Response of Tetrahymena pyriformis to stress induced by starvation

mRNA synthesis was studied in exponentially growing and starved Tetrahymena pyriformis. Poly(A)-containing RNAs separated from total RNA by affinity chromatography on oligo(dT)-cellulose were characterized by poly acrylamide gel electrophoresis; their template activity was assayed in a rabbit reticulocyte lysate system and their translation products were analysed using two-dimensional electrophoresis according to O'Farrell. Polysome profiles show that the bulk of ribosomes are in 80S monosomes in starved cells, whereas less than 8% are present in the form of monosomes in exponentially growing cells, the rest being engaged in polysomes. Polysomes are almost completely reformed 30 min after addition of enriched medium to suspensions of starved cells. This polysome reformation is dependent on mRNA synthesis since we have shown that it is inhibited by actinomycin D. Electrophoretic profiles of poly(A)-rich RNA isolated from cytoplasmic fractions of exponential and starved cells are indistinguishable except that in the latter state significant amounts of low-molecular-mass species are observed. Poly(A)-rich RNAs isolated from polysomal and non-polysomal (subpolysomal) fractions of exponential cells are equally able to promote protein synthesis. The corresponding poly(A)-rich RNAs isolated from starved cells also possess equal template activities which are, however, 15% lower than those of the poly(A)-rich RNAs of exponentially growing cells. We also present evidence that in the system used in vitro, polyadenylated RNA isolated from heavy polysomes of starved cells directs the synthesis of four sets of proteins with molecular masses around 100 kDa, 70 kDa, 50 kDa and 30 kDa. The former two groups of proteins are more abundant in the translation products of poly(A)-rich RNA of starved than of normal cells, whereas the latter two groups are present only in the translation products of poly(A)-rich RNA of starved Cells. The fluorograms of the translation products obtained in vitro from subpolysomal poly(A)-rich RNA are identical to those obtained from polysomal poly(A)-rich RNA. Studies on starved cells in vivo show that polypeptides of 100 kDa, 70 kDa and 38 kDa are more strongly labelled and also revealed the specific presence of 85 kDa, 55 kDa, 50 kDa and 25 kDa proteins. These results lead us to the conclusion that this microorganism responds to depleted environmental conditions by regulating gene expression at the transcriptional level, but also at the translational level.