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R A Singer

Publications and source records attributed to R A Singer.

At least 55 records · Page 3Linked to original sources
PubMed1988

Rapid initial cleavage of nascent pre-rRNA transcripts in yeast.

In yeast cells, as in many other eukaryotes, the initial step in the processing of the pre-rRNA primary transcript is removal of external transcribed spacer (ETS) sequences from the 5' end of the transcript. We show here, both by Northern analysis and by quantitative hybridization procedures using cloned yeast ETS sequences, that in cells growing exponentially at 23 degrees C most nascent pre-rRNA transcripts no longer contain ETS sequences. Moreover, quantitative hybridization shows that uncleaved pre-rRNA molecules that still contain ETS sequences have a half-life of only 0.5 minute, a value that supports the finding that ETS removal usually takes place before pre-rRNA transcription is complete. Under these same conditions, the half-life of ETS sequences is shown to be only 1.0 minute.

Hot Temperature↗
PubMed1987

Molecular characterization of the yeast PRT1 gene in which mutations affect translation initiation and regulation of cell proliferation.

Several temperature-sensitive cell division cycle (cdc) mutations differentially affect the regulatory step for cell proliferation in the yeast Saccharomyces cerevisiae. We recently found that one of these mutations, cdc63-1, resides in a gene called PRT1; other mutations in this gene had been previously shown to affect translation initiation. Here we report the molecular cloning and characterization of the PRT1 gene from yeast. Our results show that the PRT1 gene is an essential, single-copy gene which encodes a 2500-nucleotide polyadenylated transcript. The nucleotide sequence indicates that the gene could code for a protein product of Mr 88,000, which bears no overall amino acid sequence similarity to any other known protein but which contains similarity over a limited region to amino acid sequences involved in nucleotide binding.

Amino Acid Sequence↗
PubMed1987

Regulated arrest of cell proliferation mediated by yeast prt1 mutations.

Several temperature-sensitive cell-division-cycle (cdc) mutations differentially affect the regulatory step for cell proliferation in the yeast. Saccharomyces cerevisiae, including one mutation termed cdc63-1, which resides in a previously known gene called PRT1. Other mutations in the PRT1 gene have been shown by others to affect an initiation step in protein synthesis. Here we show that at the appropriate nonpermissive temperature each prt1 mutation can produce a uniform and concerted arrest of cell division; the prt1-1 mutation, like cdc63-1, is shown to arrest cells specifically at the regulatory step for cell proliferation. This response of cessation of cell division is different from the response of cells to an equivalent limitation of protein synthesis using cycloheximide or verrucarin A, which implies that the PRT1 gene product could separately influence both cellular growth via protein synthesis and events in the regulation of cell proliferation.

Cell Division↗
PubMed1987

A yeast mutant conditionally defective only for reentry into the mitotic cell cycle from stationary phase.

We report the isolation of a cold-sensitive mutant of the yeast Saccharomyces cerevisiae that is conditionally defective only for reentry into the mitotic cell cycle from stationary phase. Although actively dividing mutant cells shifted to the restrictive temperature continued to divide, stationary-phase mutant cells placed in fresh medium at the restrictive temperature failed to divide or even perform the cell cycle regulatory step "start" but did lose the characteristic stationary-phase properties of thermotolerance, accumulation of storage carbohydrates, and resistance to cell-wall-lytic enzymes. Order-of-function analysis indicated that the cold-sensitive defect blocked cells during reentry before start of the first mitotic cell cycle. Genetic analysis showed that the mutant phenotype is due to the interaction between two mutations, a cold-sensitive mutation gcs1 and a suppressor mutation sed1. These mutations thus provide the genetic basis for further analysis of stationary phase and the G0 state.

Cell Cycle↗
PubMed1987

Production of heat shock protein is independent of cell cycle blockage in the yeast Saccharomyces cerevisiae.

In response to certain environmental stresses, cells display a response characterized by the production of heat shock proteins. In this study we showed that blockage of cells of the yeast Saccharomyces cerevisiae at specific points in the mitotic cell cycle was not in itself a stress that induced the production of heat shock proteins. Nevertheless, cell cycle blockage did not preclude a normal heat shock response in arrested cells subjected to elevated temperatures.

Cell Cycle↗
PubMed1985

Effects of sinefungin on rRNA production and methylation in the yeast Saccharomyces cerevisiae.

The antifungal agent, Sinefungin (SF), has been shown to be an inhibitor of transmethylation reactions. We report here the effects of SF on the production and methylation of rRNA in the yeast, Saccharomyces cerevisiae. Under conditions of SF treatment which have been shown to affect the regulation of cell proliferation in this yeast, pulse-chase labeling experiments using [methyl-3H]methionine and [3H]uracil indicated that methyl incorporation into rRNA during a short labeling period was inhibited, and stable 18 S rRNA production was differentially decreased. Other experiments quantitating modified nucleotides in newly produced rRNA showed that stable molecules were methylated. Taken together, these results suggest that SF slows methylation of rRNA, and is associated with differential loss of undermethylated 18 S rRNA species.

Adenosine↗
PubMed1985

Growth and the DNA-division sequence in the yeast Saccharomyces cerevisiae.

Cells of the yeast S. cerevisiae can be cultured under conditions in which the DNA-division sequence, and not cellular growth, is the rate-limiting feature for cell proliferation. Relief of these limiting conditions, which has been shown to allow accelerated cell division, did not result in increased rates of cell mass accumulation during the time of rapid cell division. Moreover, under conditions of constant DNA-division sequence constraint, populations of smaller cells produced by slowing growth with cycloheximide gave rise to large cells when cycloheximide was removed. These observations suggest that in proliferating cells of S. cerevisiae the DNA-division sequences does not affect cellular growth.

Cell Cycle↗
PubMed1985

Indirect suppression of the wee1 mutant phenotype in Schizosaccharomyces pombe.

For S. pombe cells mutations in the wee1 regulatory gene have been shown previously to allow cells to be smaller than normal at cell division, to endow the cell with a significantly long G1 cell cycle interval, and to alter the timing in the cell cycle of certain mutationally-defined cell cycle steps in G2. We show here that situations which lengthen S phase in proliferating wee1 mutant cells 'suppress' to varying degrees these wee1-mediated cell cycle alterations. Conditions chosen to protract S phase were use of cdc22.M45 mutant cells at semipermissive temperatures, and the presence of sub-arresting concentrations of the S phase inhibitors hydroxyurea or deoxyadenosine. Proliferation in the presence of each of these inhibitors was shown directly to result in protracted S phase. Residual cell division measurements were used to measure the cell cycle timing of G1 and G2 cell-cycle steps. The indirect suppression of the wee1 phenotype shown here can be understood in terms of the proposed role of the wee1+ gene product in coordinating cell division with cellular growth.

Ascomycota↗
PubMed1985

Novel cell cycle regulation in the yeast Schizosaccharomyces pombe. The DNA-division sequence modulates mass accumulation.

For wee1 mutant cells of Schizosaccharomyces pombe the DNA-division sequence of the cell cycle can be differentially slowed by the presence of low concentrations of the S-phase inhibitor hydroxyurea, or by semipermissive temperatures for certain wee1 cdc double mutants. Under these conditions the rate of proliferation is decreased, yet still exponential. Relief of these constraints slowing the DNA-division sequence resulted in prompt increases in the exponential rates of mass accumulation, to rates greater than those normally found. These observations suggest that mass accumulation by this yeast is always modulated by performance of the DNA-division sequence.

Ascomycota↗
PubMed1984

Bud formation by the yeast Saccharomyces cerevisiae is directly dependent on "start".

Cells of the yeast Saccharomyces cerevisiae, which bear a cdc4 gene mutation, arrest early in the cell cycle but continue to produce buds in a periodic fashion. We show here that this periodic bud formation by cells already arrested at the CDC4 step is inhibited if the cell cycle regulatory step "start" is also specifically blocked by mutation or by the presence of the yeast mating pheromone alpha-factor. Thus, the characteristic periodic bud formation by cdc4 mutant cells requires the continued ability to perform start. This finding raises questions concerning the nature of start; these issues are discussed.

Cell Division↗
PubMed1983

Growth and the cell cycle of the yeast Saccharomyces cerevisiae. I. Slowing S phase or nuclear division decreases the G1 cell cycle period.

Cells of the yeast Saccharomyces cerevisiae were subjected to a number of treatments which protracted S phase without proportional effects on growth processes. These treatments allowed steady-state exponential growth but lengthened the overall generation time. Such cells exhibited larger cell sizes and earlier performance of the cell cycle regulatory event 'start' and the two prereplicative steps defined by cdc4 and cdc7 mutations. Similarly, inhibiting progress through nuclear division with sub-arresting concentrations of methyl-benzimidazole-2-yl-carbamate also caused longer steady-state cell cycle times and earlier performance of 'start'. These findings underscore and extend earlier conclusions that most of the G1 interval of the yeast cell cycle is simply a period of ongoing growth. Conditions which protract any one of the periodic events in the division process without affecting growth will lead to the virtual elimination of the G1 interval.

Benzimidazoles↗
PubMed1983

Growth and the cell cycle of the yeast Saccharomyces cerevisiae. II. Relief of cell-cycle constraints allows accelerated cell divisions.

For cells of the yeast Saccharomyces cerevisiae, conditions which limit S phase or nuclear division allow steady-state division kinetics without significant effects on growth. Such cells become unusually large. When large proliferating cells were released from any one of several conditions which slowed progress through the DNA-division sequence, they underwent a period of accelerated division with a cell cycle devoid of a G1 interval, as evidenced by low proportions of unbudded cells and shifted execution points for the 'start' cell cycle step. We interpret these results to mean that when released from conditions slowing the DNA-division sequence these large cells continue for several cell doublings to accumulate mass fast enough to eliminate the need for a G1 interval. The results support the conclusion that the yeast G1 interval is the for most part only an interval of growth.

Benzimidazoles↗
PubMed1981

Nature of the G1 phase of the yeast Saccharomyces cerevisiae.

Under conditions that protract the S phase for Saccharomyces cerevisiae without affecting steady-state rates of cell growth or proliferation, there were striking decreases in the length of the G1 period. These decreases were localized in the period between mitosis and the start event that initiates a new cell cycle. We conclude that this major fraction of the G1 period has no functional role in the DNA-division sequence of cell cycle events.

Cell Division↗
PubMed1980

Ribosomal precursor RNA metabolism and cell division in the yeast Saccharomyces cerevisiae.

When shifted from 23 degrees C to 36 degrees C, cells of a non-temperature-sensitive strain of yeast arrest transiently in G1 before continuation of the cell division cycle. When shifted to 36 degrees C, cells harboring a temperature-sensitive rna mutation behave similarly. Others have shown that temperature shift transiently decreases the rates of production and processing of ribosomal precursor RNA (rpreRNA). Production of rpreRNA is soon restored to normal levels in these strains, but normal processing of these repreRNA transcripts is restored only in non-temperature-sensitive strains. Therefore these experiments serve to eliminate from cell cycle considerations the involvement of processing of rpreRNA, while maintaining the established correlation between cell cycle behavior and rpreRNA production.

Cell Division↗
PubMed1980

Transient cell cycle arrest of Saccharomyces cerevisiae by amino acid analog beta-2-DL-thienylalanine.

When treated with the amino acid analog beta-2-DL-thienylalanine, cells of the yeast Saccharomyces cerevisiae accumulated in the G1 portion of the cell cycle at the "start" event. This G1 arrest was accompanied by a rapid decrease in the rate of labeling of ribonucleic acid (RNA) with little effect on the rate of labeling of protein. When we examined which aspect of RNA metabolism was most affected by beta-2-DL-thienylalanine treatment, we found a dramatic decrease in the production of ribosomal precursor RNA. These results are consistent with previous findings which show a correlation between G1 arrest and reduced ribosomal precursor RNA production. The G1 arrest brought about be beta-2-DL-thienylalanine was transient; cells remain arrested in G1 only for several hours. Release from G1 arrest appeared to be accompanied either by metabolism or sequestration of the analog.

Alanine↗
PubMed1980

Ornithine decarboxylase activity and cell cycle regulation in Saccharomyces cerevisiae.

In the yeast Saccharomyces cerevisiae, the specific activity of the enzyme ornithine decarboxylase (ODC) was correlated with overall growth status. The activity of ODC was highest in actively growing cells, whereas the specific activity was lower in slow-growing cultures limited for nitrogen or inhibited by low concentrations of cycloheximide. Specific activities of ODC were also low in cultures arrested in the stationary phase (in the G1 portion of the cell cycle) by starvation for required nutrients. Although correlated with overall growth, ODC activity was not required for growth or cell cycle regulation. Cells continued to grow in the presence of the polyamine spermidine or spermine, which markedly reduced ODC specific activities. Thus, high levels of ODC activity were not necessary for growth, nor were decreased ODC specific activities sufficient to cause cells to arrest in G1. Conversely, one agent (o-phenanthroline) which causes growing cells to arrest in G1 did so with no effect on ODC specific activity. Therefore, ODC specific activity changes are not necessary for cell cycle regulation but simply reflect the normal growth status of cells.

Carboxy-Lyases↗
PubMed1979

Nalidixic acid causes a transient G1 arrest in the yeast Saccharomyces cerevisiae.

The addition of nalidixic acid to growing cells of the yeast Saccharomyces cerevisiae resulted in a transient depression in the rate of ribosomal precursor RNA production and a transient arrest of cells in G1. Protein synthesis rates were less affected. Lower concentrations of nalidixic acid also affected RNA synthesis and progression through G1 but had no effect on protein synthesis rates. We suggest that nalidixic acid has a primary effect on RNA synthesis leading to a G1 arrest.

Cell Cycle↗