Relaxed mutant of Saccharomyces cerevisiae: proper maturation of ribosomal RNA in absence of protein synthesis.

We have characterized a relaxed yeast mutant, S. cerevisiae SY15, isolated by mutagenesis with ethylmethanesulfonate of strain A364A. Starvation for a required amino acid or treatment with cycloheximide blocks protein synthesis in both parental and mutant strains, while the synthesis of total RNA is inhibited by 72% in A364A and 23% in SY15 cells. In the absence of protein synthesis, the transcription of 37S primary precursor to rRNA is not inhibited in the SY15 mutant, and the rRNA transcripts are correctly processed, although at a reduced rate, and are almost free of ribosomal proteins. The relaxed phenotype in yeast is accompanied by alteration in the regulation of rRNA biosynthesis at transcriptional and posttranscriptional levels.