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 follow-ing chemical mutagenesis. Phenotypical characteristics are given for seven cell lines, which have a range of average cell volumes from 8000 five? to more than 100 000/an3. Wild-type Tetrahymena cells have an average cell volume of 15 000 /im3. Two of the mutagenized cell lines have comparative-ly small cells at 29°C but normal cells at 37 °C; whereas the other five lines are normal at 29°C but large at 37°C. While the small cells are poor growers, the large cells grow excellently at 37°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