A multidisciplinary intervention to prevent the readmission of elderly patients with congestive heart failure

BACKGROUND

Congestive heart failure is the most common indication for admission to the hospital among older adults. Behavioral factors, such as poor compliance with treatment, frequently contribute to exacerbations of heart failure, a fact suggesting that many admissions could be prevented.

METHODS

We conducted a prospective, randomized trial of the effect of a nurse-directed, multidisciplinary intervention on rates of readmission within 90 days of hospital discharge, quality of life, and costs of care for high-risk patients 70 years of age or older who were hospitalized with congestive heart failure. The intervention consisted of comprehensive education of the patient and family, a prescribed diet, social-service consultation and planning for an early discharge, a review of medications, and intensive follow-up.

RESULTS

Survival for 90 days without readmission, the primary outcome measure, was achieved in 91 of the 142 patients in the treatment group, as compared with 75 of the 140 patients in the control group, who received conventional care (P = 0.09). There were 94 readmissions in the control group and 53 in the treatment group (risk ratio, 0.56; P = 0.02). The number of readmissions for heart failure was reduced by 56.2 percent in the treatment group (54 vs. 24, P = 0.04), whereas the number of readmissions for other causes was reduced by 28.5 percent (40 vs. 29, P not significant). In the control group, 23 patients (16.4 percent) had more than one readmission, as compared with 9 patients (6.3 percent) in the treatment group (risk ratio, 0.39; P = 0.01). In a subgroup of 126 patients, quality-of-life scores at 90 days improved more from base line for patients in the treatment group (P = 0.001). Because of the reduction in hospital admissions, the overall cost of care was $460 less per patient in the treatment group.

CONCLUSIONS

A nurse-directed, multidisciplinary intervention can improve quality of life and reduce hospital use and medical costs for elderly patients with congestive heart failure.

An essential G1 function for cyclin-like proteins in yeast

Cyclins were discovered in marine invertebrates based on their dramatic cell cycle periodicity. Recently, the products of three genes associated with cell cycle progression in S. cerevisiae were found to share limited homology with cyclins. Mutational elimination of the CLN1, CLN2, and DAF1/WHI1 products leads to cell cycle arrest independent of cell type, while expression of any one of the genes allows cell proliferation. Using strains where CLN1 was expressed conditionally, the essential function of Cln proteins was found to be limited to the G1 phase. Furthermore, the ability of the Cln proteins to carry out this function was found to decay rapidly upon cessation of Cln biosynthesis. The data are consistent with the hypothesis that Cln proteins activate the Cdc28 protein kinase, shown to be essential for the G1 to S phase transition in S. cerevisiae. Because of the apparent functional redundancy of these genes, DAF1/WHI1 has been renamed CLN3.

G1-specific cyclins of S. cerevisiae: cell cycle periodicity, regulation by mating pheromone, and association with the p34CDC28 protein kinase

The S. cerevisiae CLN genes encode cyclin homologs essential for progression from G1 to S phase. The CLN2 gene encodes a 62 kd polypeptide that accumulates periodically, peaking during G1 and decreasing rapidly thereafter, and is rapidly lost following exposure of cells to mating pheromone. Cln2 abundance can be explained by the G1-specific accumulation of the CLN2 transcript coupled with instability of the Cln2 protein. The abundance of the CLN1 and CLN2 transcripts increases greater than 5-fold during the G1 interval, decreasing dramatically as cells enter S phase. Both transcripts decrease in cells responding to mating pheromone. Finally, we demonstrate that the Cln2 polypeptide interacts with p34CDC28 to form an active protein kinase complex. This physical interaction is consistent with the genetic interaction between the CLN genes and CDC28 and suggests that Cln proteins are an essential component of the active protein kinase complex required for the G1 to S transition.

A family of cyclin homologs that control the G1 phase in yeast

Two Saccharomyces cerevisiae genes were isolated based upon their dosage-dependent rescue of a temperature-sensitive mutation of the gene CDC28, which encodes a protein kinase involved in control of cell division. CLN1 and CLN2 encode closely related proteins that also share homology with cyclins. Cyclins, characterized by a dramatic periodicity of abundance through the cell cycle, are thought to be involved in mitotic induction in animal cells. A dominant mutation in the CLN2 gene, CLN2-1, advances the G1- to S-phase transition in cycling cells and impairs the ability of cells to arrest in G1 phase in response to external signals, suggesting that the encoded protein is involved in G1 control of the cell cycle in Saccharomyces.

Cdc53 targets phosphorylated G1 cyclins for degradation by the ubiquitin proteolytic pathway

In budding yeast, cell division is initiated in late G1 phase once the Cdc28 cyclin-dependent kinase is activated by the G1 cyclins Cln1, Cln2, and Cln3. The extreme instability of the Cln proteins couples environmental signals, which regulate Cln synthesis, to cell division. We isolated Cdc53 as a Cln2-associated protein and show that Cdc53 is required for Cln2 instability and ubiquitination in vivo. The Cln2-Cdc53 interaction, Cln2 ubiquitination, and Cln2 instability all depend on phosphorylation of Cln2. Cdc53 also binds the E2 ubiquitin-conjugating enzyme, Cdc34. These findings suggest that Cdc53 is a component of a ubiquitin-protein ligase complex that targets phosphorylated G1 cyclins for degradation by the ubiquitin-proteasome pathway.

Regulation of transcription by ubiquitination without proteolysis: Cdc34/SCF(Met30)-mediated inactivation of the transcription factor Met4

Polyubiquitination of proteins by Cdc34/SCF complexes targets them for degradation by the 26S proteasome. The essential F-box protein Met30 is the substrate recognition subunit of the ubiquitin ligase SCF(Met30). The critical target of SCF(Met30) is the transcription factor Met4, as deletion of MET4 suppresses the lethality of met30 mutants. Surprisingly, Met4 is a relatively stable protein and its abundance is not influenced by Met30. However, transcriptional repression of Met4 target genes correlates with Cdc34/SCF(Met30)-dependent ubiquitination of Met4. Functionally, ubiquitinated Met4 associates with target promoters but fails to form functional transcription complexes. Our data reveal a novel proteolysis-independent function for Cdc34/SCF and indicate that ubiquitination of transcription factors can be utilized to directly regulate their activities.

A cyclin B homolog in S. cerevisiae: chronic activation of the Cdc28 protein kinase by cyclin prevents exit from mitosis

A cyclin B homolog was identified in Saccharomyces cerevisiae using degenerate oligonucleotides and the polymerase chain reaction. The protein, designated Scb1, has a high degree of similarity with B-type cyclins from organisms ranging from fission yeast to human. Levels of SCB1 mRNA and protein were found to be periodic through the cell cycle, with maximum accumulation late, most likely in the G2 interval. Deletion of the gene was found not to be lethal, and subsequently other B-type cyclins have been found in yeast functionally redundant with Scb1. A mutant allele of SCB1 that removes an amino-terminal fragment of the encoded protein thought to be required for efficient degradation during mitosis confers a mitotic arrest phenotype. This arrest can be reversed by inactivation of the Cdc28 protein kinase, suggesting that cyclin-mediated arrest results from persistent protein kinase activation.

DNA polymerase epsilon catalytic domains are dispensable for DNA replication, DNA repair, and cell viability

DNA polymerase epsilon (Pol epsilon) is believed to play an essential catalytic role during eukaryotic DNA replication and is thought to participate in recombination and DNA repair. That Pol epsilon is essential for progression through S phase and for viability in budding and fission yeasts is a central element of support for that view. We show that the amino-terminal portion of budding yeast Pol epsilon (Pol2) containing all known DNA polymerase and exonuclease motifs is dispensable for DNA replication, DNA repair, and viability. However, the carboxy-terminal portion of Pol2 is both necessary and sufficient for viability. Finally, the viability of cells lacking Pol2 catalytic function does not require intact DNA replication or damage checkpoints.

Rapid degradation of the G1 cyclin Cln2 induced by CDK-dependent phosphorylation

Cyclins regulate the major cell cycle transitions in eukaryotes through association with cyclin-dependent protein kinases (CDKs). In yeast, G1 cyclins are essential, rate-limiting activators of cell cycle initiation. G1-specific accumulation of one G1 cyclin, Cln2, results from periodic gene expression coupled with rapid protein turnover. Site-directed mutagenesis of CLN2 revealed that its phosphorylation provides a signal that promotes rapid degradation. Cln2 phosphorylation is dependent on the Cdc28 protein kinase, the CDK that it activates. These findings suggest that Cln2 is rendered self-limiting by virtue of its ability to activate its cognate CDK subunit.

Effect of a multidisciplinary intervention on medication compliance in elderly patients with congestive heart failure

PURPOSE

The objectives of this investigation were to prospectively assess medication compliance rates in elderly patients with congestive heart failure, to identify factors associated with reduced compliance, and to evaluate the effect of a multidisciplinary treatment approach on medication adherence.

PATIENTS AND METHODS

A total of 156 patients > or = 70 years of age (mean, 79.4 +/- 6.0; 67% female, 65% nonwhite) hospitalized with congestive heart failure were evaluated prospectively. Prior to discharge, patients were randomized to the study intervention (n = 80) or conventional care (n = 76). The intervention consisted of comprehensive patient education, dietary and social service consultations, medication review, and intensive postdischarge follow-up. Detailed data were collected on all prescribed medications at the time of discharge, and compliance was assessed by pill counts 30 +/- 2 days later.

RESULTS

The overall compliance rate during the first 30 days after discharge was 84.6 +/- 15.1% (range, 23.1-100%). Compliance was 87.9 +/- 12.0% in patients randomized to the study intervention, compared with 81.1 +/- 17.2% in the control group (P = 0.003). A compliance rate of > or = 80% was achieved by 85.0% of the treatment group versus 69.7% of the control group (P = 0.036). By multivariate analysis, assignment to the treatment group was the strongest independent predictor of compliance (P = 0.008). Other variables included in the model were Caucasian race (P = 0.044) and not living alone (P = 0.09).

CONCLUSIONS

A multidisciplinary treatment strategy is associated with improved medication compliance during the first 30 days following hospital discharge in elderly patients with congestive heart failure. Improved compliance may contribute to improved outcomes in these patients.

CLN3, not positive feedback, determines the timing of CLN2 transcription in cycling cells

Transcriptional activation of the budding yeast CLN1 and CLN2 genes during the late G1 phase of the cell cycle has been attributed to a positive feedback loop, wherein the transcription of both genes is stimulated by the accumulation of their protein products. We demonstrate that in cycling cells CLN2 does not play a role in determining the timing of its own transcriptional activation. First, we show that CLN3 alone is sufficient to maximally activate CLN2 transcription. Cells that lack functional CLN1 and CLN2 genes activate the CLN2 promoter with the same kinetics and at the same size as cells in which all three CLN genes are functional. In addition, CLN2 transcription is activated with similar kinetics in cells that have CLN2 as their only functional CLN gene and in CLN-deficient cells. Promoter analysis shows that CLN3-dependent activation of CLN2 transcription is directed primarily through the previously identified UAS1 region although another cis-acting region, UAS2, also can contribute to CLN2 activation under some conditions. The ability to activate transcription of CLN2 is not a unique property of CLN3 because ectopically expressed CLN2 can both activate the endogenous CLN2 promoter and induce Start. We propose that failure of the endogenous CLN2 gene to contribute significantly to activation of its own transcription results from its relative effectiveness at inducing Start, cell cycle progression and, subsequently, inactivation of CLN2 expression.

Course and prognosis in patients > or = 70 years of age with congestive heart failure and normal versus abnormal left ventricular ejection fraction

In this study of 501 patients aged > or =70 years hospitalized with congestive heart failure, 34.1% had normal left ventricular systolic function. Reduced left ventricular ejection fraction was an independent predictor of an adverse prognosis at 3 months but not at 1 year.

CLB5 and CLB6 are required for premeiotic DNA replication and activation of the meiotic S/M checkpoint

Initiation of DNA replication during the mitotic cell cycle requires the activation of a cyclin-dependent protein kinase (CDK). The B-type cyclins Clb5 and Clb6 are the primary activators of the S phase function of the budding yeast CDK Cdc28. However, in mitotically growing cells this role can be fulfilled by the other B-type cyclins Clb1-Clb4. We report here that cells undergoing meiotic development also require Clb dependent CDK activity for DNA replication. Diploid clb5/clb5 clb6/clb6 mutants are unable to perform premeiotic DNA replication. Despite this defect, the mutant cells progress into the meiotic program and undergo lethal segregation of unreplicated DNA suggesting that they fail to activate a checkpoint that restrains meiotic M phase until DNA replication is complete. We have found that a DNA replication checkpoint dependent on the ATM homolog MEC1 operates in wild-type cells during meiosis and can be invoked in response to inhibition of DNA synthesis. Although cells that lack clb5 and clb6 are unable to activate the meiotic DNA replication checkpoint, they do possess an intact DNA damage checkpoint which can restrain chromosome segregation in the face of DNA damage. We conclude that CLB5 and CLB6 are essential for premeiotic DNA replication and, consequently, for activation of a meiotic DNA replication checkpoint.

Mitotic role for the Cdc28 protein kinase of Saccharomyces cerevisiae

The Cdc28 protein kinase functions in the G1 to S phase transition of the cell cycle of the budding yeast Saccharomyces cerevisiae. This is in contrast with observations of the homologous protein kinase from a variety of metazoans, where activity and function are associated with the G2 to M phase transition. We present evidence that the Cdc28 protein kinase is also required for mitosis and that this function is executed in the G2 interval of the cell cycle. We show, in addition, that the protein kinase is highly active during this phase of the cell cycle. The dual role of the Cdc28 protein kinase in the S. cerevisiae cell cycle thus parallels that demonstrated for the cdc2 protein kinase of the fission yeast Schizosaccharomyces pombe.

Control of the yeast cell cycle is associated with assembly/disassembly of the Cdc28 protein kinase complex

The Saccharomyces cerevisiae gene CDC28 encodes a protein kinase required for progression from G1 to S phase in the cell cycle. We present evidence that the active form of the Cdc28 protein kinase is a complex of approximately 160 kd containing an endogenous substrate, p40, and possibly other polypeptides. This complex phosphorylates p40 and exogenous histone H1 in vitro. Cell cycle arrest during G1 results in inactivation of the protein kinase accompanied by the disassembly of the complex. Furthermore, assembly of the complex is regulated during the cell cycle, reaching a maximum during G1. Partial complexes thought to be intermediates in the assembly process phosphorylate histone H1 but not p40. Addition of soluble factors to these partial complexes in vitro restores p40 phosphorylation and causes the complex to increase to the mature size. A model is presented in which p40 phosphorylation is required during G1 for cells to initiate a new cell cycle.

Microalbuminuria after pregnancy complicated by pre-eclampsia

BACKGROUND

Microalbuminuria is an important risk factor for underlying vascular disease. Its detection after pregnancy complicated by pre-eclampsia may have predictive value for the later development of chronic hypertension or renal disease.

METHOD

The study group consisted of 48 women in whom pregnancy had been complicated by pre-eclampsia. Urinary albumin excretion rate, blood pressure, and renal function parameters were assessed 2-4 months and 3-5 years after the pregnancy. Results were compared with those in 44 women after normal pregnancy.

RESULTS

Mean urinary albumin excretion rate was significantly higher in the study group than in the controls both at 2-4 months after delivery (27.0 +/- 33 vs 6.1 +/- 3.3 mg/24 h) and at 3-5 years after delivery (23.5 +/- 26.8 vs 6.7 +/- 2.8 mg/24 h) (P = 0.001). The rate of occurrence of microalbuminuria was not significantly different between the early (58%) and late (42%) time-points within the study group or between the nulliparous and the multiparous women.

CONCLUSIONS

A history of pregnancy complicated by pre-eclampsia is associated with a high occurrence of microalbuminuria. Whether the presence of microalbuminuria reflects a possible underlying vascular disease in affected patients needs to be further investigated in large-scale studies.

The Saccharomyces cerevisiae CKS1 gene, a homolog of the Schizosaccharomyces pombe suc1+ gene, encodes a subunit of the Cdc28 protein kinase complex

The Saccharomyces cerevisiae gene CDC28 encodes a protein kinase required for cell cycle initiation. In an attempt to identify genes encoding proteins that interact with the Cdc28 protein kinase, high-copy plasmid suppressors of a temperature-sensitive cdc28 mutation were isolated. One such suppressor, CKS1, was found to encode an 18-kilodalton protein that shared a high degree of homology with the suc1+ protein (p13) of Schizosaccharomyces pombe (67% amino acid sequence identity). Disruption of the chromosomal CKS1 gene conferred a G1 arrest phenotype similar to that of cdc28 mutants. The presence of the 18-kilodalton Cks1 protein in yeast lysates was demonstrated by using Cks-1 specific antiserum. Furthermore, the Cks1 protein was shown to be physically associated with active forms of the Cdc28 protein kinase. These data suggest that Cks1 is an essential component of the Cdc28 protein kinase complex.

F-box protein Grr1 interacts with phosphorylated targets via the cationic surface of its leucine-rich repeat

The flexibility and specificity of ubiquitin-dependent proteolysis are mediated, in part, by the E3 ubiquitin ligases. One class of E3 enzymes, SKp1/cullin/F-box protein (SCF), derives its specificity from F-box proteins, a heterogeneous family of adapters for target protein recognition. Grr1, the F-box component of SCF(Grr1), mediates the interaction with phosphorylated forms of the G(1) cyclins Cln1 and Cln2. We show that binding of Cln2 by SCF(Grr1) was dependent upon its leucine-rich repeat (LRR) domain and its carboxy terminus. Our structural model for the Grr1 LRR predicted a high density of positive charge on the concave surface of the characteristic horseshoe structure. We hypothesized that specific basic residues on the predicted concave surface are important for recognition of phosphorylated Cln2. We show that point mutations that converted the basic residues on the concave surface but not those on the convex surface to neutral or acidic residues interfered with the capacity of Grr1 to bind to Cln2. The same mutations resulted in the stabilization of Cln2 and Gic2 and also in a spectrum of phenotypes characteristic of inactivation of GRR1, including hyperpolarization and enhancement of pseudohyphal growth. It was surprising that the same residues were not important for the role of Grr1 in nutrient-regulated transcription of HXT1 or AGP1. We concluded that the cationic nature of the concave surface of the Grr1 LRR is critical for the recognition of phosphorylated targets of SCF(Grr1) but that other properties of Grr1 are required for its other functions.

Yeast G1 cyclins are unstable in G1 phase

In most eukaryotes, commitment to cell division occurs in late G1 phase at an event called Start in the yeast Saccharomyces cerevisiae, and called the restriction point in mammalian cells. Start is triggered by the cyclin-dependent kinase Cdc28 and three rate-limiting activators, the G1 cyclins Cln1, Cln2 and Cln3. Cyclin accumulation in G1 is driven in part by the cell-cycle-regulated transcription of CLN1 and CLN2, which peaks at Start. CLN transcription is modulated by physiological signals that regulate G1 progression, but it is unclear whether Cln protein stability is cell-cycle-regulated. It has been suggested that once cells pass Start, Cln proteolysis is triggered by the mitotic cyclins Clb1, 2, 3 and 4. But here we show that G1 cyclins are unstable in G1 phase, and that Clb-Cdc28 activity is not needed fgr G1 cyclin turnover. Cln instability thus provides a means to couple Cln-Cdc28 activity to transcriptional regulation and protein synthetic rate in pre-Start G1 cells.

Subcellular localization of a protein kinase required for cell cycle initiation in Saccharomyces cerevisiae: evidence for an association between the CDC28 gene product and the insoluble cytoplasmic matrix

The product of the Saccharomyces cerevisiae gene CDC28, a protein kinase required for initiation of the cell division cycle, was localized within yeast cells. By using immunofluorescence methods, the CDC28 product was shown to be primarily cytoplasmic in distribution. The gene product was localized largely to the particulate fraction by differential centrifugation after mechanical disruption in aqueous buffers. The particulate association was not affected by the presence of nonionic detergent. To refine this localization further, a procedure was developed for the preparation of yeast cytoplasmic matrices which resemble the cytoskeletons of vertebrate cells on the basis of methodology, immunochemistry, and gross ultrastructure. A portion of the CDC28 product was found to be tightly associated with these detergent-insoluble cytoplasmic matrices by both immunofluorescence and immunoblotting procedures. Although, for technical reasons, precise quantitation was not possible, it is estimated that a minimum of 2-15% of the total CDC28 product pool is involved in the association with the insoluble matrix. Alcohol dehydrogenase, a soluble cytoplasmic protein, was found not to be associated with the cytoplasmic matrices at any detectable level, whereas, in contrast, approximately 10-40% of the total cellular actin, a bonafide cytoskeletal protein, was present in these structures. The proportion of CDC28 gene product associated with the particulate fraction, and perhaps the insoluble matrix, appears to be substantially decreased during the preparation of spheroplasts.

Differential regulation of lung endothelial permeability in vitro and in situ

In the lungs, increased vascular permeability can lead to acute lung injury. Because vascular permeability is regulated primarily by endothelial cells, many researchers have studied endothelial cell monolayers in culture, in order to understand the pathomechanisms of pulmonary edema. Such studies are based on the assumption that endothelial cells in culture behave like endothelial cells in situ. Here we show that this assumption is largely unfounded. Cultured endothelial cells show profound differences compared to their physiological counterparts, including a dysregulated calcium homeostasis. They fail to reproduce the pulmonary responses to agents such as platelet-activating factor. In contrast, they respond in a Rho-kinase depend fashion to thrombin, LPS or TNF. This is a striking finding for three reasons: (i) in the lungs, none of these agents increases vascular permeability by a direct interaction with endothelial cells; (ii) The endothelial Rho-kinase pathway seems to play little role in the development of pulmonary edema; (iii) This response pattern is similar for many endothelial cells in culture irrespective of their origin, which is in contrast to the stark heterogeneity of endothelial cells in situ. It appears that most endothelial in culture tend to develop a similar phenotyp that is not representative of any of the known endothelial cells of the lungs. We conclude that at present cultured endothelial cells are not useful to study the pathomechanisms of pulmonary edema.

Rme1, a negative regulator of meiosis, is also a positive activator of G1 cyclin gene expression

Control of G1 cyclin expression in Saccharomyces cerevisiae is mediated primarily by the transcription factor SBF (Swi4/Swi6). In the absence of Swi4 and Swi6 cell viability is lost, but can be regained by ectopic expression of the G1 cyclin encoding genes, CLN1 or CLN2. Here we demonstrate that the RME1 (regulator of meiosis) gene can also bypass the normally essential requirement for SBF. RME1 encodes a zinc finger protein which is able to repress transcription of IME1 (inducer of meiosis) and thereby inhibit cells from entering meiosis. We have found that expression of RME1 from a high copy number plasmid can specifically induce CLN2 expression. Deletion of RME1 alone shows no discernible effect on vegetative growth, however, deletion of RME1 in a swi6 delta swi4ts strain results in a lowering of the non-permissive temperature for viability. This suggests that Rme1 plays a significant but ancillary role in SBF in inducing CLN2 expression. We show that Rme1 interacts directly with the CLN2 promoter and have mapped the region of the CLN2 promoter required for Rme1-dependent activation. Consistent with Rme1 having a cell cycle role in G1, we have found that RME1 mRNA is synthesized periodically in the cell cycle, with maximum accumulation occurring at the M/G1 boundary. Thus Rme1 may act both to promote mitosis, by activating CLN2 expression, and prevent meiosis, by repressing IME1 expression.

Regulation of cell size by glucose is exerted via repression of the CLN1 promoter

Yeast cells are keenly sensitive to the availability and quality of nutrients. Addition of glucose to cells growing on a poorer carbon source elicits a cell cycle delay during G1 phase and a concomitant increase in the cell size. The signal is transduced through the RAS-cyclic AMP pathway. Using synchronized populations of G1 cells, we show that the increase in cell size required for budding depends upon CLN1 but not other G1 cyclins. This delay in cell cycle initiation is associated specifically with transcriptional repression of CLN1. CLN2 is not repressed. Repression of CLN1 is not limited to the first cycle following glucose addition but occurs in each cell cycle during growth on glucose. A 106-bp fragment of the CLN1 promoter containing the three MluI cell cycle box (MCB) core elements responsible for the majority of CLN1-associated upstream activation sequence activity is sufficient to confer glucose-induced repression on a heterologous reporter. A mutant CLN2 promoter that is rendered dependent upon its three MCB core elements due to inactivation of its Swi4-dependent cell cycle box (SCB) elements is also repressed by glucose. The response to glucose is partially suppressed by inactivation of SWI4, but not MBP1, which is consistent with the dependence of MCB core elements upon the SCB-binding transcription factor (SBF). We suggest that differential regulation of CLN1 and CLN2 by glucose results from differences in the capacity of SBF to activate transcription driven by SCB and MCB core elements. Finally, we show that transcriptional repression is sufficient to explain the cell cycle delay that occurs in response to glucose.

Plugging it in: signaling circuits and the yeast cell cycle

Signal transduction pathways provide the means to transmit information and elicit specific responses. Modulation of the cell cycle machinery is one such response. Molecular genetic approaches with budding yeast have been instrumental in elucidating the components of these complex signaling pathways and the inter-relationships among these components. Recent progress has revealed pathways that link extracellular signals with the machinery governing both cell cycle progression and morphogenesis. The nature of the interface between nutritional and checkpoint signals with the cell cycle apparatus is just now emerging.

Cell cycle-dependent transcription of CLN2 is conferred by multiple distinct cis-acting regulatory elements

The budding yeast Saccharomyces cerevisiae CLN1, CLN2, and CLN3 genes encode functionally redundant G1 cyclins required for cell cycle initiation. CLN1 and CLN2 mRNAs accumulate periodically throughout the cell cycle, peaking in late G1. We show that cell cycle-dependent fluctuation in CLN2 mRNA is regulated at the level of transcriptional initiation. Mutational analysis of the CLN2 promoter revealed that the major cell cycle-dependent upstream activating sequence (UAS) resides within a 100-bp fragment. This UAS contains three putative SWI4-dependent cell cycle boxes (SCBs) and two putative MluI cell cycle boxes (MCBs). Mutational inactivation of these elements substantially decreased CLN2 promoter activity but failed to eliminate periodic transcription. Similarly, inactivation of SWI4 decreased CLN2 transcription without affecting its periodicity. We have identified a second UAS in the CLN2 upstream region that can promote cell cycle-dependent transcription with kinetics similar to that of the intact CLN2 promoter. Unlike the major CLN2 UAS, this newly identified UAS promotes transcription in cells arrested in G1 by inactivation of cdc28. This novel UAS is both necessary and sufficient for regulated transcription driven by a CLN2 promoter lacking functional SCBs and MCBs. Although this UAS itself contains no SCBs or MCBs, its activity is dependent upon SWI4 function. The characteristics of this novel UAS suggest that it might have a role in initiating CLN2 expression early in G1 to activate the positive feedback loop that drives maximal Cln accumulation.