The processing of ribosomal RNA has been studied in a temperature sensitive mutant of the Syrian hamster cell line BHK 21. At 39°C, these cells are unable to synthesize 28S RNA, and 60S ribosomal subunits, while 18S RNA, and 40S subunits are produced at both temperatures. At 39°C the 45S RNA precursor is transcribed and processed as in wild type cells. The processing of the RNA precursors becomes defective after the cleavage of the 41S RNA, and the separation of the 18S and 28S RNAs sequences in two different RNA molecules. The 36S RNA precursor, which is always present in very small quantity in the nucleoli of wild type cells and of the mutant at 33°C, is found in very large amounts in the mutant at 39°C. The 36S RNA can be, however, slowly processed to 32S RNA. The 32S RNA cannot be processed at 39°C, and it is degraded soon after its formation. Only a small proportion accumulates in the nucleoli. The 32S RNA synthesized at 39°C cannot be processed to 28S RNA upon shift to the permissive temperature, even when the processing of the newly synthesized rRNA has returned to normal. The data suggest that the 36S and 32S RNAs are contained in aberrant ribonucleoprotein particles, leading to a defective processing of the particles as a whole.
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