MECHANISMS CONTROLLING DEATH, SURVIVAL AND PROLIFERATION IN A MODEL UNICELLULAR EUKARYOTE TETRAHYMENA-THERMOPHILA

Below a critical cell density of 750 cells ml(-1), and in a manner fam iliar throughout much of cell culture technology, Tetrahymena thermoph ila die within a few hours despite being supported by a nutritionally complete synthetic medium, SSM, in which a supracritical inoculum (100 0 cells ml(-1)) nevertheless proliferates and quickly reaches 1 x 10(6 ) cells ml(-1). The kinetics of cell death, and the conditions require d to keep cells alive at and below the critical density have now been more fully investigated. Interestingly, cell death follows first order kinetics, with a half-life of less than two hours at 250 cells ml(-1) . Survival can be extended by an order of magnitude, however, when pro tein synthesis is reduced by inoculation of cells at this density in: (a) Tris/HCl-buffer; (b) SSM deficient in an essential amino acid (arg inine or phenylalanine); or, (c) SSM containing cycloheximide, In the presence of actinomycin D, the critical density required for prolifera tion can be lowered to 100 cells ml(-1). These results are discussed i n relation to the capacity of Tetrahymena to produce and release signa l molecules (loosely referred to as growth factors), which need to be present above a certain threshold level before proliferation occurs, T he evidence for the demise of cells at low density being active - in t erms of requirement for, or dependency on, new transcriptional and tra nslational processes - is discussed, along with more general implicati ons of the findings for the control of cell death in populations of 'f ree-living' unicellular organisms in culture compared with their norma l habitats.