The yeast cell cycle: coordination of growth and division rates

Asymmetric cell division requires specific mechanisms for adjusting global transcription

Most cells divide symmetrically into two approximately identical cells. There are many examples, however, of asymmetric cell division that can generate sibling cell size differences. Whereas physical asymmetric division mechanisms and cell fate consequences have been investigated, the specific problem caused by asymmetric division at the transcription level has not yet been addressed. In symmetrically dividing cells the nascent transcription rate increases in parallel to cell volume to compensate it by keeping the actual mRNA synthesis rate constant. This cannot apply to the yeast Saccharomyces cerevisiae, where this mechanism would provoke a never-ending increasing mRNA synthesis rate in smaller daughter cells. We show here that, contrarily to other eukaryotes with symmetric division, budding yeast keeps the nascent transcription rates of its RNA polymerases constant and increases mRNA stability. This control on RNA pol II-dependent transcription rate is obtained by controlling the cellular concentration of this enzyme.

Metabolic control of glucose degradation in yeast and tumor cells

Regulation of glucose degradation in both yeasts and tumor cells is very similar in many respects. In both cases it leads to excretion of intermediary metabolites (e.g., ethanol, lactate) in those cell types where uptake of glucose is unrestricted (Saccharomyces cerevisiae, Bowes melanoma cells). The similarities between glucose metabolism observed in yeast and tumor cells is explained by the fact that cell transformation of animal cells leads to inadequate expression of (proto-)oncogenes, which force the cell to enter the cell cycle. These events are accompanied by alterations at the signal transduction level, a marked increase of glucose transporter synthesis, enhancement of glycolytic key enzyme activities, and slightly reduced respiration of the tumor cell. In relation to homologous glucose degradation found in yeast and tumor cells there exist strong similarities on the level of cell division cycle genes, signal transduction and regulation of glycolytic key enzymes. It has been demonstrated that ethanol and lactate excretion in yeast and tumor cells, respectively, result from an overflow reaction at the point of pyruvate that is due to a carbon flux exceeding the capacity of oxidative breakdown. Therefore, the respiratory capacity of a cell determines the amount of glycolytic breakdown products if ample glucose is available. This restricted flux is also referred to as the respiratory bottleneck. The expression "catabolite repression", which is often used in textbooks to explain ethanol and acid excretion, should be abandoned, unless specific mechanisms can be demonstrated. Furthermore, it was shown that maximum respiration and growth rates are only obtained under optimum culture conditions, where the carbon source is limiting.

The decisive role of the Saccharomyces cerevisiae cell cycle behaviour for dynamic growth characterization

The dynamic behaviour of the cell cycle and the physiology of Saccharomyces cerevisiae was monitored in transient experiments. Frequent flow cytometric analyses of the DNA (nuclear phase state) and the cell size enabled us to characterize the proliferation properties of yeast cells under well controlled and undisturbed cultivation conditions. Preliminarily, the correlation between flow cytometric light scattering measurements and the cell size was attested for yeasts. These flow cytometric results are compared with the physiological behaviour of the culture that was detected by high resolution on-line analyses and off-line measurements. The presented results focus on the importance of the yeast cell cycle behaviour for the dynamic growth characterization. Any kind of transients in yeast cultures induced partial synchronization. The characteristics and the time course of the yeast cell cycle were found to be strongly dependent on the physiological environment.

Changes in activities of several enzymes involved in carbohydrate metabolism during the cell cycle of Saccharomyces cerevisiae

Activity changes of a number of enzymes involved in carbohydrate metabolism were determined in cell extracts of fractionated exponential-phase populations of Saccharomyces cerevisiae grown under excess glucose. Cell-size fractionation was achieved by an improved centrifugal elutriation procedure. Evidence that the yeast populations had been fractionated according to age in the cell cycle was obtained by examining the various cell fractions for their volume distribution and their microscopic appearance and by flow cytometric analysis of the distribution patterns of cellular DNA and protein contents. Trehalase, hexokinase, pyruvate kinase, phosphofructokinase 1, and fructose-1,6-diphosphatase showed changes in specific activities throughout the cell cycle, whereas the specific activities of alcohol dehydrogenase and glucose-6-phosphate dehydrogenase remained constant. The basal trehalase activity increased substantially (about 20-fold) with bud emergence and decreased again in binucleated cells. However, when the enzyme was activated by pretreatment of the cell extracts with cyclic AMP-dependent protein kinase, no significant fluctuations in activity were seen. These observations strongly favor posttranslational modification through phosphorylation-dephosphorylation as the mechanism underlying the periodic changes in trehalase activity during the cell cycle. As observed for trehalase, the specific activities of hexokinase and phosphofructokinase 1 rose from the beginning of bud formation onward, finally leading to more than eightfold higher values at the end of the S phase. Subsequently, the enzyme activities dropped markedly at later stages of the cycle. Pyruvate kinase activity was relatively low during the G1 phase and the S phase, but increased dramatically (more than 50-fold) during G2. In contrast to the three glycolytic enzymes investigated, the highest specific activity of the gluconeogenic enzyme fructose-1, 6-diphosphatase 1 was found in fractions enriched in either unbudded cells with a single nucleus or binucleated cells. The observed changes in enzyme activities most likely underlie pronounced alterations in carbohydrate metabolism during the cell cycle.

A second growth state for Schizosaccharomyces pombe

The kinetics of volume increase in individual cells of Schizosaccharomyces pombe were determined by phase microscopy at osmolalities lower than those reported in the literature. At the highest osmolality, 550 mmol/kg, all cells followed a biphasic pattern of growth, in which cell volumes increased to their maximum values approximately four-fifths of the way through the growth cycle. At lower osmolalities (400-420 mmol/kg), many or most of the cells followed a different growth pattern, with a linear increase in cell volume throughout the cycle. The following evidence indicates that a different regulatory mechanism is responsible for the linear growth pattern: (1) Regulation of cell length and diameter differed for the two cases. During biphasic growth, cell length also increased biphasically and cell diameters remained essentially constant during the cycle, whereas during linear growth, both cell length and diameter increased linearly until formation of the cell plate very late in cycle. (2) The two different growth states were observed for cells growing on two very different kinds of medium. (3) Frequency distributions of the two growth patterns showed that there were two distinct groups of growing cells, with and without a cell volume plateau; these results rule out a single growth state in which plateaus are graded from large to infinitesimally small. (4) Linear regressions fitted to the data for linear growth did not differ significantly from the theoretical model for linear growth without a terminal plateau. These results reveal the operation of a second regulatory system for cell growth in S. pombe at osmolalities closer to those in liquid medium. The occurrence of transitions between the two growth states in successive generations and the agreement between several growth parameters for the two modes suggest that the growth states are closely related.

Regulation of CDC9, the Saccharomyces cerevisiae gene that encodes DNA ligase

We have cloned CDC9, the structural gene for Saccharomyces cerevisiae DNA ligase, and investigated its transcriptional regulation both as a function of cell cycle stage and after UV irradiation. The steady-state level of DNA ligase mRNA increases at least fourfold in late G1, after the completion of start but before S phase. This high level of CDC9 mRNA then decays with an apparent half-life of ca. 20 min and remains at a low basal level throughout the rest of the cell cycle. The accumulation of CDC9 mRNA in late G1 is dependent upon the completion of start but not the CDC7 and CDC8 functions. Exposure of cells to UV light elicits an eightfold increase in DNA ligase mRNA levels.

Cell cycle-dependent expression of thymidylate synthase in Saccharomyces cerevisiae

Synchronous populations of Saccharomyces cerevisiae cells, generated by two independent methods, have been used to show that thymidylate synthase, in contrast to the vast majority of cellular proteins thus far examined, fluctuates periodically during the S. cerevisiae cell cycle. The enzyme, as assayed by two different methods, accumulated during S period and peaked in mid to late S phase, and then its level dropped. These observations suggest that both periodic synthesis and the instability of the enzyme contribute to the activity profile seen during the cell cycle. Accumulation of thymidylate synthase is determined at the level of its transcript, with synthase-specific mRNA levels increasing at least 10-fold to peak near the beginning of S period and then falling dramatically to basal levels after the onset of DNA synthesis. This mRNA peak coincided with the time during the cell cycle when thymidylate synthase levels were increasing maximally and immediately preceded the peak of DNA synthesis, for which the enzyme provides precursor dTMP.

Regulation of CDC9, the Saccharomyces cerevisiae gene that encodes DNA ligase

We have cloned CDC9, the structural gene for Saccharomyces cerevisiae DNA ligase, and investigated its transcriptional regulation both as a function of cell cycle stage and after UV irradiation. The steady-state level of DNA ligase mRNA increases at least fourfold in late G1, after the completion of start but before S phase. This high level of CDC9 mRNA then decays with an apparent half-life of ca. 20 min and remains at a low basal level throughout the rest of the cell cycle. The accumulation of CDC9 mRNA in late G1 is dependent upon the completion of start but not the CDC7 and CDC8 functions. Exposure of cells to UV light elicits an eightfold increase in DNA ligase mRNA levels.

Cell cycle-dependent expression of thymidylate synthase in Saccharomyces cerevisiae

Synchronous populations of Saccharomyces cerevisiae cells, generated by two independent methods, have been used to show that thymidylate synthase, in contrast to the vast majority of cellular proteins thus far examined, fluctuates periodically during the S. cerevisiae cell cycle. The enzyme, as assayed by two different methods, accumulated during S period and peaked in mid to late S phase, and then its level dropped. These observations suggest that both periodic synthesis and the instability of the enzyme contribute to the activity profile seen during the cell cycle. Accumulation of thymidylate synthase is determined at the level of its transcript, with synthase-specific mRNA levels increasing at least 10-fold to peak near the beginning of S period and then falling dramatically to basal levels after the onset of DNA synthesis. This mRNA peak coincided with the time during the cell cycle when thymidylate synthase levels were increasing maximally and immediately preceded the peak of DNA synthesis, for which the enzyme provides precursor dTMP.

Partial characterization of the mitogenic action of pp60v-src, the oncogenic protein product of the src gene of avian sarcoma virus

NRK cells infected with a temperature-sensitive, transformation-defective mutant of avian sarcoma virus (ASV), tsLA23, are transformed at 36 degrees C, but at 40 degrees C they behave as nontransformed cells because of the inactivation of the abnormally thermolabile pp60v-src product of the virus' transforming src gene. At 40 degrees C, these tsLA23-NRK cells were arrested in G1/G0 by severe serum deprivation. They were induced to enter G1, initiate DNA synthesis 7 or 10 hours later, and then divide as (1) nontransformed cells by adding serum or platelet-derived growth factor (PDGF) at 40 degrees C, or (2) transformed cells by lowering the temperature to a pp60v-src-activating 36 degrees C without adding exogenous growth factor(s). The level of pp60v-src kinase activity rose dramatically in these serum-deprived cells within 30 minutes of lowering the temperature to the permissive 36 degrees C, and it fell just as rapidly when the cells were returned to the restrictive 40 degrees C. As little as a 2-hour exposure to 36 degrees C, with an attendant 2-hour burst of pp60v-src kinase activity, was enough to stimulate serum-deprived tsLA23-NRK cells to transit G1 and initiate DNA replication, but not to divide. Much more prolonged pp60v-src activity was needed for these serum-deprived cells to complete their cycle and divide. The prereplicative development of quiescent tsLA23-NRK cells stimulated by serum or PDGF was accompanied by greatly increased protein synthesis and slightly decreased protein degradation, but the pp60v-src-stimulated cells progressed through G1 and initiated DNA replication without appreciably affecting the protein synthetic machinery of the cell. The cells stimulated by the mitogenic action of pp60v-src, like the cells stimulated by serum, needed to activate early prereplicative genes in order to initiate DNA replication. The needed RNA transcripts induced by serum and pp60v-src were produced with comparable efficiency, although it took longer for pp60v-src-stimulated cells to translate these transcripts and to initiate DNA replication, probably because of their unstimulated protein-synthetic machinery.