Preliminary characterization of a temperature-sensitive mutant of Moloney murine leukemia virus that produces particles at the restrictive temperature.

The isolation and preliminary characterization of a new temperature-sensitive mutant of Moloney murine leukemia virus, designated ts7, are reported. The infectivity of ts7, determined by a focus-forming unit assay, was reduced at least 100-fold when the virus was assayed at the nonpermissive temperature (39 degrees C) as compared with assay at the permissive temperature (34 degrees C). However, several lines of evidence indicated that the diminution of ts7 titer at 39 degrees C is not due to its inability to form virus particles at that temperature. The supernatant from ts7-infected cells grown at 39 degrees C showed significant infectivity when assayed at 34 degrees C; only small reductions in reverse transcriptase activity and fusion ability were observed when compared with supernatant from 34 degrees C ts7-infected cultures. That particles are produced at the nonpermissive temperature was confirmed by transmission electron microscopy of the supernatant from ts7-infected cells at 39 degrees C and by transmission electron microscope observations of mature particles trapped in the intercellular spaces of pelleted thin cell section. A possible explanation for the productivity at 39 degrees C of particles that are infectious at 34 degrees C but not at 39 degrees C is that the virus is heat labile at the nonpermissive temperature. Consistent with this hypothesis is the extreme heat lability of virus harvested at 34 degrees C. Such virus, when incubated at 39 degrees C, has a half-life one-sixth that of identical virus incubated at 34 degrees C, or that of wild-type virus at either temperature.