Conditionally lethal mutations in chinese hamster cells. I. Isolation of a temperature‐sensitive line and its investigation by cell cycle studies

The isolation of a temperature sensitive cell line from the Chinese hamster line CCL39 of the American Type Culture Collection is described. At the nonpermissive temperature (39°C) the cells become attached to the surface of tissue culture dishes, but no microscopically observable colonies are formed upon prolonged incubation. Exposure to the high temperature for more than 24 hours leads to an almost complete loss in viability. A karyotypic analysis showed that this new line has lost one of the medium‐sized metacentric chromosomes, although no proof is available so far to show that this loss is not simply coincidental. In nonsynchronized cultures transferred to 39°C DNA synthesis stops first, RNA synthesis shortly thereafter, while protein synthesis (turnover) continues for a longer time. After such a shift the cell number increases by less than 15% as measured with the Coulter counter. Studies with synchronized cultures give the following results: (1) one round of DNA synthesis can occur at 39°C when the cells are released from serum starvation or a hydroxyurea block, or when mitotic cells are placed at 39°C; (2) the entry of cells into metaphase of mitosis at 39°C is almost normal when the preceding time interval at 39°C is only eight hours (release of cells from G 1 /S boundary), but considerably reduced when the cells spend an additional 12 to 15 hours at 39°C in G 1 (release from serum starvation). Infection by SV40 virus temporarily induces DNA synthesis after it has come to a stop at the nonpermissive temperature, but cells permanently transformed by SV40 still exhibit the temperature‐sensitive phenotype.