Temperature-sensitive mutants of mouse hepatitis virus type 3 (MHV-3): isolation, biochemical and genetic characterization.

Mouse hepatitis virus 3 (MHV-3) is highly hepatotropic in sensitive mice. Temperature-sensitive mutants (ts mutants) induced by N-methyl-N'-nitrosoguanidine and 5-fluorouracil were isolated. Twelve mutants which were able to induce the formation of syncytia at 33 degrees C but not at the restrictive temperature (39.5 degrees C) were selected for detailed study. No viral RNA synthesis was detected after infection at the restrictive temperature with six of the mutants (RNA-) whereas six others were RNA+, although they displayed RNA synthesis which was generally reduced. No differences have been detected in the size of the genome or the viral-intracellular RNA species found in wild type virus or ts mutant infected cells at permissive temperature. The pattern of virus-induced proteins analyzed after immunoprecipitation by SDS-PAGE was similar in wild type virus and RNA+ mutant infected cells at 39.5 degrees C. Complementation experiments between ts mutants enabled us to distinguish five groups. Three of the groups contained RNA- mutants and two of them RNA+. Plaques made by mutants in one group displayed characteristic features that distinguished them from the wild type.