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.

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Wejksnora PJ, Haber JE

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