Long-term effects of restrictive culture conditions on Saccharomyces cerevisiae sec7 cells

Long-term effects of restrictive conditions on the temperature-sensitive S. cerevisiae sec7 mutant were studied. By microscopic analysis no cell lysis could be detected of cells cultured for up to 19 days at 37 degrees C. The optical density as well as the cell number remained constant during the whole period under restrictive conditions. However, restrictive conditions decreased the incorporation of 35S-methionine into intracellular proteins in a reversible manner indicating that protein biosynthesis was inhibited whereas the cells remained alive. Northern blot experiments revealed that restrictive conditions did not markedly decrease the ratio of the mRNA levels to total RNA for the genes TEF1, TEF2, SUC2, and BGL2 up to 73 hours. However the content of total RNA decreased drastically with increasing incubation times at restrictive temperature. In spite of the reduced total RNA content, cells are capable of new synthesis of mRNA under restrictive conditions which was shown by incubation of the cells in the presence of actinomycin D--an inhibitor of the mRNA synthesis. Most of the cells which survived a long-term incubation at 37 degrees C are not able to divide and to form colonies immediately after their transfer to permissive conditions.