Induction of a heat-shock-type response in Saccharomyces cerevisiae following glucose limitation

Phosphorylations of Sds23/Psp1/Moc1 by stress-activated kinase and cAMP-dependent kinase are essential for regulating cell viability in prolonged stationary phase

Under nutritional deprivation caused by prolonged culture, actively growing cells prepare to enter stationary phase. We showed here that Sds23/Psp1/Moc1 was phosphorylated by both cAMP-dependent kinase and stress-activated MAP kinase Sty1 upon entry into stationary phase. Overexpression of the phosphorylation-defective mutant Sds23/Psp1/Moc1 induced cell death in prolonged culture and blocked re-entry into the cell division cycle. These phosphorylations are likely to be required for cell survival during stationary phase and for binding of Ufd2, a Schizosaccharomyces pombe homologue of multi-ubiquitin chain assembly factor E4. Deletion of the Ufd2 gene and overexpression of Sds23/Psp1/Moc1 increased cell viability in prolonged stationary phase. These results suggested that Ufd2 induces cell death in prolonged nutrient deprivation, that Sds23/Psp1/Moc1 may be a target protein of the ubiquitin-fusion degradation pathway for regulation of cell viability under this stress condition, and that Sty1 and PKA activity in stationary phase is essential for interaction between Sds23/Psp1/Moc1 and Ufd2.

Proteomic analysis of a nutritional shift-up in Saccharomyces cerevisiae identifies Gvp36 as a BAR-containing protein involved in vesicular traffic and nutritional adaptation

Yeast cells undergoing a nutritional shift-up from a poor to a rich carbon source take several hours to adapt to the novel, richer carbon source. The budding index is a physiologically relevant "global" parameter that reflects the complex links between cell growth and division that are both coordinately and deeply affected by nutritional conditions. We used changes in budding index as a guide to choose appropriate, relevant time points during an ethanol to glucose nutritional shift-up for preparation of samples for the analysis of proteome by two-dimensional electrophoresis/mass spectrometry. About 600 spots were detected. 90 spots, mostly comprising proteins involved in intermediary metabolism, protein synthesis, and response to stress, showed differential expression after glucose addition. Among modulated proteins we identified a protein of previously unknown function, Gvp36, showing a transitory increase corresponding to the drop of the fraction of budded cells. A gvp36Delta strain shares several phenotypes (including general growth defects, heat shock, and high salt sensitivity, defects in polarization of the actin cytoskeleton, in endocytosis and in vacuolar biogenesis, defects in entering stationary phase upon nutrient starvation) with secretory pathway mutants and with mutants in genes encoding the two previously known yeast BAR proteins (RSV161 and RSV167). We thus propose that Gvp36 represents a novel yeast BAR protein involved in vesicular traffic and in nutritional adaptation.

The Snf1 protein kinase controls the induction of genes of the iron uptake pathway at the diauxic shift in Saccharomyces cerevisiae

In Saccharomyces cerevisiae the transition between the fermentative and the oxidative metabolism, called the diauxic shift, is associated with major changes in gene expression. In this study, we characterized a novel family of five genes whose expression is induced during the diauxic shift. These genes, FET3, FTR1, TIS11, SIT1, and FIT2, are involved in the iron uptake pathway. We showed that their induction at the diauxic shift is positively controlled by the Snf1/Snf4 kinase pathway. The transcriptional factor Aft1p, which is known to control their induction in response to iron limitation, is also required for their induction during the diauxic shift. The increase of the extracellular iron concentration does not affect this induction, indicating that glucose exhaustion by itself would be the signal. The possibility that the Snf1/Snf4 pathway was also involved in the induction of the same set of genes in response to iron starvation was considered. We demonstrate here that this is not the case. Thus, the two signals, glucose exhaustion and iron starvation, use two independent pathways to activate the same set of genes through the Aft1p transcriptional factor.

The fission yeast ES2 homologue, Bis1, interacts with the Ish1 stress-responsive nuclear envelope protein

In fission yeast, nutrient starvation induces physiological, biochemical, and morphological changes that enable survival. Collectively these changes are referred to as stationary phase. We have used a green fluorescent protein random insertional mutagenesis system to isolate two novel stress-response proteins required in stationary phase. Ish1 is a nuclear envelope protein that is present throughout the cell cycle and whose expression is increased in response to stresses such as glucose and nitrogen starvation, as well as osmotic stress. Expression of Ish1 is regulated by the Spc1 MAPK pathway through the Atf1 transcription factor. Although overexpression of Ish1 is lethal, cells lacking ish1 exhibit reduced viability in stationary phase. Bis1 is a novel interacting partner of Ish1. Bis1 is the Schizosaccharomyces pombe member of the ES2 nuclear protein family found in Mus musculus, Drosophila melanogaster, Homo sapiens, and Arabidopsis thaliana. Overexpression of Bis1 results in a cell elongation phenotype, whereas bis1(-) cells exhibit a reduced viability in stationary phase similar to that seen in ish1(-) cells.

Fermentative capacity after cold storage of baker's yeast is dependent on the initial physiological state but not correlated to the levels of glycolytic enzymes

Growth and starvation of baker's yeast was monitored by on-line microcalorimetry and cells originating from four different physiological states were stored at low temperature (4 degrees C) for up to 26 days. The different physiological states were designated F (respiro-Fermentative phase of growth), R (initial Respiratory phase of growth), -N (non-growing state because of Nitrogen depletion), and -NC (non-growing state because of both Nitrogen and Carbon depletion). The cells were tested before and after cold storage for their fermentative capacity, and characterised by 2D gel analysis (and subsequent quantitative silver staining and image analysis with software PDQUEST) for their levels of six enzymes of the glycolytic pathway (hexokinase 2 (Hxk2p), fructose bisphosphate aldolase (Fba1p), glyceraldehyde-3-phosphate dehydrogenase (Tdh3p), enolase A (Enolp), enolase B (Eno2p), and triose phosphate isomerase (Tpi1p)) and two enzymes of the fermentative branch (pyruvate decarboxylase (Pdc1p) and alcohol dehydrogenase (Adh1p)). The enzymes Hxk2p, Tdh3p, Eno2p, Pdc1p and Adh1p were down-regulated by 25-80% during the transition between the F and R states. During the transition to non-growing states (-N and -NC states), the levels of Hxk2p, Tdh3p and Eno2p were further reduced. However, after cold storage, the glycolytic and fermentative enzymes of the different physiological states were expressed to the same extent. In contrast, the fermentative capacity differed between the states; the R-state cells were superior compared to cells from the other states tested and preserved more than 50% of their initial fermentative capacity (6 mmol ethanol per gram dry weight and hour). Our data therefore clearly demonstrate that persistence of fermentative capacity during total starvation at low temperature after as long as 1 month is strongly dependent on the physiological state from which the cells originate. However, the level of expression of the glycolytic enzymes could not explain the difference in fermentative capacity of the different physiological states after cold storage.

The transcriptional activator Cat8p provides a major contribution to the reprogramming of carbon metabolism during the diauxic shift in Saccharomyces cerevisiae

In yeast, the transition between the fermentative and the oxidative metabolism, called the diauxic shift, is associated with major changes in gene expression and protein synthesis. The zinc cluster protein Cat8p is required for the derepression of nine genes under nonfermentative growth conditions (ACS1, FBP1, ICL1, IDP2, JEN1, MLS1, PCK1, SFC1, and SIP4). To investigate whether the transcriptional control mediated by Cat8p can be extended to other genes and whether this control is the main control for the changes in the synthesis of the respective proteins during the adaptation to growth on ethanol, we analyzed the transcriptome and the proteome of a cat8 Delta strain during the diauxic shift. In this report, we demonstrate that, in addition to the nine genes known as Cat8p-dependent, there are 25 other genes or open reading frames whose expression at the diauxic shift is altered in the absence of Cat8p. For all of the genes characterized here, the Cat8p-dependent control results in a parallel alteration in mRNA and protein synthesis. It appears that the biochemical functions of the proteins encoded by Cat8p-dependent genes are essentially related to the first steps of ethanol utilization, the glyoxylate cycle, and gluconeogenesis. Interestingly, no function involved in the tricarboxylic cycle and the oxidative phosphorylation seems to be controlled by Cat8p.

Biotechnological applications of the disaccharide trehalose

Trehalose is a disaccharide present in a variety of anhydrobiotic organisms which have the ability to promptly resume their metabolism after addition of water. It has been successfully used as a nontoxic cryoprotectant of enzymes, membranes, vaccines, animal and plant cells and organs for surgical transplants. It has been predicted that trehalose can also be used as an ingredient for dried and processed food. Therefore, the recent biotechnological applications of trehalose have imposed the standardization of methods for its production, as well as for its specific quantification.

Msn2p and Msn4p control a large number of genes induced at the diauxic transition which are repressed by cyclic AMP in Saccharomyces cerevisiae

The multicopy suppressors of the snf1 defect, Msn2p and Msn4p transcription factors (Msn2/4p), activate genes through the stress-responsive cis element (CCCCT) in response to various stresses. This cis element is also the target for repression by the cyclic AMP (cAMP)-signaling pathway. We analyzed the two-dimensional gel electrophoresis pattern of protein synthesis of the msn2 msn4 double mutant and compared it with that of the wild-type strain during exponential growth phase and at the diauxic transition. Thirty-nine gene products (including those of ALD3, GDH3, GLK1, GPP2, HSP104, HXK1, PGM2, SOD2, SSA3, SSA4, TKL2, TPS1, and YBR149W) are dependent upon Msn2/4p for their induction at the diauxic transition. The expression of all these genes is repressed by cAMP. Thirty other genes identified during this study are still inducible in the mutant. A subset of these genes were found to be superinduced at the diauxic transition, and others were subject to cAMP repression (including ACH1, ADH2, ALD6, ATP2, GPD1, ICL1, and KGD2). We conclude from this analysis that Msn2/4p control a large number of genes induced at the diauxic transition but that other, as-yet-uncharacterized regulators, also contribute to this response. In addition, we show here that cAMP repression applies to both Msn2/4p-dependent and -independent control of gene expression at the diauxic shift. Furthermore, the fact that all the Msn2/4p gene targets are subject to cAMP repression suggests that these regulators could be targets for the cAMP-signaling pathway.

Osmoresponsive proteins and functional assessment strategies in Saccharomyces cerevisiae

Cells respond to increased external osmolarities by enhanced accumulation of compatible solutes. In yeast-cells, mainly exemplified by Saccharomyces cerevisiae, the premier compatible solute is the polyhydroxy-alcohol glycerol, the production of which is accompanied by overall metabolic changes. By applying two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) coupled to computerized image quantification, a large body of valuable physiological information relating to this stress-adaptation has been gathered. One of the presumed key-enzymes in the production of glycerol in the cell is glycerol 3-phosphate dehydrogenase encoded by the GPD1 gene. The amount of this protein is enhanced during saline stress, and from 2-D analysis linked to microsequencing it became apparent that the osmo-regulated from contained a putative presequence. Sequence analysis of another salt-induced spot in the 2-D pattern revealed identity to a gene, YER062c, with previously unknown function. Biochemical characterization of this protein, including standard purification via chromatography and subsequent activity/specificity measurements, identified this salt-regulated protein as the missing protein/gene in glycerol production, namely the glycerol 3-phosphatase. The sequence of another salt regulated protein resolved in the 2-D gel revealed identity to a bacterial dihydroxyacetone kinase, thus indicating salt induced glycerol dissimilation. Comparing Northern data to the 2-D generated expression pattern revealed a strong correlation, indicating mainly regulation at the transcriptional level. In addition, altered expression during saline growth of some of the glycolytic enzymes was also apparent. Signalling mutants, either in the cAMP-dependent protein kinase A pathway or in a protein kinase cascade, have been analyzed during osmotic stress via 2-D PAGE, grouping proteins/genes apparently regulated via similar mechanismus. Proteome analysis has proven invaluable in the unravelling of the molecular physiology of yeast cells during adaptation and growth under osmotic stress, identifying vital components not selected by purely genetic approaches.

Metabolic and regulatory changes associated with growth of Saccharomyces cerevisiae in 1.4 M NaCl. Evidence for osmotic induction of glycerol dissimilation via the dihydroxyacetone pathway

The salt-instigated protein expression of Saccharomyces cerevisiae during growth in either 0.7 or 1.4 M NaCl was studied by two-dimensional polyacrylamide gel electrophoresis. The 73 protein spots that were identified as more than 3-fold responsive in 1.4 M NaCl were further grouped by response class (halometric, low-salt, and high-salt regulation). Roughly 40% of these responsive proteins were found to decrease in expression, while at higher magnitudes of change (>8-fold) only induction was recorded. Enolase 1 (Eno1p) was the most increasing protein by absolute numbers per cell, but not by -fold change, and the enzymes involved in glycerol synthesis, Gpd1p and Gpp2p, were also induced to a similar degree as Eno1p. We furthermore present evidence for salt induction of glycerol dissimilation via dihydroxyacetone and also identify genes putatively encoding the two enzymes involved; dihydroxyacetone kinase (DAK1 and DAK2) and glycerol dehydrogenase (YPR1 and GCY1). The GPD1, GPP2, GCY1, DAK1, and ENO1 genes all displayed a halometric increase in the amount of transcript. This increase was closely linked to the salt-induced rate of protein synthesis of the corresponding proteins, indicating mainly transcriptional regulation of expression for these genes. A consensus element with homology to the URS sequence of the ENO1 promoter was found in the promoters of the GPD1, GPP2, GCY1, and DAK1 genes.

A stationary-phase gene in Saccharomyces cerevisiae is a member of a novel, highly conserved gene family

The regulation of cellular growth and proliferation in response to environmental cues is critical for development and the maintenance of viability in all organisms. In unicellular organisms, such as the budding yeast Saccharomyces cerevisiae, growth and proliferation are regulated by nutrient availability. We have described changes in the pattern of protein synthesis during the growth of S. cerevisiae cells to stationary phase (E. K. Fuge, E. L. Braun, and M. Werner-Washburne, J. Bacteriol. 176:5802-5813, 1994) and noted a protein, which we designated Snz1p (p35), that shows increased synthesis after entry into stationary phase. We report here the identification of the SNZ1 gene, which encodes this protein. We detected increased SNZ1 mRNA accumulation almost 2 days after glucose exhaustion, significantly later than that of mRNAs encoded by other postexponential genes. SNZ1-related sequences were detected in phylogenetically diverse organisms by sequence comparisons and low-stringency hybridization. Multiple SNZ1-related sequences were detected in some organisms, including S. cerevisiae. Snz1p was found to be among the most evolutionarily conserved proteins currently identified, indicating that we have identified a novel, highly conserved protein involved in growth arrest in S. cerevisiae. The broad phylogenetic distribution, the regulation of the SNZ1 mRNA and protein in S. cerevisiae, and identification of a Snz protein modified during sporulation in the gram-positive bacterium Bacillus subtilis support the hypothesis that Snz proteins are part of an ancient response that occurs during nutrient limitation and growth arrest.

High cAMP levels antagonize the reprogramming of gene expression that occurs at the diauxic shift in Saccharomyces cerevisiae

In order to analyse the involvement of the cAMP pathway in the regulation of gene expression in Saccharomyces cerevisiae, we have examined the effect of cAMP on protein synthesis by using two-dimensional gel electrophoresis. cAMP had only a minor effect on the protein pattern of cells growing exponentially on glucose. However, it interfered with the changes in gene expression normally occurring upon glucose exhaustion in yeast cultures, maintaining a protein pattern typical of cells growing on glucose. This effect was accompanied by a delay before growth recovery on ethanol. We propose a model in which the cAMP-signalling pathway has a role in the maintenance of gene expression, rather than in the determination of a specific programme. A decrease of cAMP would then be required for metabolic transitions such as the diauxic phase.

Global changes in protein synthesis during adaptation of the yeast Saccharomyces cerevisiae to 0.7 M NaCl

Exponentially growing Saccharomyces cerevisiae was challenged to increased salinity by transfer to 0.7 M NaCl medium, and changes in protein synthesis were examined during the 1st h of adaptation by use of two-dimensional gel electrophoresis coupled to computerized quantification. An impressive number of proteins displayed changes in the relative rate of synthesis, with most differences from nonstressed cells being found at between 20 and 40 min. During this period, 18 proteins exhibited more than eightfold increases in their rates of synthesis and were classified as highly NaCl responsive. Only two proteins were repressed to the same level. Most of these highly NaCl-responsive proteins seemed to constitute gene products not earlier reported to respond to dehydration. Applying a selection criterion to subsequent samples of a twofold change in the relative rate of synthesis, 14 different regulatory patterns were discerned. Most identified glycolytic enzymes exhibited a delayed response, and their rates of synthesis did not change until the middle phase of adaptation, with only a minor decrease in the rate of production. A slight salt-stimulated response was observed for some members of the HSP70 gene family. Overall, the data presented indicate complex intracellular signalling as well as involvement of diverse regulatory mechanisms during the period of adaptation to NaCl.

Differential synthesis of glyceraldehyde-3-phosphate dehydrogenase polypeptides in stressed yeast cells

Three unlinked genes, TDH1, TDH2 and TDH3, encode the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (triose-phosphate dehydrogenase; TDH) in the yeast Saccharomyces cerevisiae. We demonstrate that the synthesis of the three encoded TDH polypeptides (TDHa, TDHb and TDHc, respectively) is not co-ordinately regulated and that TDHa is only synthesised as cells enter stationary phase, due to glucose starvation, or in heat-shocked cells. Furthermore, the synthesis of TDHb, but not TDHc, is strongly repressed by a heat shock. Hence, the TDHa enzyme may play a cellular role, distinct from glycolysis, that is required by stressed cells.

Protein synthesis in long-term stationary-phase cultures of Saccharomyces cerevisiae

We are interested in characterizing the process of entry into and the maintenance of the stationary phase. To identify proteins that are induced during growth to stationary phase, we examined protein synthesis in long-term stationary-phase cultures using two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). Although the total rate of protein synthesis declined when growth ceased after the postdiauxic phase, the pattern of proteins synthesized remained similar throughout the experimental period (28 days), except at the diauxic shift. At the diauxic shift most proteins detectable by 2D-PAGE undergo a transient reduction in their relative rate of synthesis that ends when cells resume growth during the postdiauxic phase. We conclude from this that the transient repression of protein synthesis at the diauxic shift is not directly associated with stationary-phase arrest. A number of proteins that are synthesized after exponential phase have been identified by 2D-PAGE. These proteins could be divided into three temporal classes depending upon when their synthesis became detectable. One postexponential protein, designated p35, was induced later than all other proteins, and its relative rate of synthesis increased throughout stationary phase. Unlike most postexponential proteins, p35 was not regulated by heat shock or glucose repression. We also observed that a direct correlation between steady-state mRNA accumulation and protein synthesis for another postexponential protein (Ssa3p) or two closely related constitutive proteins (Ssa1p and Ssa2p) did not exist. We conclude from this result that synthesis of proteins in stationary phase is regulated by mechanisms other than the control of steady-state mRNA accumulation.

Trehalose metabolism in Saccharomyces cerevisiae during heat-shock

When different strains of Saccharomyces cerevisiae grown at 23 degrees C were transferred to 36 degrees C, trehalose and glycogen were accumulated. Glycogen accumulation was less extensive and its synthesis started at least 15 min after initiation of trehalose synthesis. The steady-state intracellular concentration of trehalose increased simultaneously with the activities of the enzymes trehalose-6P synthase, UDPG-pyrophosphorylase, phosphoglucomutase and trehalase. A small but significant change was observed in hexokinase activity. Our results directly implicate isoform PII of hexokinase and the minor isoform of phosphoglucomutase in the pathway of trehalose formation during heat-shock. We also showed that the major isoform of phosphoglucomutase increased in activity but was not essential for trehalose accumulation. Studies with the glucose uptake system indicated that trehalose accumulation could be primarily determined by intracellular availability of substrates due to the increase in the rate of glucose uptake. The increased uptake appears to have two components: a kinetic effect of temperature upon glucose transporters and an increase in the numbers of molecules of the transporters, probably mediated by synthesis de novo.

A portion of RNA polymerase II molecules has a component essential for stress responses and stress survival

Cells respond to stress by altering gene expression, and these adjustments facilitate stress tolerance. Although transcriptional changes are integral to most stress responses, little is known about the mechanisms that permit the transcription apparatus itself to tolerate stress. Here we report that a major role of the RNA polymerase II subunit RPB4 is to permit appropriate transcriptional responses during stress. Yeast cells lacking RPB4 have essentially wild-type growth rates at moderate temperatures (18 to 22 degrees C), but their growth rates are substantially reduced at temperatures outside this range. When subjected to a heat shock, cells lacking RPB4 rapidly lose the ability to transcribe genes and experience a dramatic loss in viability. When cells lacking RPB4 are subjected to the nutrient stress that accompanies entry into stationary phase, they also exhibit a substantial decline in mRNA synthesis and in viability relative to wild-type cells. Interestingly, the portion of RNA polymerase II molecules that contain RPB4 is small in log phase but increases substantially as cells enter stationary phase. We propose that the association of RPB4 with the other RNA polymerase II subunits increases the tolerance of the enzyme to stress.

A portion of RNA polymerase II molecules has a component essential for stress responses and stress survival

Cells respond to stress by altering gene expression, and these adjustments facilitate stress tolerance. Although transcriptional changes are integral to most stress responses, little is known about the mechanisms that permit the transcription apparatus itself to tolerate stress. Here we report that a major role of the RNA polymerase II subunit RPB4 is to permit appropriate transcriptional responses during stress. Yeast cells lacking RPB4 have essentially wild-type growth rates at moderate temperatures (18 to 22 degrees C), but their growth rates are substantially reduced at temperatures outside this range. When subjected to a heat shock, cells lacking RPB4 rapidly lose the ability to transcribe genes and experience a dramatic loss in viability. When cells lacking RPB4 are subjected to the nutrient stress that accompanies entry into stationary phase, they also exhibit a substantial decline in mRNA synthesis and in viability relative to wild-type cells. Interestingly, the portion of RNA polymerase II molecules that contain RPB4 is small in log phase but increases substantially as cells enter stationary phase. We propose that the association of RPB4 with the other RNA polymerase II subunits increases the tolerance of the enzyme to stress.

A growth rate-limiting process in the last growth phase of the yeast life cycle involves RPB4, a subunit of RNA polymerase II

Saccharomyces cerevisiae cells, grown on a fermentable carbon source, display two growth phases before they enter the stationary phase: a rapid phase (log phase) followed by a slow phase. It was previously shown that a subunit of the yeast RNA polymerase II, RPB4, positively affects the activity of the enzyme in post-log phases but has little or no effect on its activity in log phase. Here, I show that RPB4 level limits the growth rate during the slow growth phase. Thus, a small increase in RPB4 protein level, in cells carrying multiple copies of the RPB4 gene, results in an almost twofold increase in the growth rate during this phase. Furthermore, RPB4 expression is differentially regulated in the two growth phases. During the slow growth phase, a posttranscriptional process which controls the RPB4 level and thus can control growth rate becomes active. These results reveal a complex growth control mechanism, in which the transcriptional apparatus is probably a limiting element, operating in the last stages of the yeast growth.

Molecular events associated with acquisition of heat tolerance by the yeast Saccharomyces cerevisiae

The heat shock response is an inducible protective system of all living cells. It simultaneously induces both heat shock proteins and an increased capacity for the cell to withstand potentially lethal temperatures (an increased thermotolerance). This has lead to the suspicion that these two phenomena must be inexorably linked. However, analysis of heat shock protein function in Saccharomyces cerevisiae by molecular genetic techniques has revealed only a minority of the heat shock proteins of this organism having appreciable influences on thermotolerance. Instead, physiological perturbations and the accumulation of trehalose with heat stress may be more important in the development of thermotolerance during a preconditioning heat shock. Vegetative S. cerevisiae also acquires thermotolerance through osmotic dehydration, through treatment with certain chemical agents and when, due to nutrient limitation, it arrests growth in the G1 phase of the cell cycle. There is evidence for the activities of the cAMP-dependent protein kinase and plasma membrane ATPase being very important in thermotolerance determination. Also, intracellular water activity and trehalose probably exert a strong influence over thermotolerance through their effects on stabilisation of membranes and intracellular assemblies. Future investigations should address the unresolved issue of whether the different routes to thermotolerance induction cause a common change to the physical state of the intracellular environment, a change that may result in an increased stabilisation of cellular structures through more stable hydrogen bonding and hydrophobic interactions.

Isolation and sequence of HSP30, a yeast heat-shock gene coding for a hydrophobic membrane protein

Using differential hybridization, a gene preferentially expressed during entry into stationary phase has been isolated. Subsequent analysis indicated that this gene corresponds to a heat-shock gene. The nucleotide sequence has been determined. It revealed a 332 amino-acid protein. No similarities to any previously known protein have been noted. The protein is very hydrophobic and is predicted to have a membraneous localisation. In agreement with this hypothesis, the analysis of membrane proteins from stationary-phase cells showed that a strain carrying this gene on a multicopy vector overproduces a protein of 30 kDa. This protein was recognized by antibodies directed against the N-terminal portion of the gene product. Considering its induction in response to heat shock and the apparent molecular weight of its product, this gene was designated HSP30.

The RAS-adenylate cyclase pathway and cell cycle control in Saccharomyces cerevisiae

The cell cycle of Saccharomyces 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'.