A novel temperature-sensitive mammalian cell line exhibiting defective prophase progression.

A temperature-sensitive mammalian cell line has been isolated which grows and divides normally at the permissive temperature of 33 degrees C. When incubated at 39 degrees C, the nonpermissive temperature, interphase cells continue to enter a prophase-like state. Chromatin-like material condenses and coalesces into dark-staining clumps rather than into discernible chromosomes. Disappearance of the nuclear boundary is observed, but re-formation of the boundary around the clumps fails to occur. In corporation of labeled precursors reveals a decrease in protein synthesis which is accompanied by a slower decrease in DNA synthesis. Approximately 0.2% of the mutant cells revert in their capability of growth and cell division at 39 degrees C. These "revertants" are found to contain a higher number of chromosomes. The isolation of this mutant is based on the initial observation that the cells become rounded at the nonpermissive temperature. The cell-rounding process characteristic of mitotic cells should serve as a useful marker in the isolation of mitotic mutants.