The Aspergillus nidulans MAPK module AnSte11-Ste50-Ste7-Fus3 controls development and secondary metabolism

The sexual Fus3 MAP kinase module of yeast is highly conserved in eukaryotes and transmits external signals from the plasma membrane to the nucleus. We show here that the module of the filamentous fungus Aspergillus nidulans (An) consists of the AnFus3 MAP kinase, the upstream kinases AnSte7 and AnSte11, and the AnSte50 adaptor. The fungal MAPK module controls the coordination of fungal development and secondary metabolite production. It lacks the membrane docking yeast Ste5 scaffold homolog; but, similar to yeast, the entire MAPK module's proteins interact with each other at the plasma membrane. AnFus3 is the only subunit with the potential to enter the nucleus from the nuclear envelope. AnFus3 interacts with the conserved nuclear transcription factor AnSte12 to initiate sexual development and phosphorylates VeA, which is a major regulatory protein required for sexual development and coordinated secondary metabolite production. Our data suggest that not only Fus3, but even the entire MAPK module complex of four physically interacting proteins, can migrate from plasma membrane to nuclear envelope.

Aggregate clearance of α-synuclein in Saccharomyces cerevisiae depends more on autophagosome and vacuole function than on the proteasome

Parkinson disease is the second most common neurodegenerative disease. The molecular hallmark is the accumulation of proteinaceous inclusions termed Lewy bodies containing misfolded and aggregated α-synuclein. The molecular mechanism of clearance of α-synuclein aggregates was addressed using the bakers' yeast Saccharomyces cerevisiae as the model. Overexpression of wild type α-synuclein or the genetic variant A53T integrated into one genomic locus resulted in a gene copy-dependent manner in cytoplasmic proteinaceous inclusions reminiscent of the pathogenesis of the disease. In contrast, overexpression of the genetic variant A30P resulted only in transient aggregation, whereas the designer mutant A30P/A36P/A76P neither caused aggregation nor impaired yeast growth. The α-synuclein accumulation can be cleared after promoter shut-off by a combination of autophagy and vacuolar protein degradation. Whereas the proteasomal inhibitor MG-132 did not significantly inhibit aggregate clearance, treatment with phenylmethylsulfonyl fluoride, an inhibitor of vacuolar proteases, resulted in significant reduction in clearance. Consistently, a cim3-1 yeast mutant restricted in the 19 S proteasome regulatory subunit was unaffected in clearance, whereas an Δatg1 yeast mutant deficient in autophagy showed a delayed aggregate clearance response. A cim3-1Δatg1 double mutant was still able to clear aggregates, suggesting additional cellular mechanisms for α-synuclein clearance. Our data provide insight into the mechanisms yeast cells use for clearing different species of α-synuclein and demonstrate a higher contribution of the autophagy/vacuole than the proteasome system. This contributes to the understanding of how cells can cope with toxic and/or aggregated proteins and may ultimately enable the development of novel strategies for therapeutic intervention.

Dissection of mitotic functions of the yeast cyclin Clb2

Progression through mitosis requires the activity of cyclin-dependent kinases (CDKs) associated with regulatory cyclin subunits. In the yeast Saccharomyces cerevisiae, Clb2 has the most important role among the four mitotic cyclins, Clb1-4, manifested by data showing that simultaneous deletion of the CLB1, CLB3 and CLB4 genes has only minor effects on mitosis. Thus, Clb2 alone is sufficient for all essential CDK functions in mitosis, such as the assembly of bipolar spindles and spindle elongation. Here, we show that a modification of Clb2, by the C-terminal addition of a Myc12 epitope, causes the loss of one specific mitotic function of Clb2. Strains carrying CLB2-MYC12 are nonviable in the absence of the CLB3 and CLB4 genes, because the modified Clb2 version fails to promote assembly of the mitotic spindle. In contrast, Clb2-Myc12 has no apparent defects in late mitotic functions and, furthermore, induces the switch from polarized to isotropic growth with similar efficiency as the endogenous Clb2. Thus, the presence of the Myc12 epitope selectively inactivates Clb2's capacity to promote spindle formation. Clb2-Myc12 represents therefore the first version of Clb2 impaired in one specific mitotic function. We conclude that the major mitotic functions of this cyclin can be unequivocally dissected.

The Ime2 protein kinase family in fungi: more duties than just meiosis

Ime2 of the budding yeast Saccharomyces cerevisiae belongs to a family of conserved protein kinases displaying sequence similarities to both cyclin-dependent kinases and mitogen-activated protein kinases. Ime2 has a pivotal role for meiosis and sporulation. The involvement of this protein kinase in the regulation of various key events in meiosis, such as the initiation of DNA replication, the expression of meiosis-specific genes and the passage through the two consecutive rounds of nuclear divisions has been characterized in detail. More than 20 years after the identification of the IME2 gene, a recent report has provided the first evidence for a function of this gene outside of meiosis, which is the regulation of pseudohyphal growth. In the last few years, Ime2-related protein kinases from various fungal species were studied. Remarkably, these homologues are not generally required for meiosis, but instead have other specific tasks. In filamentous ascomycete species, Ime2 homologues are involved in the inhibition of fruiting body formation in response to environmental signals. In the pathogenic basidiomycetes Ustilago maydis and Cryptococcus neoformans, members of this kinase family apparently have primary roles in regulating mating. Thus, Ime2-related kinases exhibit an amazing variety in controlling sexual developmental programs in fungi.

The anaphase promoting complex is required for memory function in mice

Learning and memory processes critically involve the orchestrated regulation of de novo protein synthesis. On the other hand it has become clear that regulated protein degradation also plays a major role in neuronal plasticity and learning behavior. One of the key pathways mediating protein degradation is proteosomal protein destruction. The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase that targets proteins for proteosomal degradation by the 26S proteasome. While the APC/C is essential for cell cycle progression it is also expressed in postmitotic neurons where it has been implicated with axonal outgrowth and neuronal survival. In this study we addressed the role of APC/C in learning and memory function by generating mice that lack the essential subunit APC2 from excitatory neurons of the adult forebrain. Those animals are viable but exhibit a severe impairment in the ability to extinct fear memories, a process critical for the treatment of anxiety diseases such as phobia or post-traumatic stress disorder. Since deregulated protein degradation and APC/C activity has been implicated with neurodegeneration we also analyzed the effect of Apc2 deletion in a mouse model for Alzheimer's disease. In our experimental setting loss of APC2 form principle forebrain neurons did not affect the course of pathology in an Alzheimer's disease mouse model. In conclusion, our data provides genetic evidence that APC/C activity in the adult forebrain is required for cognitive function.

LaeA control of velvet family regulatory proteins for light-dependent development and fungal cell-type specificity

VeA is the founding member of the velvet superfamily of fungal regulatory proteins. This protein is involved in light response and coordinates sexual reproduction and secondary metabolism in Aspergillus nidulans. In the dark, VeA bridges VelB and LaeA to form the VelB-VeA-LaeA (velvet) complex. The VeA-like protein VelB is another developmental regulator, and LaeA has been known as global regulator of secondary metabolism. In this study, we show that VelB forms a second light-regulated developmental complex together with VosA, another member of the velvet family, which represses asexual development. LaeA plays a key role, not only in secondary metabolism, but also in directing formation of the VelB-VosA and VelB-VeA-LaeA complexes. LaeA controls VeA modification and protein levels and possesses additional developmental functions. The laeA null mutant results in constitutive sexual differentiation, indicating that LaeA plays a pivotal role in inhibiting sexual development in response to light. Moreover, the absence of LaeA results in the formation of significantly smaller fruiting bodies. This is due to the lack of a specific globose cell type (Hülle cells), which nurse the young fruiting body during development. This suggests that LaeA controls Hülle cells. In summary, LaeA plays a dynamic role in fungal morphological and chemical development, and it controls expression, interactions, and modification of the velvet regulators.

Fungal development and the COP9 signalosome

The conserved COP9 signalosome (CSN) multiprotein complex is located at the interface between cellular signaling, protein modification, life span and the development of multicellular organisms. CSN is required for light-controlled responses in filamentous fungi. This includes the circadian rhythm of Neurospora crassa or the repression of sexual development by light in Aspergillus nidulans. In contrast to plants and animals, CSN is not essential for fungal viability. Therefore fungi are suitable models to study CSN composition, activity and cellular functions and its role in light controlled development.

The C-terminal region of the meiosis-specific protein kinase Ime2 mediates protein instability and is required for normal spore formation in budding yeast

The cyclin-dependent kinase Cdk1 and the related kinase Ime2 act in concert to trigger progression of the meiotic cell cycle in the yeast Saccharomyces cerevisiae. These kinases share several functions and substrates during meiosis, but their regulation seems to be clearly different. In contrast to Cdk1, no cyclin seems to be involved in the regulation of Ime2 activity. Ime2 is a highly unstable protein, and we aimed to elucidate the relevance of Ime2 instability. We first determined the sequence elements required for Ime2 instability by constructing a set of deletions in the IME2 gene. None of the small deletions in Ime2 affected its instability, but deletion of a 241 amino acid C-terminal region resulted in a highly stabilized protein. Thus, the C-terminal domain of Ime2 is important for mediating protein instability. The stabilized, truncated Ime2 protein is highly active in vivo. Replacement of the IME2 gene with the truncated IME2DeltaC241 in diploid strains did not interfere with meiotic nuclear divisions, but caused abnormalities in spore formation, as manifested by the appearance of many asci with a reduced spore number such as triads and dyads. The truncated Ime2 caused a reduction of spore number in a dominant manner. We conclude that downregulation of Ime2 kinase activity mediated by the C-terminal domain is required for the efficient production of normal four-spore asci. Our data suggest a role for Ime2 in spore number control in S. cerevisiae.

A process independent of the anaphase-promoting complex contributes to instability of the yeast S phase cyclin Clb5

Proteolytic destruction of many cyclins is induced by a multi-subunit ubiquitin ligase termed the anaphase promoting complex/cyclosome (APC/C). In the budding yeast Saccharomyces cerevisiae, the S phase cyclin Clb5 and the mitotic cyclins Clb1-4 are known as substrates of this complex. The relevance of APC/C in proteolysis of Clb5 is still under debate. Importantly, a deletion of the Clb5 destruction box has little influence on cell cycle progression. To understand Clb5 degradation in more detail, we applied in vivo pulse labeling to determine the half-life of Clb5 at different cell cycle stages and in the presence or absence of APC/C activity. Clb5 is significantly unstable, with a half-life of approximately 8-10 min, at cell cycle periods when APC/C is inactive and in mutants impaired in APC/C function. A Clb5 version lacking its cyclin destruction box is similarly unstable. The half-life of Clb5 is further decreased in a destruction box-dependent manner to 3-5 min in mitotic or G(1) cells with active APC/C. Clb5 instability is highly dependent on the function of the proteasome. We conclude that Clb5 proteolysis involves two different modes for targeting of Clb5 to the proteasome, an APC/C-dependent and an APC/C-independent mechanism. These different modes apparently have overlapping functions in restricting Clb5 levels in a normal cell cycle, but APC/C function is essential in the presence of abnormally high Clb5 levels.

Preventing Fatal Destruction: Inhibitors of the Anaphase-Promoting Complex in Meiosis

The anaphase-promoting complex/cyclosome (APC/C) is a multi-subunit ubiquitin-ligase whose major functions in the cell cycle are the initiation of sister chromatid separation and the inactivation of cyclin-dependent kinases. This complex is also essential for meiosis, a specialized form of the cell cycle characterized by two consecutive rounds of chromosome segregation. To ensure a proper meiotic cell cycle, the activity of APC/C needs to be tightly controlled. It is now evident that inhibitors of APC/C play pivotal roles to avert its untimely activation. During prophase I, this ubiquitin-ligase must be kept inactive to prevent precocious sister chromatid separation. Studies in yeast showed that this inhibition is mediated by a specific subunit of the complex. Accurate chromosome segregation in meiosis I depends on spindle checkpoint proteins such as Mad2 which delay APC/C activation in response to an erroneous spindle attachment of chromosomes. Additional APC/C antagonists are known to block complete cyclin destruction between meiosis I and II, thereby ensuring that cyclin dependent kinases remain active and that DNA replication does not occur. Inhibitors of APC/C also mediate the cytostatic factor induced metaphase II arrest of oocytes. This review highlights the current knowledge about the role and relevance of these diverse regulators of the meiotic APC/C.

Smt3/SUMO and Ubc9 are required for efficient APC/C-mediated proteolysis in budding yeast

Ubiquitin-mediated proteolysis triggered by the anaphase-promoting complex/cyclosome (APC/C) is essential for sister chromatid separation and the mitotic exit. Like ubiquitylation, protein modification with the small ubiquitin-related modifier SUMO appears to be important during mitosis, because yeast cells impaired in the SUMO-conjugating enzyme Ubc9 were found to be blocked in mitosis and defective in cyclin degradation. Here, we analysed the role of SUMOylation in the metaphase/anaphase transition and in APC/C-mediated proteolysis in Saccharomyces cerevisiae. We show that cells depleted of Ubc9 or Smt3, the yeast SUMO protein, mostly arrested with undivided nuclei and with high levels of securin Pds1. This metaphase block was partially relieved by a deletion of PDS1. The absence of Ubc9 or Smt3 also resulted in defects in chromosome segregation. Temperature-sensitive ubc9-2 mutants were delayed in proteolysis of Pds1 and of cyclin Clb2 during mitosis. The requirement of SUMOylation for APC/C-mediated degradation was tested more directly in G1-arrested cells. Both ubc9-2 and smt3-331 mutants were defective in efficient degradation of Pds1 and mitotic cyclins, whereas proteolysis of unstable proteins that are not APC/C substrates was unaffected. We conclude that SUMOylation is needed for efficient proteolysis mediated by APC/C in budding yeast.

Controlling transcription by destruction: the regulation of yeast Gcn4p stability

The Gcn4 protein, a member of the AP-1 family of transcription factors, is involved in the expression of more than 500 genes in the budding yeast Saccharomyces cerevisiae. A key role of Gcn4p is the increased expression of many amino acid biosynthesis genes in response to amino acid starvation. The accumulation of this transcription activator is mainly induced by efficient translation of the GCN4 ORF and by stabilisation of the Gcn4 protein. Under normal growth conditions, Gcn4p is a highly unstable protein, thereby resembling many eukaryotic transcription factors, including mammalian Jun and Myc proteins. Gcn4p is degraded by ubiquitin-dependent proteolysis mediated by the Skp1/cullin/F-box (SCF) ubiquitin ligase, which recognises specifically phosphorylated substrates. Two cyclin-dependent protein kinases, Pho85p and Srb10p, have crucial functions in regulating Gcn4p phosphorylation and degradation. The past few years have revealed many novel insights into these regulatory processes. Here, we summarise current knowledge about the factors and mechanisms regulating Gcn4p stability.

Synergistic inhibition of APC/C by glucose and activated Ras proteins can be mediated by each of the Tpk1-3 proteins in Saccharomyces cerevisiae

Proteolysis triggered by the anaphase-promoting complex/cyclosome (APC/C) is essential for the progression through mitosis. APC/C is a highly conserved ubiquitin ligase whose activity is regulated during the cell cycle by various factors, including spindle checkpoint components and protein kinases. The cAMP-dependent protein kinase (PKA) was identified as negative regulator of APC/C in yeast and mammalian cells. In the yeast Saccharomyces cerevisiae, PKA activity is induced upon glucose addition or by activated Ras proteins. This study shows that glucose and the activated Ras2(Val19) protein synergistically inhibit APC/C function via the cAMP/PKA pathway in yeast. Remarkably, Ras2 proteins defective in the interaction with adenylate cyclase fail to influence APC/C, implying that its function is regulated exclusively by PKA, but not by alternative Ras pathways. Furthermore, it is shown that the three PKAs in yeast, Tpk1, Tpk2 and Tpk3, have redundant functions in regulating APC/C in response to glucose medium. Single or double deletions of TPK genes did not prevent inhibition of APC/C, suggesting that each of the Tpk proteins can take over this function. However, Tpk2 seems to inhibit APC/C function more efficiently than Tpk1 and Tpk3. Finally, evidence is provided that Cdc20 is involved in APC/C regulation by the cAMP/PKA pathway.

Cyclin destruction in mitosis: a crucial task of Cdc20

Proteolytic destruction of cyclins is a fundamental process for cell division. At the end of mitosis, degradation of mitotic cyclins results in the inactivation of cyclin-dependent kinases. Cyclin proteolysis is triggered by the anaphase-promoting complex/cyclosome (APC/C), a multi-subunit complex which contains ubiquitin ligase activity. Recent data in yeast demonstrated that a partial degradation of the mitotic cyclin Clb2, mediated by APC/C and its activator protein Cdc20, is essential and sufficient for the mitotic exit. Remarkably, a complete inactivation of cyclin-dependent kinases seems to be not essential. This review discusses recent novel insights into cyclin destruction and its implications for the mitotic exit.

Amino acid-dependent Gcn4p stability regulation occurs exclusively in the yeast nucleus

The c-Jun-like transcriptional activator Gcn4p controls biosynthesis of translational precursors in the yeast Saccharomyces cerevisiae. Protein stability is dependent on amino acid limitation and cis signals within Gcn4p which are recognized by cyclin-dependent protein kinases, including Pho85p. The Gcn4p population within unstarved yeast consists of a small relatively stable cytoplasmic fraction and a larger less stable nuclear fraction. Gcn4p contains two nuclear localization signals (NLS) which function independently of the presence or absence of amino acids. Expression of NLS-truncated Gcn4p results in an increased cytoplasmic fraction and an overall stabilization of the protein. The same effect is achieved for the entire Gcn4p in a yrb1 yeast mutant strain impaired in the nuclear import machinery. In the presence of amino acids, controlled destabilization of Gcn4p is triggered by the phosphorylation activity of Pho85p. A pho85delta mutation stabilizes Gcn4p without affecting nuclear import. Pho85p is localized within the nucleus in the presence or absence of amino acids. Therefore, there is a strict spatial separation of protein synthesis and degradation of Gcn4p in yeast. Control of protein stabilization which antagonizes Gcn4p function is restricted to the nucleus.

Inhibition of APC-mediated proteolysis by the meiosis-specific protein kinase Ime2

Proteolysis triggered by the anaphase-promoting complex (APC) is needed for sister chromatid separation and the exit from mitosis. APC is a ubiquitin ligase whose activity is tightly controlled during the cell cycle. To identify factors involved in the regulation of APC-mediated proteolysis, a Saccharomyces cerevisiae GAL-cDNA library was screened for genes whose overexpression prevented degradation of an APC target protein, the mitotic cyclin Clb2. Genes encoding G1, S, and mitotic cyclins were identified, consistent with previous data showing that the cyclin-dependent kinase Cdk1 associated with different cyclins is a key factor for inhibiting APC(Cdh1) activity from late-G1 phase until mitosis. In addition, the meiosis-specific protein kinase Ime2 was identified as a negative regulator of APC-mediated proteolysis. Ectopic expression of IME2 in G1 arrested cells inhibited the degradation of mitotic cyclins and of other APC substrates. IME2 expression resulted in the phosphorylation of Cdh1 in G1 cells, indicating that Ime2 and Cdk1 regulate APC(Cdh1) in a similar manner. The expression of IME2 in cycling cells inhibited bud formation and caused cells to arrest in mitosis. We show further that Ime2 itself is an unstable protein whose proteolysis occurs independently of the APC and SCF (Skp1/Cdc53/F-box) ubiquitin ligases. Our findings suggest that Ime2 represents an unstable, meiosis-specific regulator of APC(Cdh1).

Induction of jlbA mRNA synthesis for a putative bZIP protein of Aspergillus nidulans by amino acid starvation

The jlbA (jun-like hZIP) gene of Aspergillus nidulans was isolated. The deduced amino acid motif of the C-terminal region of jlhA encodes a putative DNA-binding site composed of a basic amino acid domain and an adjacent leucine zipper motif. This region shares highest similarities to the C-terminal DNA-binding domain and the basic zipper (bZIP)-motifs of transcription factors like CPCA from A. niger, Gcn4p from Saccharomyces cerevisiae, human JUNB and c-JUN. The putative jlbA protein contains a PEST-rich region (an instability region rich in the amino acids proline, glutamic acid, serine and threonine) described to be implicated in protein stability. The jlbA mRNA formation is elevated up to 40-fold upon amino acid starvation induced by the addition of the false feedback inhibitor 3-amino-1,2,4-triazole. This induction is partially dependent and partially independent on the presence of the transcription factor CPCA. Therefore jlbA is a novel gene of A. nidulans which is transcriptionally activated by amino acid starvation conditions.

Yeast Ran-binding protein Yrb1p is required for efficient proteolysis of cell cycle regulatory proteins Pds1p and Sic1p

Ubiquitin-dependent proteolysis of specific target proteins is required for several important steps during the cell cycle. Degradation of such proteins is strictly cell cycle-regulated and triggered by two large ubiquitin ligases, termed anaphase-promoting complex (APC) and Skp1/Cullin/F-box complex (SCF). Here we show that yeast Ran-binding protein 1 (Yrb1p), a predominantly cytoplasmic protein implicated in nucleocytoplasmic transport, is required for cell cycle regulated protein degradation. Depletion of Yrb1p results in the accumulation of unbudded G(1) cells and of cells arrested in mitosis implying a function of Yrb1p in the G(1)/S transition and in the progression through mitosis. Temperature-sensitive yrb1-51 mutants are defective in APC-mediated degradation of the anaphase inhibitor protein Pds1p and in degradation of the cyclin-dependent kinase inhibitor Sic1p, a target of SCF. Thus, Yrb1p is crucial for efficient APC- and SCF-mediated proteolysis of important cell cycle regulatory proteins. We have identified the UBS1 gene as a multicopy suppressor of yrb1-51 mutants. Ubs1p is a nuclear protein, and its deletion is synthetic lethal with a yrb1-51 mutation. Interestingly, UBS1 was previously identified as a multicopy suppressor of cdc34-2 mutants, which are defective in SCF activity. We suggest that Ubs1p may represent a link between nucleocytoplasmic transport and ubiquitin ligase activity.

Glucose and ras activity influence the ubiquitin ligases APC/C and SCF in Saccharomyces cerevisiae

In budding yeast, the Ras/cAMP pathway is involved in the coordination of cell growth and cell division. Glucose-rich medium stimulates Ras/cAMP signaling, which causes an increase in the critical cell size for cell cycle entry. Here we show that glucose and activated Ras proteins also influence the function of the anaphase-promoting complex (APC/C), a ubiquitin-protein ligase required for sister chromatid separation and mitotic exit. We found that apc10-22 and other mutants defective in the APC/C are suppressed by reduced Ras signaling activity, by a deletion of the RAS2 gene, by a cdc25 mutation, by elevated levels of PDE2, or by growth without glucose. Viability of these mutants is also enhanced by decreased Cdk1 activity. In contrast, a constitutively activated RAS2(Val19) allele or shifts to glucose medium are deleterious to apc10-22 mutants. Remarkably, cdc34-2 mutants, which are impaired in SCF function, are differently affected with respect to Ras activity. Viability of cdc34-2 mutants at elevated temperatures is dependent on glucose and the RAS2 gene. We conclude that glucose and Ras proteins influence the APC/C and the SCF complex in an opposite manner. These ubiquitin ligases might represent novel targets for modulating cell division in response to growth conditions.

Two different modes of cyclin clb2 proteolysis during mitosis in Saccharomyces cerevisiae

Sister chromatid separation and mitotic exit are triggered by the anaphase-promoting complex (APC/C) which is a multi-subunit ubiquitin ligase required for proteolytic degradation of various target proteins. Cdc20 and Cdh1 are substrate-specific activators of the APC/C. It was previously proposed that Cdh1 is essential for proteolysis of the yeast mitotic cyclin Clb2. We show that Clb2 proteolysis is triggered by two different modes during mitosis. A fraction of Clb2 is degraded during anaphase in the absence of Cdh1. However, a second fraction of Clb2 remains stable during anaphase and is degraded in a Cdh1-dependent manner as cells exit from mitosis. Most of cyclin Clb3 is degraded independently of Cdh1. Our data imply that degradation of mitotic cyclins is initiated by a Cdh1-independent mechanism.

The anaphase-promoting complex is required in G1 arrested yeast cells to inhibit B-type cyclin accumulation and to prevent uncontrolled entry into S-phase

Inactivation of B-type cyclin dependent kinases due to ubiquitin-mediated cyclin proteolysis is necessary for the exit from mitosis. In Saccharomyces cerevisiae, proteolysis is initiated at the onset of anaphase and remains active until Cln1 and Cln2 cyclins appear in late G1 of the subsequent cell cycle. A large particle called the anaphase-promoting complex (APC) which is composed of the TPR proteins Cdc16p/Cdc23p/Cdc27p and other proteins is required for B-type cyclin ubiquitination in both anaphase and during G1 phase. The APC has an essential role for the separation of sister chromatids and for the exit from mitosis, but until now it was unclear whether the persistence of APC activity throughout G1 had any physiological role. We show here that the APC is needed in G1 arrested cells to inhibit premature appearance of B-type cyclins and to prevent unscheduled initiation of DNA replication. When pheromone arrested cells of cdc16 and cdc23 mutants were shifted to the restrictive temperature, they underwent DNA replication in the presence of pheromone. DNA replication also occurred in a G1 arrest induced by G1 cyclin (Cln) depletion, indicating that mutant cells with a defective APC initiate DNA replication without the Cln G1 cyclins, which are normally needed for the onset of S-phase. Degradation of Clb2p, Clb3p and Clb5p depends on the APC. This suggests that accumulation of any one of the six B-type cyclin proteins could account for the precocious replication of cdc16 and cdc23 mutants.

Sequence requirements of the bidirectional yeast TRP4 mRNA 3'-end formation signal

The yeast TRP4 3'-end formation signal functions in both orientations in an in vivo test system. We show here that the TRP4 3'-end formation element consists of two functionally different sequence regions. One region of approximately 70 nucleotides is located in the untranslated region between the translational stop codon and the major poly(A) site. The major poly(A) site is not part of this region and can be deleted without a decrease in TRP4 3'-end formation. 5'and 3'deletions and point mutations within this region affected 3'-end formation similarly in both orientations. In the center of this region the motif TAGT is located on the antisense strand. Point mutations within this motif resulted in a drastic reduce of 3'-end formation activity in both orientations. A second region consists of the 3'-end of the TRP4 open reading frame and is required for 3'-end formation in forward orientation. A single point mutation in a TAGT motif of the TRP4 open reading frame abolished TRP4 mRNA 3'-end formation in forward orientation and had no effect on the reverse orientation.

Genes involved in sister chromatid separation are needed for B-type cyclin proteolysis in budding yeast

B-type cyclin destruction is necessary for exit from mitosis and the initiation of a new cell cycle. Through the isolation of mutants, we have identified three essential yeast genes, CDC16, CDC23, and CSE1, which are required for proteolysis of the B-type cyclin CLB2 but not of other unstable proteins. cdc23-1 mutants are defective in both entering and exiting anaphase. Their failure to exit anaphase can be explained by defective cyclin proteolysis. CDC23 is required at the metaphase/anaphase transition to separate sister chromatids, and we speculate that it might promote proteolysis of proteins that hold sister chromatids together. Proteolysis of CLB2 is initiated in early anaphase, but a fraction of CLB2 remains stable until anaphase is complete.

Closing the cell cycle circle in yeast: G2 cyclin proteolysis initiated at mitosis persists until the activation of G1 cyclins in the next cycle

It is thought that DNA replication and mitosis in yeast are triggered by oscillations in the level of G1-specific (CLN1 and CLN2) and G2-specific (CLB1-CLB4) cyclins, which determine the substrate specificity of the CDC28 protein kinase. It is not understood how the time and order of appearance of different cyclin types are determined. We show here that CLB2 proteolysis, which is important for transition from mitosis to G1, is not confined to a narrow window at the end of mitosis as previously thought but continues until reactivation of CDC28 by CLN cyclins toward the end of the subsequent G1 period. Thus, cell cycle-regulated proteolysis prevents accumulation of G2-specific CLB cyclins during G1 and thereby ensures that the CLN-associated forms of the CDC28 kinase are activated without interference from CLB cyclins. Accumulation of CLN cyclins leads to inactivation of CLB cyclin proteolysis, which is a precondition for subsequent activation of G2-specific B-type cyclins.

Saturation mutagenesis of a polyadenylation signal reveals a hexanucleotide element essential for mRNA 3' end formation in Saccharomyces cerevisiae

The cis-acting signal sequences required for mRNA 3' end formation are highly conserved and well characterized in higher eukaryotes. However, the situation in the yeast Saccharomyces cerevisiae is still unclear. Several sequences have been proposed which share only limited similarities. One difficulty in identifying yeast polyadenylylation signals might be the presence of redundant signal sequences in the 3' region of yeast genes. To circumvent this problem we have analyzed the heterologous 3' region from cauliflower mosaic virus which contains a yeast polyadenylylation signal. We have performed a saturation mutagenesis of the key element TAG-TATGTA, which is a condensed version of the polyadenylylation signal TAG ... TATGTA ... (TTT) which had previously been proposed. Each of the nine nucleotides was replaced by the three other possible nucleotides and all resulting 1-bp mutants were tested for their capacity to specify mRNA 3' end formation in yeast cells. The first three nucleotides of this condensed sequence are not required, but mutagenesis of the other six nucleotides had distinct effects on mRNA 3' end formation. All mutants that were significantly functional had the sequence TAYRTA, and the sequence TATATA had the best capacity for mRNA 3' end formation. The two thymidine residues at the first and fifth positions are the most essential nucleotides in this sequence. Our results suggest that a degenerate hexanucleotide is essential for mRNA 3' end formation in yeast. This is reminiscent of the conserved polyadenylylation signal in higher eukaryotes, AATAAA.

Messenger RNA 3'-end formation of a DNA fragment from the human c-myc 3'-end region in Saccharomyces cerevisiae

We have tested the functioning of the human c-myc polyadenylation signal in Saccharomyces cerevisiae. A DNA fragment containing the two AATAAA polyadenylation signals of the c-myc gene was inserted into a plasmid designed for the in-vivo testing of polyadenylation signals in yeast. The c-myc fragment had a partial capacity for directing mRNA 3'-end formation in yeast. The 3'-endpoints were 50-100 bp distant from the mRNA 3'-ends mapped in humans. This human DNA fragment is therefore unspecifically functional in yeast, indicating that other sequence elements than the human polyadenylation signal, AATAAA, are necessary for 3'-end formation.

The yeast actin intron contains a cryptic promoter that can be switched on by preventing transcriptional interference

We show that the single intron of the actin gene of the yeast Saccharomyces cerevisiae contains a cryptic promoter for transcription of the second exon. This promoter is inactive in the normal actin gene, but can be activated when the actin gene promoter is deleted. An identical activation was induced by placing efficient transcriptional terminators at position 61 of the 309 bp intron. In all cases transcripts with identical 5' ends close to the boundary of the intron and the second exon were produced. These results indicate that the cryptic promoter in the actin intron is occluded in the normal actin gene by transcriptional interference with the actin gene promoter. Transcription initiation near the intron/exon 2 boundary is enabled by protection from traversing polymerases, that initiated transcription at the upstream located actin gene promoter. A partial promoter protection using leaky terminators resulted in small amounts of transcripts initiated from the cryptic promoter. Although we do not know any function of the cryptic promoter in actin gene expression, it is tentative to speculate that the cryptic intron promoter might be a relict of a promoter that was functional earlier in evolution.

Cloning, primary structure and regulation of the ARO4 gene, encoding the tyrosine-inhibited 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Saccharomyces cerevisiae

In Saccharomyces cerevisiae, two differently regulated 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase (DAHPS; EC 4.1.2.15) isoenzymes carry out the first step in the shikimate pathway. Mutations in both genes are necessary to cause aromatic amino acid (aa) auxotrophy, since one isoenzyme alone is sufficient to produce enough DAHP for normal growth of the cells. The phenylalanine-inhibited DAHPS is encoded by the previously isolated and characterized ARO3 gene. Here, we report the cloning and characterization of the ARO4 gene, encoding the second DAHPS, which is inhibited by tyrosine. The aa sequence of the ARO4 gene product reveals 76% similarity to the ARO3-encoded isoenzyme and 66 and 73% to the three DAHPS isoenzymes from Escherichia coli. ARO4 gene expression is regulated by the general control system of aa biosynthesis. As in the case of the ARO3 gene, a single GCN4-recognition element in the promoter is responsible for derepression of the ARO4 gene under aa starvation conditions. However, in contrast to the situation in the isogene, ARO3, GCN4 does not contribute to the basal level of ARO4 transcription under nonderepressing conditions.

Different sequence elements are required for function of the cauliflower mosaic virus polyadenylation site in Saccharomyces cerevisiae compared with in plants

We show that the polyadenylation site derived from the plant cauliflower mosaic virus (CaMV) is specifically functional in the yeast Saccharomyces cerevisiae. The mRNA 3' endpoints were mapped at the same position in yeast cells as in plants, and the CaMV polyadenylation site was recognized in an orientation-dependent manner. Mutational analysis of the CaMV 3'-end-formation signal revealed that multiple elements are essential for proper activity in yeast cells, including two upstream elements that are situated more than 100 and 43 to 51 nucleotides upstream of the poly(A) addition site and the sequences at or near the poly(A) addition site. A comparison of the sequence elements that are essential for proper function of the CaMV signal in yeast cells and plants showed that both organisms require a distal and a proximal upstream element but that these sequence elements are not identical in yeast cells and plants. The key element for functioning of the CaMV signal in yeast cells is the sequence TAGTATGTA, which is similar to a sequence previously proposed to act in yeast cells as a bipartite signal, namely, TAG ... TATGTA. Deletion of this sequence in the CaMV polyadenylation signal abolished 3'-end formation in yeast cells, and a single point mutation in this motif reduced the activity of the CaMV signal to below 15%. These results indicate that the bipartite sequence element acts as a signal for 3'-end formation in yeast cells but only together with other cis-acting elements.

Different classes of polyadenylation sites in the yeast Saccharomyces cerevisiae

This report provides an analysis of the function of polyadenylation sites from six different genes of the yeast Saccharomyces cerevisiae. These sites were tested for their ability to turn off read-through transcription into the URA3 gene in vivo when inserted into an ACT-URA3 fusion gene. The 3' ends of all polyadenylation sites inserted into the test system in their natural configuration are identical to the 3' ends of the chromosomal genes. We identified two classes of polyadenylation sites: (i) efficient sites (originating from the genes GCN4 and PHO5) that were functional in a strict orientation-dependent manner and (ii) bidirectional sites (derived from ARO4, TRP1, and TRP4) that had a distinctly reduced efficiency. The ADH1 polyadenylation site was efficient and bidirectional and was shown to be a combination of two polyadenylation sites of two convergently transcribed genes. Sequence comparison revealed that all efficient unidirectional polyadenylation sites contain the sequence TTTTTAT, whereas all bidirectional sites have the tripartite sequence TAG...TA (T)GT...TTT. Both sequence elements have previously been proposed to be involved in 3' end formation. Site-directed point mutagenesis of the TTTTTAT sequence had no effect, whereas mutations within the tripartite sequence caused a reduced efficiency for 3' end formation. The tripartite sequence alone, however, is not sufficient for 3' end formation, but it might be part of a signal sequence in the bidirectional class of yeast polyadenylation sites. Our findings support the assumption that there are at least two different mechanisms with different sequence elements directing 3' end formation in yeast.

Localization and functional analysis of the regulated promoter from the Streptomyces glaucescens mel operon

The transcription initiation site of the mel operon from Streptomyces glaucescens, determined by S1 mapping and primer elongation experiments, lies 32 to 34 bp upstream of the translation initiation codon of the first open reading frame. A total of 172 to 219 bp upstream of the transcription start point are necessary for a fully active and regulated mel promoter. Deletion analysis, gel retardation assays and DNAse I footprint experiments facilitated division of the promoter into three functional domains, which include the RNA polymerase recognition site up to nucleotides -33 to -42, the binding region of a protein of assumed regulatory function between nucleotides -65 and -93, and an upstream activator site, located between positions -158 and -219.