Use of yeast populations fractionated by zonal centrifugation to study the cell cycle

Identification of proteins whose synthesis is modulated during the cell cycle of Saccharomyces cerevisiae

We examined the synthesis and turnover of individual proteins in the Saccharomyces cerevisiae cell cycle. Proteins were pulse-labeled with radioactive isotope (35S or 14C) in cells at discrete cycle stages and then resolved on two-dimensional gels and analyzed by a semiautomatic procedure for quantitating gel electropherogram-autoradiographs. The cells were obtained by one of three methods: (i) isolation of synchronous subpopulations of growing cells by zonal centrifugation.; (ii) fractionation of pulse-labeled steady-state cultures according to cell age; and (iii) synchronization of cells with the mating pheromone, alpha-factor. In confirmation of previous studies, we found that the histones H4, H2A, and H2B were synthesized almost exclusively in the late G1 and early S phases. In addition, we identified eight proteins whose rates of synthesis were modulated in the cell cycle, and nine proteins (of which five, which may well be related, were unstable, with half-lives of 10 to 15 min) that might be regulated in the cell cycle by periodic synthesis, modification, or degradation. Based on the time of maximal labeling in the cell cycle and on experiments with alpha-factor and hydroxyurea, we assigned the cell cycle proteins to two classes: proteins in class I were labeled principally in early G1 phase and at a late stage of the cycle, whereas those in class II were primarily synthesized at times ranging from late G1 to mid S phase. At least one major control point for the cell cycle proteins occurred between "start" and early S phase. A set of stress-responsive proteins was also identified and analyzed. The rates of synthesis of these proteins were affected by certain perturbations that resulted during selection of synchronous cell populations and by heat shock.

Identification of proteins whose synthesis is modulated during the cell cycle of Saccharomyces cerevisiae

We examined the synthesis and turnover of individual proteins in the Saccharomyces cerevisiae cell cycle. Proteins were pulse-labeled with radioactive isotope (35S or 14C) in cells at discrete cycle stages and then resolved on two-dimensional gels and analyzed by a semiautomatic procedure for quantitating gel electropherogram-autoradiographs. The cells were obtained by one of three methods: (i) isolation of synchronous subpopulations of growing cells by zonal centrifugation.; (ii) fractionation of pulse-labeled steady-state cultures according to cell age; and (iii) synchronization of cells with the mating pheromone, alpha-factor. In confirmation of previous studies, we found that the histones H4, H2A, and H2B were synthesized almost exclusively in the late G1 and early S phases. In addition, we identified eight proteins whose rates of synthesis were modulated in the cell cycle, and nine proteins (of which five, which may well be related, were unstable, with half-lives of 10 to 15 min) that might be regulated in the cell cycle by periodic synthesis, modification, or degradation. Based on the time of maximal labeling in the cell cycle and on experiments with alpha-factor and hydroxyurea, we assigned the cell cycle proteins to two classes: proteins in class I were labeled principally in early G1 phase and at a late stage of the cycle, whereas those in class II were primarily synthesized at times ranging from late G1 to mid S phase. At least one major control point for the cell cycle proteins occurred between "start" and early S phase. A set of stress-responsive proteins was also identified and analyzed. The rates of synthesis of these proteins were affected by certain perturbations that resulted during selection of synchronous cell populations and by heat shock.

Identification of different daughter and parent subpopulations in an asynchronously growing Saccharomyces cerevisiae population

Under all growth conditions, a growing Saccharomyces cerevisiae yeast population is extremely heterogeneous, since individual cells differ in their cell size; this is due to their position in the cell division cycle and their genealogical age. To gain insight into the structure of a growing yeast population, we used a recently developed flow cytometric approach which enables, in asynchronously growing S. cerevisiae populations, tagging of both the cell age and the protein content of individual cells. This approach enabled the identification of daughter cells belonging to different cell cycle positions (i.e. newborn, G1, S + G2 + M + G1*, and dividing), thus yielding information about the relative fraction in the whole population, cell size and variability. More limited information could be obtained for the parent subpopulation; however, we were able to identify and characterize the dividing parent cells. The coefficient of variation (CV) of the protein content distribution for the dividing parents (27) was much higher than the CV of dividing daughters (18). Further findings obtained indicated a large overlap between the cell protein content distributions of daughter and parent cells as well as between the protein content of cells of the same subpopulation but belonging to different stages of the cell division cycle. The analysis of these differences enables a better understanding of the complex structure of an asynchronously growing yeast population.

A double flow cytometric tag allows tracking of the dynamics of cell cycle progression of newborn Saccharomyces cerevisiae cells during balanced exponential growth

Studies on the dynamics of growth of single eukaryotic cells and their relationships with cell cycle regulations are generally carried out following cell synchronization procedures or, on a relatively low number of cells, by time-lapse studies. Establishment of both time-lapse studies and synchronous cell populations usually requires elaborate experimental efforts and is prone to perturb the physiological state of the cell. In this paper we use a new flow cytometric approach which allows, in asynchronous growing Saccharomyces cerevisiae populations, tagging of both the cell age and the cell protein content of a cohort of daughter cells at the different cell cycle set points. Since the cell protein content is a good estimation of the cell size, it is possible to follow the kinetics of the cell size increase during cell cycle progression. The experimental findings obtained indicate an exponential increase of the cell size during growth, that the daughter and the parent subpopulations grow with the same specific growth rate, that the average cell size increase rate of each individual cell is almost identical to the specific growth rate of the overall population and provide the opportunity to estimate the cell cycle length for the daughter cell population as well as the identification of the complex structure of asynchronously growing yeast populations.

Protein synthesis during germination of heterothallic yeast ascospores

Protein synthesis during ascospore germination of the heterothallic Saccharomyces cerevisiae strain AP-3 was investigated. Protein synthesis in the germinating ascospores appeared to begin approximately 20 min following glucose initiation. Since RNA synthesis did not start until approximately 70 min after the onset of germination, strain AP-3 ascospores must contain RNA which is ready for immediate translation. Both trehalase and glyceraldehyde-3-phosphate dehydrogenase activities were found to be affected by the onset of germination. Trehalase activity was found to increase severalfold following 60 min of spore germination but remained relatively constant over the subsequent 120 min examined. Dehydrogenase activity was not detectable in AP-3 ascospores but was measurable in germinating ascospores.

Dynamics of activation of a galactose-inducible promoter in Saccharomyces cerevisiae

We have investigated the dynamics of accumulation of the Escherichia coli beta-galactosidase (beta-gal) under the control of a promoter containing the galactose-inducible upstream activating sequence (UASG) in single Saccharomyces cerevisiae cells. The accumulation of beta-gal in single cells following the addition of the inducer, galactose, was determined using an in situ combined DNA and immunofluorescent stain in conjunction with flow cytometry. Two strains were studied, D603/2i, which has two copies of the galactose-inducible fusion gene integrated into its genome, and D603/pLGSD5, which carries a 2 microns-based plasmid containing the fusion gene. Flow cytometry results indicate that accumulation of beta-gal within the first three hours following the addition of galactose is dependent on cell cycle position. Two proposed mechanisms explaining this observed behavior are (1) the cell-cycle-dependent synthesis of the fusion protein or (2) the unequal partitioning of the protein at cell division between mother and daughter cells.

Control of cell growth and division in Saccharomyces cerevisiae

Considerable advances have been made in recent years in our understanding of the biochemistry of protein and nucleic acid synthesis and, particularly, the molecular biology of gene expression in eukaryotes. The yeast Saccharomyces cerevisiae, and to a lesser extent Schizosaccharomyces pombe, has had a preeminent role as a focus for these studies, principally because of the facility with which these organisms can be experimentally manipulated biochemically and genetically. This review will be designed to critically examine and integrate recent advances in several vital areas of regulatory control of enzyme synthesis in yeast: structure and organization of DNA, transcriptional regulation, post-transcriptional modification, control of translation, post-translational modification and secretion, and cell-cycle modulation. It will attempt to emphasize and illustrate, where detailed information is available, principal underlying molecular mechanisms, and it will attempt to make relevant comparisons of this material to inferred and demonstrated facets of regulatory control of enzyme and protein synthesis in higher eukaryotes.

Synthesis of specific identified, phosphorylated, heat shock, and heat stroke proteins through the cell cycle of Saccharomyces cerevisiae

The methods of centrifugal elutriation, two-dimensional gel electrophoresis, and dual isotopic labeling were applied to the study and identification of a number of purified yeast proteins. The location of polypeptide spots corresponding to specific proteins was determined on two-dimensional gels. A dual-label method was used to determine the rates of synthesis through the cell cycle of the identified proteins as well as to confirm the results of previous studies from our laboratory on unidentified proteins. The identified proteins, and the more generally defined phosphorylated, heat shock, and heat stroke proteins were found to follow the general pattern of exponential increase in rate of synthesis through the cell cycle. In addition, colorimetric enzyme activity assays were used to examine the catabolic enzyme alpha-glucosidase (EC 3.2.1.20). Both the activity and synthesis of alpha-glucosidase were found to be nonperiodic with respect to the cell cycle. These data contrast with earlier reports of periodicity, which employed induction and selection synchrony to study enzyme expression through the yeast cell cycle.

Changes in the activity and properties of trehalase during early germination of yeast ascospores: correlation with trehalose breakdown as studied by in vivo 13C NMR

The regulation of trehalose breakdown during dormancy and the induction of germination in yeast ascospores was studied both by in vivo high-resolution NMR spectroscopy and in vitro assays of trehalase activity. Natural-abundance (13)C NMR spectra taken during the induction of germination with glucose and phosphate showed a rapid breakdown of part of the trehalose content. The presence of both glucose and phosphate was important for maximal trehalose breakdown. The (13)C NMR spectra showed that the externally added glucose and the internal trehalose were metabolized mainly to glycerol and ethanol. Under these conditions of nitrogen deprivation, full germination is not possible and trehalose breakdown stopped after approximately 1 hr. At this moment resynthesis of trehalose occurred while glycerol and ethanol production from the exogenous glucose continued. In complex media where full spore germination can occur, trehalose breakdown was more pronounced. Measurements of trehalase activity in spore extracts made after addition of varying amounts of glucose and phosphate to the spores revealed a sudden 10-fold increase in the activity of trehalase, within the first minutes of spore germination. The activation was transient: after reaching a maximum between 5 and 10 min, the activity declined back to low values during the next hours. The increase in trehalase activity was not inhibited by cycloheximide or by anaerobic conditions. The decline in trehalase activity that occurred after the initial activation could be correlated with the extent of trehalose breakdown as measured by (13)C NMR. In addition to the increase in trehalase activity, differences in the control properties were found between the enzymes from dormant and germinating spores. Trehalase from dormant spores was strongly inhibited by ATP at a concentration of approximately 0.5 mM, which corresponds with the ATP concentration found in dormant spores. On the other hand, trehalase from germinating spores was not inhibited by ATP up to the much higher ATP concentrations that are found in germinating spores. It is suggested that the low activity and the stringent ATP feedback inhibition of trehalase from dormant spores are responsible for the very slow mobilization of the huge amount of trehalose in dormant spores. Therefore, dormancy seems to be caused primarily by extreme curtailment of the energy production within the spore at one selective and primary point. The switch towards high activity and low ATP inhibition upon induction of germination is suggested to be responsible for the breaking of dormancy and for the rapid breakdown of trehalose that occurs during the initial phase of germination.

Synthesis of specific identified, phosphorylated, heat shock, and heat stroke proteins through the cell cycle of Saccharomyces cerevisiae

The methods of centrifugal elutriation, two-dimensional gel electrophoresis, and dual isotopic labeling were applied to the study and identification of a number of purified yeast proteins. The location of polypeptide spots corresponding to specific proteins was determined on two-dimensional gels. A dual-label method was used to determine the rates of synthesis through the cell cycle of the identified proteins as well as to confirm the results of previous studies from our laboratory on unidentified proteins. The identified proteins, and the more generally defined phosphorylated, heat shock, and heat stroke proteins were found to follow the general pattern of exponential increase in rate of synthesis through the cell cycle. In addition, colorimetric enzyme activity assays were used to examine the catabolic enzyme alpha-glucosidase (EC 3.2.1.20). Both the activity and synthesis of alpha-glucosidase were found to be nonperiodic with respect to the cell cycle. These data contrast with earlier reports of periodicity, which employed induction and selection synchrony to study enzyme expression through the yeast cell cycle.

Small-sized mutants of Saccharomyces cerevisiae

The isolation of mutants of Saccharomyces cerevisiae that divide at approximately half the size of the wild type is described. Three mutants have been isolated in which the small size at bud initiation is due to a mutation in a single nuclear gene.

Reparable and irreparable damage in yeast cells induced by sparsely ionizing radiation

It is shown that in diploid yeast there are significant differences in the extent of irreparable damage after irradiation with X-rays, 60Co-gamma-rays and 30 MeV electrons. At extremely low dose rates, 60Co-gamma-rays were found to produce almost no irreparable damage at least up to 1200 Gy. X-rays, however, at the same low dose rate caused irreparable damage in the same dose range yielding a surviving fraction of 0.25 at 1200 Gy. For irradiations at high dose rate followed by liquid holding recovery the relative biological effectiveness of X-rays amounted to at least 4 for absorbed doses of up to 1000 Gy. With 30 MeV electrons at high dose rates an accumulation of sublethal and potentially lethal damage resulting in irreparable damage occurred above 1000 Gy. It is suggested that irreparable damage in yeast is due to a cooperative effect of neighbouring track ends.

Synthesis of 1,3-beta-glucanases in Saccharomyces cerevisiae during the mitotic cycle, mating, and sporulation

Upon fractionating Saccharomyces cerevisiae asynchronous cultures by sucrose density gradient centrifugation in a zonal rotor and examining the exo-1,3-beta-glucanase and deoxyribonucleic acid content of the cells, a periodic step increase in the activity of this enzyme was observed, indicating a discontinuous pattern of synthesis or activation of exo-1,3-beta-glucanase during the mitotic cycle at the transition from the S to the G(2) phase. Similar results were obtained for endo-1,3-beta-glucanase by assaying activity against oxidized laminarin in permeabilized cells, suggesting that the synthesis of endo-1,3-beta-glucanase is controlled in the same way. When a and alpha strains were mated, the specific activity of cell extracts against laminarin, oxidized laminarin, and pustulan remained constant while zygote formation was taking place. However, when growth resumed, active synthesis of 1,3-beta-glucanases took place as shown by the occurrence of a significant increase in the specific activity against the three substrates. Specific changes in the level of glucan degradative enzymes, not observed in a haploid parental strain, occurred when the diploid S. cerevisiae AP-1 was induced to sporulate. The sporulation process triggered the activation of first the pustulan degradative capacity and then the capacity to hydrolyze oxidized laminarin. The specific activity against this substrate was 10 times higher than that against pustulan.

Synthesis and modification of proteins during the cell cycle of the yeast Saccharomyces cerevisiae

We have used a novel technique to study the synthesis, modification and degradation of proteins during the cell cycle in Saccharomyces cerevisiae. Logarithmically growing cells were pulse-labeled twice, with the pulses separated in time by more than one generation. Subsequently, the cells were fractionated as to their position in the cell cycle by centrifugal elutriation, and for different proteins the ratio of radioactive material from the two pulses was then determined. Periodic degradation, synthesis, or modification would produce periodic variations in the ratio of counts. Two-dimensional gel electrophoresis was used to examine 110 different proteins at different times of the cell cycle. All but two proteins had a constant ratio of counts through the cell cycle. This indicates that the rate of synthesis of individual proteins increases exponentially during the cell cycle and that periodic degradation or modification of proteins is not a general feature of the cell cycle in S. cerevisiae.

Regulation of RNA synthesis in yeast. III. Synthesis during the cell cycle

Centrifugal elutriation was used to separate cells in different stages of the cell cycle from a culture of Saccharomyces cerevisiae in balanced exponential growth. The rate of DNA and RNA synthesis was determined using a pulse--long-term label technique that is capable of distinguishing between exponential, linear, and periodic variations in the rate of synthesis through the cell cycle. It was found that while the rate of DNA synthesis varies periodically through the cell cycle, the rate of synthesis of mRNA, rRNA, and tRNA increases exponentially through the cell cycle. The implications of these findings for the control of RNA synthesis are discussed.

Cell-cycle variation in the induction of lethality and mitotic recombination after treatment with UV and nitrous acid in the yeast, Saccharomyces cerevisiae

Exponentially growing yeast cultures separated into discrete periods of the cell cycle by zonal rotor centrifugation show cyclic variation in both UV and nitrous acid induced cell lethality, mitotic gene conversion and mitotic crossing-over. Maximum cell survival after UV treatment was observed in the S and G2 phases of the cell cycle at a time when UV induction of both types of mitotic recombination was at a minimum. In contrast, cell inactivation by the chemical mutagen nitrous acid showed a single discrete period of sensitivity which occurred in S phase cells which are undergoing DNA synthesis. Mitotic gene conversion and mitotic crossing-over were induced by nitrous acid in cells at all stages of the cell cycle with a peak of induction of both events occurring at the time of maximum cell lethality. The lack of correlation observed between maximum cell and the maximum induction of mitotic intragenic recombination suggest that other DNA-repair mechanisms besides DNA-recombination repair are involved in the recovery of inactivated yeast cells during the cell cycle.

Rate of macromolecular synthesis through the cell cycle of the yeast Saccharomyces cerevisiae

Centrifugal elutriation was used to separate cells of Saccharomyces cerevisiae in balanced exponential growth according to position in the cell cycle. Macromolecular synthesis was examined. DNA synthesis was found to be periodic, but RNA and protein synthesis showed an exponential increase in rate. Two-dimensional electrophoresis was used to determine the rate of synthesis of individual proteins, with 111 of the more abundant cellular proteins selected for analysis from among the more than 1000 proteins that migrate in the system. All the examined proteins showed an exponentially increasing rate of synthesis.

Relationship between polyamines and macromolecules in germinating yeast ascospores

The accumulation of spermidine and/or spermine was not necessary for normal macromolecule biosynthesis or germination and outgrowth of Saccharomyces cerevisiae spores.

Midcycle doubling of uptake rates of adenine and serine in Saccharomyces cerevisiae

Rates of uptake of serine and of adenine were measured as a function of cell size, and therefore age, in asynchronous, exponential phase cultures of diploid Saccharomyces cerevisiae strain Y55. In both cases, uptake rates were constant during the initial third of the cell cycle and doubled during the S period in the middle part of the cycle to a constant value during the final third. Cell size and age at mid-step doubling were indistinguishable for serine and adenine uptake, and occurred during the period of DNA synthesis. The results extend an earlier hypothesis of constancy of cell growth rates (mass accumulation rates) and rates of uptake of all or almost all compounds into cells in exponential phase growth to one of piecewise constancy, with an abrupt doubling of growth and uptake rates during DNA synthesis.

Mutagenic action of hydrostatic pressure on yeast: a cell cycle analysis

Pressure-treated log growth cultures (14,000 psi equivalent to 966 x 10(5) N/m2 for 4 h) of Saccharomyces cerevisiae were fractionated across a linear Ficoll gradient by zonal rotor centrifugation. This procedure separated the yeast cells on the basis of size and volume into a continuum of cell cycle ages. Cell survival and petite mutation frequency were determined for several zonal fractions. Survival of yeast cells after pressure treatment was maximal in zonal fractions obtained from either the top (single cells in G1) or the botton ("doublets") of the gradient. Intermediate zonal fractions showed more lethality, with minimal survival occurring in zonal fractions containing a large proportion of yeast cells in which buds were just beginning to emerge (initiation of S phase). The petite mutation frequency was minimal in zonal fractions from the top (single cells in G1) and bottom ("doublets") of the gradient. Induction increased to a maximum in those fractions containing cells in S phase.

Function of S-adenosylmethionine in germinating yeast ascospores

Germination and outgrowth of ascospores of Saccharomyces cerevisiae 4579 require both methionine and adenine, whereas leucine is only required for outgrowth. The methionine requirement may be satisfied by S-adenosylmethionine, but this sulfonium compound will not substitute for adenine. Between 30 and 70 min of protein synthesis is initially required for the completion of germination in strain 4579. The inhibition of S-adenosylmethionine synthetase by trifluoromethionine prevents both germination and protein synthesis. During the initial stages of germination, the S-adenosylmethionine synthetase, S-adenosylmethionine decarboxylase, and transfer ribonucleic acid methyltransferases increased significantly, indicating that polyamines and/or the methylation of transfer ribonucleic acid are required for the initiation of germination.

Molecular specificity of x-radiation and its repair in Saccharomyces cerevisiae

Molecular specificity of soft X-radiation has been studied in yeast by analyzing the transitions UAA in equilibrium UAG and nonsense leads to sense mutations in the codon tyr7-1. Synchronized cell populations in the most radiosensitive and radioresistant stages were compared: they did not show any qualitative or quantitative differences in their sensitivities to the mutagenic action of X-rays. We conclude that repair mechanisms, which remain unexpressed in the sensitive cells, do not affect point mutations of the base-substitution type.

Fractionation of Saccharomyces cerevisiae cell populations by centrifugal elutriation

An exponential population of Saccharomyces cerevisiae cells was fractionated by centrifugal elutriation, using water as the elutriating liquid. Evidence that the population had been fractionated according to age in the cell cycle was obtained by examining the fractions for their size distribution, their microscopic appearance after Giemsa staining, and their ability to initiate synchronous growth.

Rate of synthesis of polyadenylate-containing ribonucleic acid during the yeast cell cycle

The rate of synthesis of polyadenylate-containing ribonucleic acid is constant throughout the cell cycle of Saccharomyces cerevisiae.

Germ tube formation from zonal rotor fractions of Candida albicans

Homogenous cell populations of increasing cell volume may have been isolated from exponential and stationary culture of Candida albicans by centrifugation on a sucrose gradient. Observations of the yeast-mycelial transition using these populations showed the following. (i) No fraction from early logarithmic phase cells was unable to undergo morphological transition. (ii) The time of initiation of germ tube production was correlated with cell size in stationary-phase cultures. (iii) The rate of appearance of germ tubes was nearly identical in all fractions measured. (iv) Addition of N-acetyl-D-glucosamine to homogeneous cell populations decreased the time of initial appearance of germ tubes but did not affect the rate of appearance after initiation.

beta-D-galactosidase activity in single yeast cells during cell cycle of Saccharomyces lactis

Single Saccharomyces lactis cells taken from a random population were assayed for beta-D-galactosidase activity under a microscope equipped for fluorogenic measurements. The cells were also photographed, and enzymatic activity was correlated to the size of cell buds. A perodic pattern of enzyme synthesis was found during the cell cycle.

Nuclear and mitochondrial deoxyribonucleic acid replication during mitosis in Saccharomyces cerevisiae

To study nuclear and mitochondrial deoxyribonucleic acid (DNA) synthesis during the cell cycle, a 15N-labeled log-phase population of Saccharomyces cervisiae was shifted to 14N medium. After one-half generation, the cells were centrifuged on a sorbitol gradient in a zonal rotor to fractionate the population according to cell size and age into fractions representing the yeast cell cycle. DNA samples isolated from the zonal rotor cell samples were centrifuged to equilibrium in CsC1 in an analytical ultracentrifuge to separate the nuclear and mitochondrial DNA components. The amount of 14N incorporated into each 15N-labeled DNA species was measured. The extent of nuclear DNA replication per sample was obtained by measuring the amount of hybrid DNA. The percentage of hybrid nuclear DNA increased from 6 to 68% and then decreased to 44% during the cell cycle. Upon ultracentrifugation, mitochondrial DNA banded as a unimodal peak in all zonal rotor samples. Mitochondrial DNA replication could be ascertained only by the 14N level in each mitochondrial peak and not, as with nuclear DNA, by hybrid DNA level. In contrast to the nuclear incorporation pattern, the 14N percentage in mitochondrial DNA remained effectively constant during the cell cycle. Comparison of the data to theoretical distributions showed that nuclear DNA was replicated discontinuously during the cell cycle, whereas mitochondrial DNA was replicated continuously throughout the entire mitotic cycle.

Cell cycle-dependent induction of mutations along a yeast chromosome

The relation between DNA replication and the action of the mutagen N-methyl-N'-nitro-N-nitroso-guanidine has been studied in Saccharomyces cerevisiae. The frequenceis of reversion to prototrophy of six auxotrophic markers located along one arm of chromosome VII were examined as a function of the vegetative cell cycle. Exponentially growing cells were treated with nitrosoguanidine and then separated by zonal rotor centrifugation into fractions equivalent to stages in the cell cycle. The frequency of reversion for five of the six markers is greatest during the period of DNA replication. Each marker has a single point of maximum reversion, approximately 10-fold greater than the frequency observed at other points in the cell cycle. For any one marker the effect of nitrosoguanidine is restricted to an interval shorter than the period of DNA replication. The two markers most distant from each other, ade5 and leul, both have their highest reversion frequency early during DNA replication. The peak reversion frequency for lys5 is somewhat later, while the peaks for tyr3 and trp5 occur near the end of DNA replication. The results indicate that nitrosoguanidine acts primarily during DNA replication and that different markers appear to be affected at different intervals during the DNA biosynthetic period. If nitrosoguanidine does act at the growing point of DNA replication, these observations indicate that the initiation of DNA replication occurs at specific times during the period of DNA synthesis and at specific initiation sites. Further, there must be more than one point of initiation of DNA replication on one arm of chromosome VII.

Control of RNA synthesis in yeast

During the cell cycle in Saccharomyces cerevisiae there is an ordered appearance of a number of enzymes and various physiological properties but a continuous increase in the rate of rRNA synthesis. A detailed study of rRNA synthesis has shown that there are reiterated genes for rRNA which are largely clustered on chromosome I and appear to be transcribed continuously during the cell cycle. However, the level of activity of polymerase I is proportional to the level of rRNA during the cell cycle and is correlated with the growth rate of the culture. In contrast, the level of polymerase II, thought to be involved in mRNA synthesis, increases during a definite period of the cell cycle characteristic of step enzymes in yeast. It would appear that the level of the activity of polymerase I is involved in the regulation of rRNA synthesis. Possible other mechanisms for the regulation of rRNA are discussed.