Regulation of rRNA metabolism in Tetrahymena pyriformis. I. Nutritional shift-down

Changes of rates of rRNA transcription and accumulation as well as of stability of rRNA precursor fractions and mature cytoplasmic rRNAs were determined in Tetrahymena after shift-down to a non-nutrient buffer. During the initial period (4-6 hours) an enhanced degradation of pre-existing cytoplasmic rRNA and a successive reduction of rRNA transcription, processing and nucleo-cytoplasmic transport was detected. Thereafter, a "residual" rRNA metabolism at low turnover is maintained in the cells which has the following characteristics: (1) The rate of pre-rRNA transcription, as measured in vitro in isolated macronuclei, is about 3 to 5% of the rate in optimally growing cells, indicating regulatory processes at the level of initiation of the nucleolar RNA polymerases. The strong reduction of in vitro pre-rRNA synthesis can partially be reversed by pre-treatment of the starved cells with low concentrations of cycloheximide, but not with puromycin.--(2) The processing time of nuclear pre-rRNA is considerably prolonged and the introduction of the central hidden break into newly synthesized cytoplasmic 26S rRNA is strongly delayed, as shown by methionine pulse-chase experiments.--(3) The accumulation rate of cytoplasmic rRNA is 1 to 2% of the rate in optimally growing cells, as determined from the specific radioactivity of ATP at saturation with labelled exogeneous adenosine and the changes of the specific radioactivity of the AMP residues in rRNA as well as from the rRNA turnover rate.