Methionine-Dependent Synthesis of Ribosomal Ribonucleic Acid During Sporulation and Vegetative Growth of Saccharomyces cerevisiae

Methionine limitation during growth and sporulation of a methionine-requiring diploid of Saccharomyces cerevisiae causes two significant changes in the normal synthesis of ribonucleic acid (RNA). First, whereas 18S ribosomal RNA is produced, there is no significant accumulation of either 26S ribosomal RNA or 5.8S RNA. The effect of methionine on the accumulation of these RNA species occurs after the formation of a common 35S precursor molecule which is still observed in the absence of methionine. During sporulation, diploid strains of S. cerevisiae produce a stable, virtually unmethylated 20S RNA which has previously been shown to be largely homologous to methylated 18S ribosomal RNA. The appearance of this species is not affected by the presence or absence of methionine from sporulation medium. However, when exponentially growing vegetative cells are starved for methionine, unmethylated 20S RNA is found. The 20S RNA, which had previously been observed only in cells undergoing sporulation, accumulates at the same time as a methylated 18S RNA. These effects on ribosomal RNA synthesis are specific for methionine limitation, and are not observed if protein synthesis is inhibited by cycloheximide or if cells are starved for a carbon source or for another amino acid. The phenomena are not marker specific as analogous results have been obtained for both a methionine-requiring diploid homozygous for met13 and a diploid homozygous for met2. The results demonstrate that methylation of ribosomal RNA or other methionine-dependent events plays a critical role in the recognition and processing of ribosomal precursor RNA to the final mature species.     

Alteration in the processing of ribosomal ribonucleic acid inSaccharomyces cerevisiae caused by ethidium bromide

Ethidium bromide in a concentration of 200 μg/ml causes a full inhibition of RNA synthesis in aSaccharomyces cerevisiae ρ° strain, while protein synthesis continues at a reduced rate. Under these conditions, processing of rRNA is slowed down and part of the 37S rRNA precursor molecules are cleaved to a 32S RNA fraction (molecular weight 2.15×10⁶). The 32S RNA accumulates in cells treated with ethidium bromide but cannot be processed to mature 25S and 18S rRNA and is degraded. The 32S RNA fraction also appears when processing of rRNA occurs in cells starved for required amino acids. The degradation of 37S precursor molecules through 32S RNA may be a regulatory mechanism of rRNA biosynthesis in yeast, which operates when excess rRNA must be wasted.