mutant appears to be altered in the synthesis of a protein that is produced early in the infection and is required for viral RNA synthesis. The other mutant produces as much infectious RNA as the wild type at 42 degrees C and appears to be altered in the synthesis of a protein produced late in the infection.