Introduction of Hidden Breaks during rRNA Maturation and Ageing in <em>Tetrahymena pyriformis</em>.

The stability of Tetrahymena pyriformis cytoplasmic rRNAs and nuclear rRNA precursors has been studied by polyacrylamide gel electrophoresis under partly and completely denaturing conditions. Cytoplasmic 17-S rRNA (M_r=0.66×10⁶) consists of a continuous polynucleotide chain throughout its lifetime, whereas the bulk of 26-S rRNA (M_r=1.27×10⁶) dissociates upon denaturation. Two large fragments (F₁, F₂) of somewhat different molecular weights (M_r 0.63×10⁶ and 0.58×10⁶) and the small 5.8-S rRNA fragment (M_r about 50000) are regularly observed. Some additional distinct minor fragments F₃-F₆) are noted under certain preparative conditions, suggestive of artifactual origin. The following conclusions were made from the data obtained. (a) Newly synthesized 26-S rRNA molecules do not contain the ‘central’ hidden break (separating F₁ and F₂) until about 15 rain after their appearance in the cytoplasm; however, they release during denaturation the 5.8-S and/or a short-lived 7-S fragment (M_r about 75000) which might represent a direct precursor to the 5.8-S rRNA. (b) The immediate nuclear precursor to the 26-S rRNA (M_r 1.39×10⁶) releases a small fragment of similar size (7 S). (c) The largest stable transcription product of the rDNA (pre-rRNA) does not contain any hidden break. [ABSTRACT FROM AUTHOR]