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°C but not at the restrictive temperature (39.5°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°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.