Abstract
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 (Mr = 0.66 X 10(6) consists of a continuous polynucleotide chain throughout its lifetime, whereas the bulk of 26-S rRNA (Mr = 1.2m X 10(6) dissociates upon denaturation. Two large fragments (F1, F2) of somewhat different molecular weights (Mr 0.63 X 10(6) and 0.58 X 10(6) and the small 5.8-S rRNA fragment (Mr about 50 000) 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 min after their appearance in the cytoplasm; however, they release during denaturation the 5.8-S and/or a short-lived 7-S fragment (Mr about 75 000) which might represent a direct precursor to the 5.8-S rRNA. (b) The immediate nuclear precursor to the 26-S rRNA (Mr 1.39 X 10(6) 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.
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