Regulation of ribosomal RNA methylation in a temperature-sensitive mutant of BHK cells.

Abstract

CONSERVATION of methylated sequences of mammalian ribosomal precursor RNA (pre-rRNA) into mature cytoplasmic ribosomes suggests an essential structural or regulatory function for these modified bases1–3, although the role of post-transcriptional methylation is not clear. At the non-permissive temperature (39 °C), ts 422E, a temperature-sensitive mutant of BHK 21/13 cells4–6, is deficient in 28S rRNA maturation and assembly of 60S ribosomal subunits, although transcription of 45S pre-rRNA and cytoplasmic appearance of 18S rRNA and 40S subunits remains unimpaired3. To examine the relationship between RNA methylation and the processing of ribosomes, we have studied this mutant and find it contains methyl-deficient pre-rRNA. Our results show an expanded pool of nuclear 45S pre-rRNA molecules in the mutant at 39 °C, suggesting an inhibition of its processing to 32S RNA. In addition, 45S and 32S precursor molecules in ts 422E are substantially undermethylated relative to the pre-rRNAs of wild-type BHK cells at 39 °C but identical at the permissive temperature (33 °C). The residual amount of mature 28S rRNA that appears in ts 422E cytoplasm at 39 °C is markedly methyl deficient.