Identification of ten genes that control ribosome formation in yeast

Isolation of cloned DNA sequences containing ribosomal protein genes from Saccharomyces cerevisiae

Yeast mRNA enriched for ribosomal protein mRNA was obtained by isolating poly(A)+ small mRNA from small polysomes. A comparison of cell-free translation of this small mRNA and total mRNA, and electrophoresis of the products on two-dimensional gels which resolve most yeast ribosomal proteins, demonstrated that a 5-10 fold enrichment for ribosomal protein mRNA was obtained. One hundred different recombinant DNA molecules possibly containing ribosomal protein genes were selected by differential colony hybridization of this enriched mRNA and unfractionated mRNA to a bank of yeast pMB9 hybrid plasmids. After screening twenty-five of these candidates, five different clones were found which contain yeast ribosomal protein gene sequences. The yeast mRNAs complementary to these five plasmids code for 35S-methionine-labeled polypeptides which co-migrate on two-dimensional gels with yeast ribosomal proteins. Consistent with previous studies on ribosomal protein mRNAs, the amounts of mRNA complementary to three of these cloned genes are controlled by the RNA2 locus. Although two of the five clones contain more than one yeast gene, none contain more than one identifiable ribosomal protein gene. Thus there is no evidence for "tight" linkage of yeast ribosomal protein genes. Two of the cloned ribosomal protein genes are single-copy genes, whereas two other cloned sequences contain two different copies of the same ribosomal protein gene. The fifth plasmid contains sequences which are repeated in the yeast genome, but it is not known whether any or all of the ribosomal protein gene on this clone contains repetitive DNA.

A cold-sensitive mutant of Saccharomyces cerevisiae defective in ribosome processing

Cold-sensitive mutants of Saccharomyces cerevisiae isolated by tritium suicide were screened for defects in ribosome biosynthesis. The biochemical defects of mutant dip-1 (defective in processing) were characterized; it is defective in ribosome biosynthesis at the level of production of the primary 35S transcript. At restrictive conditions mutant dip-1 accumulates abnormal rRNA in addition to wild-type rRNA. In the mutant the first observable transcription product was a 14SRNA species which had sequence homologies to 18S rDNA and was the major rRNA component of the 40S ribosomal subunit. In addition, the ribonucleoprotein particles of dip-1 harbored RNA molecules with homologies to yeast rDNA which comprises the spacer region between 18S and 25S rDNA cistrons. Possible causes for the defective production of rRNA and its assembly into subunits are discussed.

Isolation and characterisation of a strain of Saccharomyces cerevisiae deficient in in vitro RNA polymerase B(II) activity

Two hundred strains of Saccharomyces cerevisiae temperature sensitive for RNA synthesis were selected and screened in crude extracts for DNA-dependent RNA polymerase activities. One strain was isolated which had only residual in vitro RNA polymerase B activity. In normal growth conditions total RNA, poly A+ RNA and protein synthesis were indistinguishable from those of the wild type strain at 23 degrees C and after shift to 37 degrees C. A temperature sensitive phenotype was detected only when rpoB containing strains were grown in adverse conditions. The mutant character showed mendelian segregation and was coexpressed with the wild type character in heterozygous diploids. Residual enzyme activity was characterised in crude extracts using synthetic polymers and natural templates in different ionic conditions.

Concerted repression of the synthesis of the arginine biosynthetic enzymes by aminoacids: a comparison between the regulatory mechanisms controlling aminoacid biosyntheses in bacteria and in yeast

It has been shown that in bacteria, besides specific regulatory mechanisms, the synthesis of aminoacid biosynthetic enzymes is also controlled by the endogenous aminoacid pool. The latter regulates the intracellular level of ppGpp, a positive effector of RNA messenger transcription. A similar regulatory control exists in yeast but does not appear to involve the same general effector. This was established by the observation that derepression of the enzymes belonging to several aminoacid biosynthetic pathways follows aminoacid starvation or tRNA discharging. We now report the repression of the arginine pathway by the total aminoacid pool. New mutations affecting the repressibility of the arginine enzymes as well as enzymes belonging to other aminoacid biosyntheses, when cells are grown in the presence of an excess of aminoacids, were identified.

Ribosomal RNA transcription in a mutant of Saccharomyces cerevisiae defective in ribosomal protein synthesis

In Saccharomyces cerevisiae, the transcription of ribosomal precursor RNA is severely inhibited in the absence of protein synthesis. However, such transcription is not dependent on the synthesis of ribosomal proteins, nor on the synthesis of mRNA for ribosomal proteins, nor on the processing of ribosomal precursor RNA.

Mutants of yeast with depressed DNA synthesis

Seven temperature-sensitive mutants have been isolated in Saccharomyces cerevisiae which show a reproducible defect in DNA synthesis at the restrictive temperature. One of these is allelic with rna11 (Hartwell et al., 1970) but the remaining mutants define six complementation groups and probably represent six different genes. The gene symbol dds (for depressed DNA synthesis) is proposed. At the restrictive temperature, rna11-2, dds2-1 and dds6-1 show a rapid and almost total cessation of DNA and RNA synthesis, whilst protein synthesis continues for several hours. The remaining dds mutants show a reduced rate of DNA synthesis from the time of temperature shift (dd1, dds3, dds4) or a cessation of DNA synthesis at a later time (dds5). In some cases, RNA synthesis is affected concomitantly with, or soon after, the depression in DNA synthesis. Possible reasons for the phenotypes of these mutants, and for the relative absence of yeast mutants which are unambiguously and specifically affected in DNA synthesis, are discussed. In addition, we report the isolation of seven new alleles of known cdc genes and ten new mutants with a cell cycle phenotype that complement those already known.

Changes in regulation of ribosome synthesis during different stages of the life cycle of Saccharomyces cerevisiae

Diploid strains of Saccharomyces cerevisiae, each homozygous for one of the temperature sensitive mutations rna2, rna3, rna4, rna6 or rna8, are temperature sensitive for ribosome synthesis during vegetative growth, but are not inhibited for ribosomal synthesis at the restrictive temperature under sporulation conditions. The continued ribosome biosynthesis at the restrictive temperature (34 degrees C) during sporulation includes de novo synthesis of both ribosomal RNA and ribosomal proteins. This lack of inhibition of ribosome biosynthesis is found even when cells committed to complete sporulation are returned to vegetative growth medium. The ribosomes synthesized at 34 degrees C are apparently functional, as they are found in polyribosomes. Although the rna mutants do not regulate ribosome synthesis during sporulation, all of these diploid strains fail to complete sporulation at 34 degrees C. The cells are arrested after the second meiotic nuclear division but before ascus formation. The failure to complete sporulation at the restrictive temperature and the inhibition of ribosome biosynthesis during growth are caused by the same mutation, because revertants selected for temperature independent growth were also able to sporulate at 34 degrees C.

Molecular basis for the maximum growth temperature of an obligately psychrophilic yeast, Leucosporidium stokesii

Cells of the obligately psychrophilic yeast Leucosporidium stokesii were subjected to permissive (15 and 20 degrees C) and restrictive (23 and 25 degrees C) temperatures to determine the event(s) responsible for the low maximum growth temperature of this organism. An investigation of subcellular morphology by nuclear staining revealed that buds formed at 20 degrees C were anucleate but showed nuclear migration within the parent cell. Cells incubated initially at 23 degrees C and then shifted down to a permissive growth temperature of 15 degrees C in the presence of a deoxyribonucleic acid (DNA) synthesis inhibitor, hydroxyurea, confirmed the observation that the anucleate condition of atypical buds was the result of temperature-sensitive DNA synthesis. Concomitantly, the incorporation of labeled adenine into DNA was inhibited at 23 and 25 degrees C. The synthesis of ribonucleic acid, however, was enhanced at 23 degrees C but impaired at 25 degres C. Similarly, protein synthesis was unaffected at either restrictive temperature.

Ribosomal subunit entry into polysomes in yeast

The kinetics of entry of newly synthesized 40 S and 60 S ribosomal subunits into yeast polysomes is described. The entry times for 40 S and 60 S subunits were found to be 3 and 8 min, respectively. The kinetics of entry of 40 S subunits into large polysomes is found to be different from the kinetics of entry of 60 S subunits into large polysomes.

Coordinate regulation of the synthesis of eukaryotic ribosomal proteins

We have developed a method of r the direct measurement, in eukaryotic cells, of the synthesis of ribosomal proteins, irrespective of the synthesis of ribosomes. In this way the synthesis of ribosomal proteins has been examined in mutant strains of Saccharomyces cerevisiae, which are unable to synthesize ribosomes under nonpermissive conditions. The results suggest that the synthesis of more than 40 ribosomal proteins is under coordinate control. Under nonpermissive conditions,the synthesis of each h protein declines exponentially to a basal level which is 10-20% fo normal. The kinetics of that decline suggest that an early, if not primary, result of the nonpermissive conditions is the cessation of production of new mRNA for eac of the ribosomal proteins. The coordinate regulation appears not to be influenced directly by the rate of transcription of ribosomal precursor RNA.

A yeast mutant defective in the processing of 27S r-RNA precursor

Among a group of 31 ts- yeast mutants screened electrophoretically for heat-sensitive synthesis of each stable RNA species, only mutant ts351 failed to accumulate 25S RNA at 36 degrees C. Pulse-labeling experiments at 36 degrees C showed that 35S and 27S precursors RNA and mature 18S r-RNA molecules are synthetized by ts351 cells but that 25S and 5.8S RNA species are not made and new 60S ribosomal sub-units are not assembled. The mutant is blocked at a specific point in r-RNA processing: the cutting of 27S to form 25S and 5.8S r-RNA.

Thermosensitive mutations affecting ribonucleic acid polymerases in Saccharomyces cerevisiae

Among 150 temperature-sensitive Saccharomyces cerevisiae mutants which we have isolated, 15 are specifically affected in ribonucleic acid (RNA) synthesis. Four of these mutants exhibit particularly drastic changes and were chosen for a more detailed study. In these four mutants, RNA synthesis is immediately blocked after a shift at the nonpermissive temperature (37 C), protein synthesis decays at a rate compatible with messenger RNA half-life, and deoxyribonucleic acid synthesis increases by about 40%. All the mutations display a recessive phenotype. The segregation of the four allelic pairs ts-/ts+ in diploids is mendelian, and the four mutants belong to three complementation groups. The elution patterns (diethylaminoethyl-Sephadex) of the three RNA polymerases of the mutants grown at 37 C for 3.5 h show very low residual activities. The in vitro thermodenaturation confirms the in vivo results; the half-lives of the mutant activities at 45 C are 10 times smaller than those of the wild-type enzymes. Polyacrylamide gel electrophoresis shows that the synthesis of all species of RNA is thermosensitive. The existence of three distinct genes, which are each indispensable for the activity of the three RNA polymerases in vivo as well as in vitro, strongly favors the hypothesis of three common subunits in the three RNA polymerases.

Isolation of cold-sensitive Chinese hamster cells

Six cold-sensitive variants have been isolated from Chinese hamster ovary cells by the BUdR-visible light selection technique. The properties of one of these lines have been studied in detail. This line stops dividing immediately after a shift from 39 degrees C to 33 degrees C though its doubling time at 39 degrees C is only slightly longer than that of wild-type cells. The rates of DNA and protein synthesis are severely reduced at 33 degrees C, but the rate of RNA synthesis is not significantly different from wild-type cells. This line may be defective in protein synthesis, but the results of sedimentation analysis indicate that it probably has normal ribosomal subunit assembly.

Dominant and semidominant mutations leading to thermosensitivity of ribonucleic acid biosynthesis in Saccharomyces cerevisiae

Different dominant thermosensitive mutations affecting the same gene were selected in Saccharomyces cerevisiae. Ribonucleic acid (RNA) synthesis decreased rapidly and markedly at 37 C in all the mutants whether they were in a homozygous or a heterozygous state. Protein biosynthesis was at first unaffected and then decreased slowly, stopping after 5 h. Measurements of RNA biosynthesis in isolated nuclei as well as in vitro activities of RNA polymerases A and B at 22 and 37 C failed to reveal any difference between mutants and the wild type. Analysis of the nature of the residual RNAs synthesized at the high temperature in the mutants showed a small relative increase in the messenger RNA fraction, but it was not sufficient to indicate a specific inactivation of RNA polymerase A activity. The results suggest an impairment in a common regulatory element for all RNA polymerases acting at the level of the initiation of transcription. Similar mutants with a semidominant phenotype were obtained in which the lesions were in two other unlinked loci.

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.

Neurospora crassa cytoplasmic ribosomes: isolation and characterization of a cold-sensitive mutant defective in ribosome biosynthesis

Twenty-seven cold-sensitive mutants of Neurospora crassa were isolated by mutagenesis of wild-type conidia followed by filtration enrichment in complete medium at the nonpermissive temperature (10 C). Zone sedimentation analyses of cytoplasmic ribosomes isolated from the wild-type strain and from 14 of the mutant strains grown at 10 C indicate that one cold-sensitive mutant is defective in ribosome biosynthesis at that temperature: instead of the 2.3:1 mass ratio of 60S:37S ribosomal subunits characteristic of wild type, the mutant strain PJ30201 (called crib-1 for cytoplasmic ribosome biosynthesis) exhibits a mass ratio of approximately 7.2:1. Ribosomal subunits synthesized by strain PJ30201 at 25 C are present in wild-type proportions. The cold-sensitive and ribosomal phenotypes segregate together in tetrads isolated from crosses between strain PJ30201 and the wild type indicating that a single nuclear gene mutation is probably responsible for both mutant phenotypes. The crib-1 locus lies near the centromere in linkage group IV.

Isolation of polyribosomes from yeast during sporulation and vegetative growth

Exponentially growing and sporulating cells of Saccharomyces cerevisiae have been subjected to a variety of conditions which mechanically disrupt the cell in an effort to establish conditions which permit the recovery of intact polyribosomes. Grinding cells for 10 s with glass beads in a Bronwill cell homogenizer was sufficiently gentle to yield a polyribosome content in exponentially growing cells which was similar to values obtained from yeast spheroplasts. Polyribosome patterns in sporulating yeast were similar to those from exponentially growing cells. This technique is fast, reproducible over a wide range of cell concentrations, and eliminates the need to make spheroplasts to recover intact polyribosomes.

Mutation in Saccharomyces cerevisiae conferring streptomycin and cold sensitivity by affecting ribosome formation and function

A cold-sensitive, streptomycin-sensitive mutant of Saccharomyces cerevisiae accumulates a 28S ribonucleoprotein particle when grown at low temperature. This particle contains 17S ribosomal ribonculeic acid which is degraded when exposed to ribonuclease. The particle does not serve as a precursor to 60 and 40S ribosomal subunits nor is it turned over when growth is allowed to resume at the permissive temperature; rather it is only diluted by growth. That streptomycin sensitivity (allelic with cold sensitivity) is ribosomal is evidenced by the inhibition of protein synthesis in vitro by streptomycin and the binding of labeled streptomycin to the mutant but not the parental 40S ribosomal subunit.

Potentiation of rifampicin and 5-fluorocytosine as antifungal antibiotics by amphotericin B (yeast-membrane permeability-ribosomal RNA-eukaryotic cell-synergism)

Amphotericin B potentiates the antifungal effects of 5-fluorocytosine and rifampicin, probably by increasing the penetration of these agents through the fungal cytoplasmic membrane.