THE TRANSCRIPTIONAL REGULATION OF YEAST RIBOSOMAL PROTEIN BIOSYNTHESIS
KIM, CHUNG HAN
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Characteristics of the mRNA for a number of ribosomal proteins were determined using cloned genes. Transcription of ribosomal protein mRNA was measured by hybridization of RNA isolated from cells pulse-labeled with ('3)H uracil to excess cloned DNA. Analysis of the approach to equilibrium labeling of individual mRNA species demonstrated that the half-lives of the mRNA for several ribosomal proteins ranges from 13 to 16 minutes in cells growing at 25(DEGREES).;Earlier experiments suggested that when wild-type cells were shifted from 23 to 36(DEGREES), the level of mRNA for ribosomal proteins declined temporarily with kinetics suggesting a brief interruption in its synthesis. Hybridization of pulse-labeled RNA now confirms that the transcription of mRNA of several individual ribosomal proteins falls to approximately 15-30% of normal within 10 minutes of a temperature shift, recovers to normal by 30 minutes and overshoots to 200% by 55 minutes. The transcription of other non-ribosomal mRNAs can either increase or remain constant after the temperature shift.;The size of the mRNAs, as determined from analysis of RNA blots, is barely large enough to code for the ribosomal proteins, suggesting that the 5' and 3' leader regions are short. In a cell carrying a ts lesion in the gene rna2, at the restrictive temperature one observes for most ribosomal protein genes the accumulation of a homologous RNA several hundred nucleotides longer than mature mRNA. Rosbash et al. Cell 24, 679-686, 1981, have demonstrated for one trancript that the mutant cell is unable to excise an intervening sequence. By extrapolation, our results suggest that most ribosomal protein genes of yeast contain introns. Introns in other yeast genes are rare; thus, previous work suggested that genes such as rna2, of which there are ten, were specifically involved in the synthesis of ribosomal protein mRNA. Whether the presence of introns is related to the coordinate synthesis of ribosomal proteins remains to be determined.