Independent existence of mutations responsible for temperature sensitivity and attenuation in a mutant of murine cytomegalovirus.

A temperature-sensitive (ts) mutant of murine cytomegalovirus, ts21, lacks pathogenicity for suckling mice. Revertants (rev21) of ts21 which could replicate at nonpermissive temperature remained attenuated, suggesting the presence of a temperature-independent factor(s) for the attenuation. Coinfection of ts21 with its parental wild-type (wt) generated recombinant-type progenies (rec21w) which lacked temperature sensitivity and were still attenuated. The results indicate that ts21 harbors at least two mutations and the mutation(s) in pathogenicity exists independently of the ts mutation(s).