Effect of nitrogen limitation and sporulation on sterol and lipid formation inSaccharomyces cerevisiae

The content of sterols and lipids was compared in the cells ofSaccharomyces cerevisiae cultivated in sporulation and the sterol-induction nitrogen-limited media. After 24 h the measured values in the two cultivations did not significantly differ. However, after subsequent 24 h, further formation of lipid globules and a corresponding increase of lipid and sterol content was detected only in the sterol-induction medium. To demonstrate the similarity of physiological state during the first day of the two cultivations, the combined cultivations were performed. Maximum sporulation, suggesting maximum similarity, of the two processes was achieved when the cells were grown in the sterol-induction medium for 15 h and then transferred to a sporulation medium.     

Electrical aspects of division control inSaccharomyces

Readily discernable changes in electrical character take place during the division cycle of the yeast,Saccharomyces cerevisiae. By using the technique of cellular spin resonance, the electrical polarizability of individual cells can be seen to change with the life cycle, as determined by cell morphology. The polarizability changes indicate that during the advance through the life cycle, the ionic double layers associated with the outer cell wall gradually tighten.     

Intracellular control of proton extrusion inSaccharomyces cerevisiae

Wild-type and mutant (glucosephosphate isomerase, pyruvate kinase and respiratory deficientrho) strains were used to determine the kinetics of substrate-induced H⁺ efflux in dilute suspensions, glucose-induced production of titratable acidity in intact cells and cell-free extracts, and kinetics of extracellular titratable acidity production (pH-stat). The results indicate that (1) initial phases of H⁺ efflux proceed at the expense of preexisting cell acidity reserves while subsequent efflux is supported by de novo formed acidity, (2) apart from regulation by pH_out the H⁺ efflux is subject to intracellular control, (3) intracellular acidity level is controlled separately from H⁺ efflux. Tentative scheme is proposed for the regulation of H⁺ fluxes inS. cerevisiae.     

The RAS-adenylate cyclase pathway and cell cycle control inSaccharomyces cerevisiae

The cell cycle ofSaccharomyces cerevisiae contains a decision point in G₁ called start, which is composed of two specific sites. Nutrient-starved cells arrest at the first site while pheromone-treated cells arrest at the second site. Functioning of the RAS-adenylate cyclase pathway is required for progression over the nutrient-starvation site while overactivation of the pathway renders the cells unable to arrest at this site. However, progression of cycling cells over the nutrient-starvation site does not appear to be triggered by the RAS-adenylate cyclase pathway in response to a specific stimulus, such as an exogenous nutrient. The essential function of the pathway appears to be limited to provision of a basal level of cAMP. cAMP-dependent protein kinase rather than cAMP might be the universal integrator of nutrient availability in yeast. On the other hand stimulation of the pathway in glucose-derepressed yeast cells by rapidly-fermented sugars, such as glucose, is well documented and might play a role in the control of the transition from gluconeogenic growth to fermentative growth. The initial trigger of this signalling pathway is proposed to reside in a glucose sensing complex which has both a function in controlling the influx of glucose into the cell and in activating in addition to the RAS-adenylate cyclase pathway all other glucose-induced regulatory pathways in yeast. Two crucial problems remaining to be solved with respect to cell cycle control are the nature of the connection between the RAS-adenylate cyclase pathway and nitrogen-source induced progression over the nutrient-starvation site of start and second the nature of the downstream processes linking the RAS-adenylate cyclase pathway to Cyclin/CDC28 controlled progression over the pheromone site of start.Abbreviations cAMP-PK cAMP-dependent protein kinase     

Effect of cyclic AMP,theophylline and caffeine on the glucose repression of sporulation inSaccharomyces cerevisiae

Summary Repression of the sporulation ability ofSaccharomyces cerevisiae by glucose present in the presporulation medium was studied. Glucose lowered sporulation ability when added to the presporulation medium containing yeast extract but did not do so when added to the presporulation medium without glucose. The glucose-repressed sporulation ability was recovered by the addition of cyclic AMP, and theophylline or caffeine to the presporulation culture. Theophylline promoted the action of cyclic AMP, but caffeine did not. The effect of caffeine to reverse glucose repression was greater than that of cyclic AMP and theophylline.     

The chitin-glucan complex inSaccharomyces cerevisiae

The content of glucosamine in the walls of daughter (without bud scars) and mother (multiscar) cells ofSaccharomyces cerevisiae was examined in a control and after treatment with dilute alkali, acid and buffer. The occurrence of chitin in the bud and birth scars is discussed. The results of IR and X-ray analysis of cell-wall fractions indicate the presence of α-chitin which is a part of the chitin-glucan complex. The size of the crystallite of α-chitin in this complex is about 60 Å.     

The integrity of the centriole inSaccharomyces

Summary The observation that the number of centrioles in the centrosome of a yeast cell correspond to the degree of ploidy supportsHarper's view that the centriole is a constant, stable, essential cell structure. The implications of this view on the function of the centriole and the integrity of the spindle fiber will be considered in subsequent communications. Besides the centriole, four other types of granules appearing in the yeast cell are described and distinguished from the central body. These are the lipolated mitochondria, the vesicular mitochondria, the intermediate form of mitochondrion and the small granules. This work has been supported by a grant from the U.S. Public Health Service C-2140.     

The genetic control of sporulation in Saccharomyces. II. Dominance and complementation of mutants of meiosis and spore formation

Summary Heat-sensitive sporulation-deficient (spo) mutants ofS. cerevisiae may be either dominant or recessive. The number of loci which can mutate to thespo phenotype has been estimated to be 48±27 from complementation studies. Comparison of the wild type and mutants by light microscopy after exposure to sporulation medium at the restrictive temperature and Giemsa staining has shown that mutant populations can not complete the meiotic nuclear divisions.Supported by NSF grants GB-8564 and GB-27688, and the Wallace C. and Clara Abbott Memorial Fund from the University of Chicago.     

Regulation of energy flow and control of the cell cycle inSaccharomyces cerevisiae

Summary Measurements of glucose utilization and ethanol production by a respiratory-deficient mutant ofS. cerevisiae in a batch culture show that during the phase of acceleration, the glucose utilization per newly formed cell,k, is higher than the average value and the fermentationF < 1. In the phase of retardation, on the other hand,k is lower andF > 1. Correlating with the changes in the physiological state of the cell population, these results indicate that a considerable fraction of the total glucose consumed is utilized for the synthesis of polysaccharides in the G₁-phase, whereas reserve carbohydrates are catabolised during (S + G₂ + M)-phases of the cell cycle.A cybernetic model for the regulation of the energy (ATP) flow during the cell cycle is presented. It is postulated that the coupling between the energy-yielding and energy-consuming processes is provided by (i) a feedback regulation of the rate of energy production by the energy level and (ii) formation and breakdown of an intracellular energy storage system with a control function in the G₁S transition during the cell cycle.     

Genetic control of fungal differentiation: The three sporulation pathways of Neurospora crassa

Sporulation in the mold Neurospora crussa can proceed along three very different pathways, leading to the production of three types of spores. Two asexual sporulation pathways that lead to the formation of macroconidia and microconidia involve budding from hyphae by two different mechanisms. A much more complex sexual reproductive pathway involves the formation of a fruiting body called a perithecium, in which meiosis takes place and ascospores are formed in sac-like cells called asci. Numerous mutations exist that affect these developmental pathways, and genes have been isolated that are expressed preferentially during sporulation. The Neurospora sporulation pathways offer a simple system with which to study mechanisms and regulation of development that are usually obscured by complex cell-cell interactions involved in animal and plant development.