Monocistronic messenger RNA in yeast

We have determined the rate of polypeptide chain synthesis on different size polysomes in yeast. The completion time for the average polypeptide chain in vivo at 23 °C is two minutes by this technique and is in good agreement with values we have determined by other independent methods.These kinetic experiments indicate that the average size of a nascent polypeptide chain on a polysome is directly related to the size of the polysome. This demonstrates that in the simple eucaryotic organism, Saccharomyces cerevisiae, mRNA is monocistronic in the sense that each mRNA molecule codes for one protein molecule which is released intact from the ribosome upon completion. The pattern of amino acid incorporation into Escherichia coli polysomes is distinctly different. These findings have a number of interesting implications for the genetics of the lower eucaryotes and indicate that the cellular mechanisms of control and co-ordination in yeast may differ from those found in procaryotes and may be similar to cellular mechanisms of control for mammalian cells.     

Kinetics of Novikoff cytoplasmic messenger RNA methylation.

Methylation patterns of Novikoff cytoplasmic mRNA were determined as a function of labeling time with L-[methyl-3H]methionine. The 5'-terminal m7G could be released from whole mRNA by treatment with nucleotide pyrophosphatase. Subsequent alkaline phosphatase treatment of this mRNA, followed by KOH digestion, yielded N'mpNp and N'mpNp from cap 1 (m7GpppN'mpN) and cap 2 (m7GpppN'mpN'mpN), respectively. Our results indicate that the relative amounts of labeled cap structures do change with time and that the amount of internal N6-methyladenosine decreases, relative to 5'-cap structures, as the cytoplasmic mRNAs age and the average size decreases. The formation of cap-2 structures by the addition of second 2'-O-methyl group at position N'm appears to be cytoplasmic event. Thus, after very short labeling times, greater than 80% of the labeled methyl groups in cap 2 are found in this position. These results, along with earlier data obtained on L-cell heterogeneous nuclear RNA methylation, are consistent with a model in which the nucleus is the cellular site of three mRNA methylation events producing 5'-terminal m7G, the first 2'-O-methylnucleoside (N'm) found in cap-1 structures and internal N6-methyladenosine. Subsequently, these nuclear methylations are followed by the cytoplasmic methylation at N'm. Analysis of the methynucleoside composition of cap-1 structures, along with comparable "core" structures (m7GpppN'm) generated from cap-2 by removal of N'm, indicates that at any single labeling time the methylnucleoside composition of a given cap-1 and the cap-2 "core" structure is remarkably similar. On the other hand, comparisons of the methylnucleoside composition of the cap structures at different labeling times indicate an increase in Cm in the first 2'-O-methylnucleoside (N'm) with time.     

Self-splicing of yeast mitochondrial ribosomal and messenger RNA precursors

We have previously shown linear and circular splicing intermediates resembling intermediates that result from self-splicing of ribosomal precursor RNA of Tetrahymena to be present in mitochondrial RNA. Here we show that splicing of yeast mitochondrial precursor RNA also occurs in vitro in the absence of mitochondrial proteins. The large ribosomal RNA gene, consisting of the intron and part of the flanking exon regions, was inserted behind the SP6 promoter in a recombinant plasmid and was transcribed in vitro. The resulting RNA shows self-catalyzed splicing via incorporation of GTP at the 5'-end of the excised intron, 5'- to 3'-exon ligation, and intron circularization. When purified mitochondrial RNA is incubated under similar conditions with alpha-32P-GTP, the excised ribosomal intron RNA is also labeled, as well as several other RNA species. Some of these RNAs are derived from excised introns from the multiply split gene coding for cytochrome oxidase subunit I.     

Secondary methylation of yeast ribosomal precursor RNA.

The timing of methylation of the ribosomal sequences of ribosomal precursor RNA (pre-rRNA) from the yeast Saccharomyces carlsbergensis was investigated by fingerprint analysis of the methylated oligonucleotides derived from the various precursors. From the total of 37 ribose and 6 base-methyl groups found in 26-S rRNA, the two copies of the base-methylated nucleoside m3U as well as the doubly methylated sequence Um-Gm psi are not yet present in 37-S RNA, the predominant common precursor of 26-S and 17-S rRNA. Introduction of these methyl groups into the ribosomal sequences appears to take place at the level of 29-S pre-rRNA, the immediate precursor to 26-S rRNA. From the total of 18 ribose-methylated and 6 base-methylated nucleosides found in 17-S rRNA, the latter group (one copy of m7G, the m62A-m62A- sequence and the hypermodified methylated nucleoside "mX") is completely missing in 37-S pre-rRNA. The methyl group of m7G is introduced into 18-S pre-rRNA, the direct precursor of 17-S rRNA, in the nucleus. The -m62A-m62A- sequence is methylated after transport of the 18-S pre-rRNA to the cytoplasm prior to the final maturation into 17-S rRNA.     

In vivo inhibition of Novikoff cytoplasmic messenger RNA methylation by S-tubercidinylhomocysteine.

The analogue S-tubercidinylhomocysteine (STH) has been used to study the methylation of mRNA in vivo. Partial inhibition of cytoplasmic poly(A)-RNA methylation was observed using a level of inhibitor which still permitted cell growth. Characterization of the partially methylated mRNA indicated the presence of cap structures lacking 2'-O-methylnucleosides, m7GpppN', which are normally not found in mammalian mRNA. Inhibition of additional methylated sites in mRNA at the second 2'-O-methynucleoside, and at internal N6-methyladenosine was also observed Methylation of 7-methylguanosine was not affected under the conditions used in these experiments. The methylnucleoside composition of cap structures differed in STH-inhibited and uninhibited cells. These results indicate that a completely methylated cap is not required for transport of mRNA into the cytoplasm. Furthermore, it may now be possible to assess in vivo the sequential nature of mRNA methylation and its potential role in mRNA processing.     

Yeast caspase 1 links messenger RNA stability to apoptosis in yeast

During the past years, yeasts have been successfully established as models to study the mechanisms of apoptotic regulation. We recently showed that mutations in the LSM4 gene, which is involved in messenger RNA decapping, lead to increased mRNA stability and apoptosis in yeast. Here, we show that mitochondrial function and YCA1, which encodes a budding yeast metacaspase, are necessary for apoptosis triggered by stabilization of mRNAs. Deletion of YCA1 in yeast cells mutated in the LSM4 gene prevents mitochondrial fragmentation and rapid cell death during chronological ageing of the culture, diminishes reactive oxygen species accumulation and DNA breakage, and increases resistance to H2O2 and acetic acid. mRNA levels in lsm4 mutants deleted for YCA1 are still increased, positioning the Yca1 budding yeast caspase as a downstream executor of cell death induced by mRNA perturbations. In addition, we show that mitochondrial function is necessary for fast death during chronological ageing, as well as in LSM4 mutated and wild-type cells.     

The methylation state of poly A-containing messenger RNA from cultured hamster cells.

The poly A-containing mRNA of cultured hamster (BHK-21) cells has been examined with regard to methylation status. Steady state-labeled mRNA was obtained by incubating cells for 20-22h in the presence of [methyl-3H]-methionine and 32Pi. The degree of methylation of this RNA was 1.8 methyl groups per 1000 nucleotides, or 4-5 methyl groups on the average per molecule. The nature of the methylated residues was determined by paper chromatography and electrophoresis of acid and alkaline hydrolysates, by DEAE cellulose chromatography of alkaline hydrolysates and of T2 RNase digests, and by examining the effect of subjecting samples to "beta-elimination." Approx. half of the methyl groups occurred in standard ("internal") linkage, 10% as m5Cp and 40% as m6Ap residues. The remainder occurred at least for the most part in "blocked" 5'-termini with the presumptive structure m7G(5')ppp(Nm)p.., where Nm was Gm, m6Am, Um, or Cm.     

The structure of yeast tRNAAsp. A model for tRNA interacting with messenger RNA

Abstract

The anticodon of yeast tRNA^Asp, GUC, presents the peculiarity to be self-complementary, with a slight mismatch at the uridine position. In the orthorhombic crystal lattice, tRNA^Asp molecules are associated by anticodon-anticodon interactions through a two-fold symmetry axis. The anticodon triplets of symmetrically related molecules are base paired and stacked in a normal helical conformation. A stacking interaction between the anticodon loops of two two-fold related tRNA molecules also exists in the orthorhombic form of yeast tRNA^Phe. In that case however the GAA anticodon cannot be base paired. Two characteristic differences can be correlated with the anticodon-anticodon association: the distribution of temperature factors as determined from the X-ray crystallographic refinements and the interaction between T and D loops. In tRNA^Asp T and D loops present higher temperature factors than the anticodon loop, in marked contrast to the situation in tRNA^Phe. This variation is a consequence of the anticodon-anticodon base pairing which rigidities the anticodon loop and stem. A transfer of flexibility to the corner of the tRNA molecule disrupts the G19-C56 tertiary interactions. Chemical mapping of the N3 position of cytosine 56 and analysis of self-splitting patterns of tRNA^Asp substantiate such a correlation.