Reexamination of the Morphogenetic Block of a Putative Late-Stage, Temperature-Sensitive Mutant of Moloney Murine Leukemia Virus

The ts 3 temperature-sensitive mutant of Moloney murine leukemia virus has been reported to have a morphogenetic block in a late stage of the budding process. As evidence, previously published electron micrographs of cells maintained at the nonpermissive temperature (39°C) revealed numerous budding virions on the cell surface. However, it appears now that these micrographs reflected budding that occurred not at 39°C, but after cells were removed from the incubator before fixation. The morphogenesis of ts 3 is actually blocked at an earlier stage of development.