Isolation and phenotypic characterization of Tetrahymena thermophila size mutants: the relationship between cell size and regulation of DNA content.

Temperature-sensitive size mutants of the ciliate Tetrahymena thermophila were selected following chemical mutagenesis. Phenotypical characteristics are given for seven cell lines, which have a range of average cell volumes from 8000 microns 3 to more than 100 000 microns 3. wild-type Tetrahymena cells have an average cell volume of 15 000 microns 3. Two of the mutagenized cell lines have comparatively small cells at 29 degrees C but normal cells at 37 degrees C; whereas the other five lines are normal at 29 degrees C but large at 37 degrees C. While the small cells are poor growers, the large cells grow excellently at 37 degrees C. Measurements of DNA, RNA and protein contents indicate a significant correlation between all parameters and cell size. However, since the cells tolerate considerably different concentrations of each class of macromolecules, the amount of any of these macromolecules cannot be tightly controlled by cell size.