Transcription of the cdc2 cell cycle control gene of the fission yeast Schizosaccharomyces pombe

Controls of cell proliferation in yeast and animals

Genetic studies using fission yeast (Schizosaccharomyces pombe) have identified a gene, cdc2, whose product (p34cdc2) is a protein kinase required for traversal of both the G1 and G2 cell cycle control points. Genetic complementation has been used to demonstrate that p34cdc2 homologues are functionally and structurally conserved in distantly related eukaryotes, and p34cdc2-related proteins are components of both maturation-promoting factor (MPF) and the M phase (growth-associated) histone H1 kinase. The p34cdc2 homologues of multicellular eukaryotes undergo potentially regulatory phosphorylation changes through the cell cycle. Phosphorylation on serine during late G1 is accompanied by a significant increase in p34cdc2 kinase activity which, by analogy with fission yeast, may betray a function related to control over entry into S phase. Phosphorylation on threonine and tyrosine in G2 precedes dephosphorylation of these residues during kinase hyperactivation and entry into mitosis. In addition, long-term control of expression of mammalian p34cdc2 homologues is likely to be exerted at the transcriptional level. These observations provide the framework of a universal model for the control of eukaryotic cell proliferation, in which the p34cdc2 protein kinase integrates multiple cues to signal the initiation of S phase and, subsequently, mitosis.

Yeast apoptosis—From genes to pathways

Yeast are eukaryotic unicellular organisms that are easy to cultivate and offer a wide spectrum of genetic and cytological tools for research. Yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe have successfully been used as models for human cell division cycle. Stress conditions, cellular ageing, failed mating, certain mutations or heterologous expression of proapoptotic genes induce yeast cell death with the characteristic markers of apoptosis. Several crucial regulators of apoptosis are conserved between metazoans and yeast. This simple model organism offers the possibility to identify conserved and new components of the apoptotic machinery and to elucidate the regulatory pathways beyond.

A 38 kb segment containing the cdc2 gene from the left arm of fission yeast chromosome II: sequence analysis and characterization of the genomic DNA and cDNAs encoded on the segment

A genomic 38 kbp segment on the c1750 cosmid clone containing the cdc2 gene, located in the left arm of chromosome II from Schizosaccharomyces pombe, was sequenced. The segment was found to have five previously known genes, pht1, cdc2, his3, act1 and mei4. Among 11 coding sequences (CDSs) predicted by the gene finding software INTRON.PLOT., four CDSs, pi007, pi010, pi014 and pi016, had considerable similarity to 40S ribosomal protein, glycosyltransferase, cdc2-related protein kinase and alpha-1, 2-mannosyltransferase, respectively. Another unusually huge open reading frame (ORF) (pi011), consisting of 2233 amino acids, existed, having significant homology to alpha-amylase, granule-bound glycogen synthase and the Sz. pombe YS 1110 clone product at the N-terminal, middle and C-terminal regions, respectively. All the predicted 11 CDSs were experimentally analysed by RACE PCR. The sequencing of the RACE products revealed that there were two small overlaps at the 3' untranslated regions (UTRs) between pi004 and pi005 (17 bp) and between pi007 and pi008 (2 bp). The distances between 5' end of the 5'UTR and the putative translation initiation codon varied from 10 to 302 nucleotides (nt) among the nine CDSs successfully analysed by 5'-RACE. The expression level of each CDS on this clone was determined. Among the 16 genes on this clone, the previously determined genes, pht1, cdc2, his3 and act1, were found to be most highly expressed. Finally, cDNAs of all the newly identified genes were detected by RACE, proving the actual expression of these genes. The nucleotide sequence has been submitted to the EMBL database under Accession No. AB004534.

A PSTTLRE-form of cdc2-like gene in the marine microalga Dunaliella tertiolecta

To understand the genetic control of algal cell division cycle that pertains to phytoplankton bloom dynamics in the sea, we cloned and analyzed a gene coding for a cyclin-dependent kinase (CDK) for the chlorophyte Dunaliella tertiolecta. The cDNA cloned, 1061 bp long, contained an open reading frame of 314 amino acids. FASTA and GAP analyses showed that this sequence was most homologous to cdc2 out of all known cdks, with an identity of 54-68% and a similarity of 65-76% to cdc2 in higher plants, animals, and yeast. Several signature domains of cdc2 were identified from this sequence, although the PSTAIRE and GDSEID motifs were replaced with PSTTLRE and GDCELQ, respectively. Southern blot hybridization demonstrated that this gene occurred as a single copy in this species, and quantitative RT-PCR showed that the transcription of this gene was constitutive. The present results suggest that the universal cdc2 is conserved in the lower eukaryote with unique structural characteristics.

Localization and properties of a silencing element near the mat3-M mating-type cassette of Schizosaccharomyces pombe

Transcription is repressed in a segment of Schizosaccharomyces pombe chromosome II that encompasses the mat2-P and mat3-M mating-type cassettes. Chromosomal deletion analysis revealed the presence of a repressor element within 500 bp of mat3-M. This element acted in synergy with the trans-acting factors Swi6, Clr1, Clr2, Clr3, and Clr4 and had several properties characteristic of silencers: it did not display promoter specificity, being able to silence not only the M mating-type genes but also the S. pombe ura4 and ade6 genes placed on the centromere-distal side of the mat3-M cassette; it could repress a gene when placed further than 2.6 kb from the promoter and it acted in both orientations, although with different efficiencies, the natural orientation repressing more stringently than the reverse. Following deletion of this element, two semistable states of expression of the mat3-M region were observed and these two states could interconvert. The deletion did not affect gene expression in the vicinity of the mat2-P cassette, 11 kb away from mat3-M. Conversely, deleting 1.5 kb on the centromere-proximal side of the mat2-P cassette, which was previously shown to partially derepress transcription around mat2-P, had no effect on gene expression near mat3-M. A double deletion removing the mat2-P and mat3-M repressor elements had the same effect as the single deletions on their respective cassettes when assayed in cells of the M mating type. These observations allow us to refine a model proposing that redundant pathways silence the mating type region of S. pombe.

Identification of a fission yeast dynamin-related protein involved in mitochondrial DNA maintenance

Members of the dynamin-related proteins family have been identified in a wide range of organisms, however their precise functions remain elusive. We have identified a new member of that GTPase family in the fission yeast Schizosaccharomyces pombe. We show that Msp1+ is an essential nuclear gene encoding a 101 kDa protein whose closest homologue is the S. cerevisiae MGM1 gene product. We also report that msp1 conditional loss of function affects the maintenance of mitochondrial DNA and leads to growth arrest associated with respiratory deficiency.

WEE1-like CDK tyrosine kinase mRNA level is regulated temporally and spatially in sea urchin embryos

A cDNA from the sea urchin Strongylocentrotus purpuratus encodes a 624 amino acid polypeptide (WEE1S.purp) with a high degree of similarity to the Mik1 and Wee1 protein tyrosine kinases. These kinases act as negative regulators of mitosis by inactivating cyclin-dependent kinases (CDK). Wee1 activity varies during the cell-cycle, and is generated only when required. The pattern of WEE1S.purp mRNA expression was examined temporally and spatially in sea urchin embryos. Only a trace amount of WEE1S.purp mRNA is present in the egg and through the fifth cell cycle post-fertilization. During the next three cycles to the mid-blastula stage, its concentration rises transiently to 2.5 x 10(4) transcripts per embryo. Its developmental profile during this early period is the inverse of that reported for cyclin mRNAs, which are at a high level in the egg and through the fifth cell cycle, then decline upon further development. WEE1S.purp mRNA in the gastrula and pluteus stages becomes restricted to cells engaged in DNA replication, including the endoderm (gut), oral ectoderm, and arm rudiments. It is absent from the aboral ectoderm, which lacks cycling cells. In the pluteus larva of the species Lytechinus pictus, WEE1 mRNA was detected in the arm rudiments during cellular proliferation and arm elongation, but not after the completion of the arms. Putative regulatory motifs in the sea urchin Wee1-like cDNA suggest a capacity for rapid turnover of both its mRNA and protein: The WEE1S.purp mRNA 3' UTR contains 13 AUUUA pentamers, which have been characterized as determinants of mRNA lability; and the N-terminal domain of the predicted WEE1S.purp polypeptide is enriched in S/TP-containing, potential kinase-target sites, as well as high-value "PEST' sequences, associated with protein lability. The developmental appearance of WEE1S.purp mRNA may coincide with the introduction of a gap phase in the cell cycle. Its spatial pattern during embryogenesis appears to reflect distinct programs of regulated cell cycling in differentiating tissues.

Pyp1 and Pyp2 PTPases dephosphorylate an osmosensing MAP kinase controlling cell size at division in fission yeast

Simultaneous inactivation of pyp1 and pyp2 PTPases in fission yeast leads to aberrant cell morphology and growth arrest. Spontaneous recessive mutations that bypass the requirement for pyp1 and pyp2 and reside in two complementation groups were isolated, sty1 and sty2. sty1- and sty2- mutant cells are substantially delayed in the timing of mitotic initiation. We have isolated the sty1 gene, which encodes a MAP kinase that is closely related to a subfamily of MAP kinases regulated by osmotic stress including Saccharomyces cervisiae HOG1 and human CSBP1. We find that sty2 is allelic to the wis1 MAP kinase kinase and that delta sty1 and delta wis1 cells are unable to grow in high osmolarity medium. Osmotic stress induces both tyrosine phosphorylation of Sty1 and a reduction in cell size at division. Pyp2 associates with and tyrosine dephosphorylates Sty1 in vitro. We find that wis1-dependent induction of pyp2 mRNA is responsible for tyrosine dephosphorylation of Sty1 in vivo on prolonged exposure to osmotic stress. We conclude that Pyp1 and Pyp2 are tyrosine-specific MAP kinase phosphatases that inactivate an osmoregulated MAP kinase, Sty1, which acts downstream of the Wis1 MAP kinase kinase to control cell size at division in fission yeast.

Dominant negative mutants of the Cdc2 kinase uncouple cell division from iterative plant development

Because plant cells do not move and are surrounded by a rigid cell wall, cell division rates and patterns are believed to be directly responsible for generating new structures throughout development. To study the relationship between cell division and morphogenesis, transgenic tobacco and Arabidopsis plants were constructed expressing dominant mutations in a key regulator of the Arabidopsis cell cycle, the Cdc2a kinase. Plants constitutively overproducing the wild-type Cdc2a or the mutant form predicted to accelerate the cell cycle did not exhibit a significantly altered development. In contrast, a mutation expected to arrest the cell cycle abolished cell division when expressed in Arabidopsis, whereas some tobacco plants constitutively producing this mutant protein were recovered. These plants had a reduced histone H1 kinase activity and contained considerably fewer cells. These cells were, however, much larger and underwent normal differentiation. Morphogenesis, histogenesis and developmental timing were unaffected. The results indicate that, in plants, the developmental controls defining shape can act independently from cell division rates.

Over-expression and amplification of the CDC2 gene in leukaemia cells

The expression and structure of the cdc2 gene, one of the master regulators of the eukaryotic cell cycle, were investigated in fresh leukaemic cells from 51 cases of various types of leukaemia. Cdc2 mRNA transcripts were detectable in approximately 40% (21/51) of cases by Northern blotting. Over-expression of cdc2 mRNA as compared to normal bone marrow cells was noted in 10/21 cases with detectable cdc2 mRNA transcripts. Amplification of the cdc2 gene was found in three cases. Cdc2 mRNA was over-expressed in these three cases, suggesting that gene amplification is a direct cause of mRNA over-expression in a subset of cases. Cell proliferative capacity was well correlated with the amount of cdc2 mRNA transcripts, i.e. 3H-thymidine incorporation was highest in cases with cdc2 mRNA over-expression and was significantly higher in cdc2-positive cases than in cdc2-negative cases. These results suggest that over-expression of CDC2, which is due to the gene amplification in some cases, might play a role in altered growth of leukaemic cells.

Studies on the promoter of the Arabidopsis thaliana cdc2a gene

The 5' flanking (promoter) region of the Arabidopsis thaliana cdc2a gene was cloned and sequenced. A number of putative regulatory motifs were identified including one Myc and three Myb protein binding sequences plus one abscisic acid and two auxin responsive elements. One of the three Myb protein binding sequences is positioned within an auxRE. Promoter-GUS fusions were introduced into plants to study the role of two promoter regions in regulating gene expression. Absence of one Myb binding sequence and the auxRE containing a Myb binding sequence resulted in a significant reduction in expression levels as did a deletion involving the Myc and the third Myb binding sequences along with the second auxRE. However, no changes in expression patterns were observed. The results were quantified using transgenic root cultures.

Isolation of the cell cycle control gene cdc2 from Paramecium tetraurelia

The polymerase chain reaction was used to amplify a cdc2 homologous sequence from Paramecium tetraurelia. The corresponding amino acid sequences exhibits about 50% similarity to the cdc2 proteins of other species. The Paramecium cdc2 encoded protein is 11 amino acids longer than S. pombe p34cdc2. There is one amino acid change in the conserved PSTAIRE region. Southern blot analysis indicates that Paramecium has multiple cdc2 genes. Northern blotting results shows that Paramecium cdc2 is much more expressed in actively dividing cells. It is almost undetectable in starved stationary cells. The mRNA level of cdc2 gene does not change during the vegetative cell cycle.

Analysis of a histone H2A variant from fission yeast: evidence for a role in chromosome stability

We have isolated and characterised the pht1 gene from the fission yeast Schizosaccharomyces pombe. The sequence of the predicted translation product has revealed a striking similarity to the family of H2A.F/Z histone variant proteins, which have been found in a variety of different organisms. Cells deleted for the pht1 gene locus grow slowly, exhibit an altered colony morphology, increased resistance to heat shock and show a significant decrease in the fidelity of segregation of an S. pombe minichromosome. We propose that the histone H2A variant encoded by the pht1 gene is important for chromosomal structure and function, possibly including a role in controlling the fidelity of chromosomal segregation during mitosis.

The Histoplasma capsulatum cdc2 gene is transcriptionally regulated during the morphologic transition

To understand the molecular mechanisms that control the reversible morphologic transition from mycelia to yeast in dimorphic fungi, we have isolated and characterized a cdc2 gene from Histoplasma capsulatum. This organism is a dimorphic pathogenic fungus that grows as a filamentous saprobic mold in soil and as a unicellular pathogenic yeast in human tissue. The cloned gene, whose protein product has a high degree of homology with other members of the cdc2 family, is split into four exons and three introns of 95, 52 and 85 nucleotides. Analyses of cDNA clones confirm the presence of the eukaryotic splice donor (GT) and acceptor (AG) sites. The spliced gene codes for a protein of 324 amino acids (aa) with a predicted molecular mass of 36.9 kDa. The H. capsulatum cdc2 product has 71% aa identity with Saccharomyces cerevisiae and 70% with Schizosaccharomyces pombe. The deduced protein contains the sequence, PSTAIRE, that is normally found in most p34cdc2 proteins. H. capsulatum cdc2 is transcriptionally regulated during the morphologic mycelium<==>yeast transitions and is more actively transcribed in the yeast than in the mycelial phase.

Study of the higher eukaryotic gene function CDK2 using fission yeast

In the fission yeast Schizosaccharomyces pombe, cdc2 function is required both in G1 to enter the cell cycle and in G2 to initiate mitosis. In higher eukaryotes, these functions appeared to be shared between several cdc2-like genes including CDK2. Temperature-sensitive mutations in S. pombe cdc2 that arrest the cell cycle in both G1 and G2 phases are not complemented by CDK2. We have used S. pombe to investigate what functions CDK2 can perform. We found that overexpression of the human homologue (HsCDK2) caused cell cycle arrest in G2/M showing that HsCDK2 interfered with mitotic events. Xenopus CDK2 (XlCDK2) overexpression did not cause cell cycle arrest and could rescue the G1 block but not the G2 block of a cdc2-M26 ts strain. A mutant XlCDK2-R33, which is inactive as a kinase, failed to rescue the G1 block, suggesting that the protein kinase activity of CDK2 is required to enter the cell cycle in these circumstances. We designed screens to select mutants that would require XlCDK2 expression for viability, hoping to isolate new gene functions interacting with, or that could be replaced by, XlCDK2 in G1, or new cdc2 mutants altered solely in their G1 role. From these screens several cell cycle mutants were selected that were XlCDK2-dependent. These were all cdc2 mutants altered only in their G2/M function. Therefore XlCDK2 can influence both the G1/S and G2/M transition points of cdc2 in S. pombe.

Analysis of the Schizosaccharomyces pombe cyclin puc1: evidence for a role in cell cycle exit

The puc1+ gene, encoding a G1-type cyclin from the fission yeast Schizosaccharomyces pombe, was originally isolated by complementation in the budding yeast Saccharomyces cerevisiae. Here, we report the molecular characterization of this gene and analyse its role in S. pombe. We fail to identify any function of this cyclin at the mitotic G1/S transition in S. pombe, but demonstrate that it does function in exit from the mitotic cycle. Expression of the puc1+ gene is increased during nitrogen starvation, and puc1 affects the timing of sexual development in response to starvation. Overexpression of the puc1 protein blocks sexual development, and rescues pat1ts cells, which would otherwise undergo a lethal meiosis. We conclude that puc1 contributes to negative regulation of the timing of sexual development in fission yeast, and functions at the transition between cycling and non-cycling cells.

cdc2a expression in Arabidopsis is linked with competence for cell division

A key regulator of the cell cycle is a highly conserved protein kinase whose catalytic subunit, p34(cdc2), is encoded by the cdc2 gene. We studied the control of the expression of the Arabidopsis cdc2a gene in cell suspensions and during plant development. In cell cultures, arrest of the cell cycle did not significantly affect cdc2a mRNA levels, but nutrient conditions were important for cdc2a expression. During plant development, the pattern of cdc2a expression was strongly correlated with the cell proliferation potential. The effects of external signals on cdc2a expression were analyzed. Wounding induced expression in leaves. Lack of light altered temporal regulation of cdc2a in the apical but not root meristem of seedlings. Differential cdc2a responses were obtained after different hormone treatments. Signals present only in intact plants were necessary to mediate these responses. Although other control levels have yet to be analyzed, these results suggest that the regulation of cdc2a expression may contribute greatly to spatial and temporal regulation of cell division in plants. Our results also show that cdc2a expression is not always coupled with cell proliferation but always precedes it. We propose that cdc2a expression may reflect a state of competence to divide, and that the release of other controls is necessary for cell division to occur.

cdc2a expression in Arabidopsis is linked with competence for cell division

A key regulator of the cell cycle is a highly conserved protein kinase whose catalytic subunit, p34(cdc2), is encoded by the cdc2 gene. We studied the control of the expression of the Arabidopsis cdc2a gene in cell suspensions and during plant development. In cell cultures, arrest of the cell cycle did not significantly affect cdc2a mRNA levels, but nutrient conditions were important for cdc2a expression. During plant development, the pattern of cdc2a expression was strongly correlated with the cell proliferation potential. The effects of external signals on cdc2a expression were analyzed. Wounding induced expression in leaves. Lack of light altered temporal regulation of cdc2a in the apical but not root meristem of seedlings. Differential cdc2a responses were obtained after different hormone treatments. Signals present only in intact plants were necessary to mediate these responses. Although other control levels have yet to be analyzed, these results suggest that the regulation of cdc2a expression may contribute greatly to spatial and temporal regulation of cell division in plants. Our results also show that cdc2a expression is not always coupled with cell proliferation but always precedes it. We propose that cdc2a expression may reflect a state of competence to divide, and that the release of other controls is necessary for cell division to occur.

Analysis of 5' flanking sequences from the Schizosaccharomyces pombe cdc2 gene

Previously, the sequence of the Schizosaccaromyces pombe cdc2 gene was reported to begin at a HindIII site, 141 nucleotides (nt) upstream from the ATG start codon [Hindley and Phear, Gene 31 (1984) 129-134]. We have extended the sequence of the 5' untranslated region of the gene to a PsI site at -822 nt. We demonstrate by primer extension analysis that transcription of the gene initiates at one major point 180 nt upstream from the ATG start codon. Since the 822-nt fragment extending from the PstI site to the start codon has been used in many studies as the promoter for cdc2 [Booher and Beach, Mol. Cell. Biol. 6 (1986) 3523-3530; Carr et al., Mol. Gen. Genet 218 (1989) 41-49; Gould and Nurse, Nature 342 (1989) 39-45], we investigated the strength of this promoter element relative to the SV40 early promoter, a promoter known to work very well in S. pombe [Jones et al., Cell 53 (1988) 659-667]. We confirm that the cdc2 gene fragment has significant promoting activity, albeit 20- to 60-fold less than the SV40 early promoter, when assayed in S. pombe.

Cell cycle dependent regulation of cdc2 mRNA in mouse fibroblasts: requirement of protein synthesis and of continued mitogenic stimulation

In the chemically transformed mouse fibroblasts (BP-A31) placed in a serum-free medium, the cdc2 mRNA content decreases in parallel with the cessation of [3H]thymidine incorporation. Extinction of the cdc2 gene expression is also observed in BP-A31 cells overexpressing the human c-myc oncogene. At quiescence, the cdc2 gene expression can be reinduced with serum or with other mitogens such as insulin or 12-O-tetradecanoyl phorbol 13-acetate (TPA). The kinetics of induction is characterized by a lag period which differs according to the mitogen used and reflects the length of the G1 phase (4-6 h with insulin or serum, 9-12 h with TPA). The cdc2 mRNA accumulation is prevented when protein synthesis is blocked with cycloheximide, also if the drug is added at a time when the synthesis of cdc2 mRNA is already under way. Similarly, removal of the mitogen leads to a cessation of the cdc2 mRNA accumulation. These results suggest that the increased expression of the cdc2 gene is mediated by (a) short-lived, growth factor-regulated protein(s).

Cell cycle effects of microinjected antisense oligodeoxynucleotides to p34cdc2 kinase

In this study the effect of antisense oligomers targeted against the mRNA transcripts of p34cdc2 kinase on G1 progression into S-phase was examined. For this purpose, antisense, sense, or nonsense oligomers were introduced directly into the cytoplasm of T98G cells grown in monolayer cultures by glass-capillary microinjection. The microinjection of antisense oligomers (but not sense or nonsense oligomers) into growth-arrested cells before serum stimulation inhibited G1 progression into S-phase. This inhibition was correlated with a reduction in the steady-state levels of nuclear p34cdc2 protein. Microinjection of antisense oligomers into cells at 2 and 6 hours after serum stimulation also resulted in a marked inhibition in the ability of cells to enter S-phase. The inhibitory effect decreased when cells were microinjected at 12 hours after serum stimulation. When cells were microinjected at 18 and 24 hours after serum stimulation, only a slight inhibition was observed. As the antisense oligomers were introduced directly into the cytoplasm of cells at each of the time points examined, the observed differences in the inhibitory effects of the antisense oligomers at later times after serum stimulation cannot be explained by differences in uptake. An alternative explanation is that after a certain threshold level of nuclear p34cdc2 protein is reached in late G1 phase; no further increase is necessary, because the cells become committed to enter S-phase. In yeast, p34cdc2 appears to play an important role in the G1/S-phase transition at a control point in late G1 phase called START (reviewed by Lewin). In mammalian cells a control point that could be equivalent to START is the "restriction point" which is defined as the time after which inhibition of protein synthesis fails to block entry into S-phase (reviewed by Pardee). The effects observed with antisense oligomers to p34cdc2 kinase are strikingly similar to what is observed when low concentrations of the drug cycloheximide are added to these cells at different times after serum stimulation; entry into S-phase is significantly inhibited when cycloheximide is added up to 12 hours postimulation. Thus, the results reported in this study are in agreement with the idea that p34cdc2 kinase plays a role in the G1/S phase transition in mammalian cells. Finally, introduction of antisense oligomers directly into the cytoplasm of cells grown in monolayer cultures by glass-capillary microinjection appears to be a viable alternative to simply adding the oligomers to the culture medium.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation and characterization of a cdc 2 cDNA from Dictyostelium discoideum

A cdc2 homologous sequence was amplified from Dictyostelium discoideum by the polymerase chain reaction and used to isolate several cDNA clones. The amino acid sequence encoded by these cDNAs exhibited approx. 60% identity to the Cdc2 proteins of other species. A cDNA containing the entire coding sequence complemented the temperature sensitive cdc28 mutant of Saccharomyces cerevisiae, although growth of the transformants was slow and limited. Southern blot analysis of restriction digests under high stringency conditions provided evidence that Dictyostelium contains a single cdc2 gene, although at lower stringency multiple fragments were detected, suggesting the existence of a cdc2 gene family. Northern blot analysis of RNA from different stages of Dictyostelium development showed that cdc2 mRNA levels increased during aggregation and then decreased to low levels by the pseudoplasmodial stage of development. By contrast, cdc2 mRNA levels remained relatively constant as cells passed from exponential growth to the stationary phase.

Cell cycle regulation of the human cdc2 gene

Transcription of the human cdc2 gene is cell cycle regulated and restricted to proliferating cells. Nuclear run-on assays show that cdc2 transcription is high in S and G2 phases of the cell cycle but low in G1. To investigate transcriptional control further, genomic clones of the human cdc2 gene containing 5' flanking sequences were isolated and shown to function as a growth regulated promoter in vivo when fused to a CAT reporter gene. In primary human fibroblasts, the human cdc2 promoter is negatively regulated by arrest of cell growth in a similar fashion to the endogenous gene. This requires specific 5' flanking upstream negative control (UNC) sequences which mediate repression. The retinoblastoma susceptibility gene product (Rb) specifically represses cdc2 transcription in cycling cells via 136 bp of 5' flanking sequence located between -245 and -109 within the UNC region. E2F binding sites in this region were shown to be essential for optimal repression. A model is proposed where Rb negatively regulates the cdc2 promoter in non-cycling and cycling G1 cells.

Five novel elements involved in the regulation of mitosis in fission yeast

Five new elements of the mitotic control in the fission yeast Schizosaccharomyces pombe were isolated from gene libraries as multicopy suppressors of the conditional lethal phenotype of win1-1 wee1ts cdc25ts triple mutant strains. These genes were designated wis1(+)-wis5+ for win suppressing, and do not correspond to win1+ or any of the previously characterised mitotic control genes. None of the wis genes is capable of suppressing the cdc phenotype of cdc25ts strains, suggesting that their effect is not simply to reverse the effect of loss of cdc25 function. wis1+ has been previously reported to encode a putative serine/threonine protein kinase that acts as a dosage-dependent inducer of mitosis. wis4+ appears to be a specific suppressor of the win1-1 mutation. wis2+ and wis3+ are capable of suppressing a wide range of cdc phenotypes arising from the combination of various mutations with wee1ts and cdc25ts, suggesting that the wis2+ and wis3+ products may interact with elements central to the mitotic control.

A cyclin A-protein kinase complex possesses sequence-specific DNA binding activity: p33cdk2 is a component of the E2F-cyclin A complex

The E2F transcription factor has been found in association with the cyclin A protein, and this complex accumulates during the S phase of the cell cycle, suggesting that E2F may play a role in cell cycle control. In independent studies, cyclin A has been shown to be associated with two other proteins, the Rb-related p107 protein and the cdc2-related p33 cdk2 protein kinase. Through an analysis of the E2F-cyclin A complex, we now find that both the p107 protein and the cdc2-related p33cdk2 kinase are components of the previously described complex. Moreover, the complex possesses H1 kinase activity. These results thus define a cyclin A-cdk2 kinase complex that possesses sequence-specific DNA binding activity. This suggests that the cdk2 kinase may phosphorylate other DNA-bound substrates, and that one role of the E2F factor may be to localize this protein kinase to the DNA.

Gene database for the fission yeast Schizosaccharomyces pombe

As an aid to the fission yeast genome project, we describe a database for Schizosaccharomyces pombe consisting of both genetic and physical information. As presented, it is therefore both an updated gene list of all the nuclear genes of the fission yeast, and provides an estimate of the physical distance between two mapped genes. Additionally, a field indicates whether the sequence of the gene is available. Currently, sequence information is available for 135 of the 501 known genes.

Human cyclin E, a new cyclin that interacts with two members of the CDC2 gene family

A new human cyclin, named cyclin E, was isolated by complementation of a triple cln deletion in S. cerevisiae. Cyclin E showed genetic interactions with the CDC28 gene, suggesting that it functioned at START by interacting with the CDC28 protein. Two human genes were identified that could interact with cyclin E to perform START in yeast containing a cdc28 mutation. One was CDC2-HS, and the second was the human homolog of Xenopus CDK2. Cyclin E produced in E. coli bound and activated the CDC2 protein in extracts from human G1 cells, and antibodies against cyclin E immunoprecipitated a histone H1 kinase from HeLa cells. The interactions between cyclin E and CDC2, or CDK2, may be important at the G1 to S transition in human cells.

Identification of a G1-type cyclin puc1+ in the fission yeast Schizosaccharomyces pombe

In rapidly growing cells of the budding yeast Saccharomyces cerevisiae, the cell cycle is regulated chiefly at Start, just before the G1-S boundary, whereas in the fission yeast Schizosaccharomyces pombe, the cycle is predominantly regulated at G2-M. Both control points are present in both yeasts, and both require the p34cdc2 protein kinase. At G2-M, p34cdc2 kinase activity in S. pombe requires a B-type cyclin in a complex with p34cdc2; this complex is the same as MPF (maturation promoting factor). The p34cdc2 activity at the G1-S transition in S. cerevisiae may be regulated by a similar cyclin complex, using one of the products of a new class of cyclin genes (CLN1, CLN2 and WHI1 (DAF1/CLN3)). At least one is required for progression through the G1-S phase, and deletion of all three leads to G1 arrest. WHI1 was isolated as a dominant allele causing budding yeast cells to divide at a reduced size and was later independently identified as DAF1, a dominant allele of which rendered the cells refractory to the G1-arrest induced by the mating pheromone alpha-factor. The dominant alleles are truncations thought to yield proteins of increased stability, and the cells are accelerated through G1. Without WHI1 function, the cells are hypersensitive to alpha-factor, enlarged and delayed in G1. Heretofore, this G1-class of cyclins has not been identified in other organisms. We have isolated a G1-type cyclin gene called puc1+ from S. pombe, using a functional assay in S. cerevisiae. Expression of puc1+ in S. pombe indicates that it has a cyclin-like role in the fission yeast distinct from the role of the B-type mitotic cyclin.

The Arabidopsis functional homolog of the p34cdc2 protein kinase

The p34cdc2 protein kinase is a key component of the eukaryotic cell cycle, which is required for G1 to S-phase transition and for entry into mitosis. Using a 380-base pair DNA fragment obtained by polymerase chain reaction amplification from an Arabidopsis thaliana flower cDNA library as a probe, we isolated and sequenced a cdc2-homologous cDNA from Arabidopsis. The encoded polypeptide has extensive homology with cdc2-like kinases. Furthermore, when expressed in a CDC28ts Saccharomyces strain, it partially restores the capacity to grow at 36 degrees C, indicating that the plant cDNA is a functional homolog of the p34cdc2 kinase. Genomic hybridization demonstrated that there is one copy of the cdc2 gene per Arabidopsis haploid genome. Using RNA gel blot analysis, we found that cdc2 mRNA is present in all plant organs.

Isolation of a human cyclin cDNA: evidence for cyclin mRNA and protein regulation in the cell cycle and for interaction with p34cdc2

This paper reports the nucleotide and predicted amino acid sequence of a human B-type cyclin. The predicted protein sequence shows strong homology to the other known cyclins in the central third of the protein. We show that the level of cyclin mRNA is regulated during the cell cycle, increasing during G2 phase to four time that present in G1. The protein accumulates steadily during G2 to at least 20 times its level in G1 and is abruptly destroyed at mitosis. In G2/M phase, cyclin is associated with p34cdc2, the human homolog of the fission yeast gene cdc2+, and this complex has histone H1 kinase activity.

Regulation of p34cdc2 protein kinase during mitosis

The cell-cycle timing of mitosis in fission yeast is determined by the cdc25+ gene product activating the p34cdc2 protein kinase leading to mitotic initiation. Protein kinase activity remains high in metaphase and then declines during anaphase. Activation of the protein kinase also requires the cyclin homolog p56cdc13, which also functions post activation at a later stage of mitosis. The continuing function of p56cdc13 during mitosis is consistent with its high level until the metaphase/anaphase transition. At anaphase the p56cdc13 level falls dramatically just before the decline in p34cdc2 protein kinase activity. The behavior of p56cdc13 is similar to that observed for cyclins in oocytes. p13suc1 interacts closely with p34cdc2; it is required during the process of mitosis and may play a role in the inactivation of the p34cdc2 protein kinase. Therefore, the cdc25+, cdc13+, and suc1+ gene products are important for regulating p34cdc2 protein kinase activity during entry into, progress through, and exit from mitosis.

Molecular cloning and sequence analysis of mutant alleles of the fission yeast cdc2 protein kinase gene: implications for cdc2+ protein structure and function

The cdc2+ gene function plays a central role in the control of the mitotic cell cycle of the fission yeast Schizosaccharomyces pombe. Recessive temperature-sensitive mutations in the cdc2 gene cause cell cycle arrest when shifted to the restrictive temperature, while a second class of mutations within the cdc2 gene causes a premature advancement into mitosis. Previously the cdc2+ gene has been cloned and has been shown to encode a 34 kDa phosphoprotein with in vitro protein kinase activity. Here we describe the cloning of 11 mutant alleles of the cdc2 gene using two simple methods, one of which is presented here for the first time. We have sequenced these alleles and find a variety of single amino acid substitutions mapping throughout the cdc2 protein. Analysis of these mutations has identified a number of regions within the cdc2 protein that are important for cdc2+ activity and regulation. These include regions which may be involved in the interaction of the cdc2+ gene product with the proteins encoded by the wee1+, cdc13+ and suc1+ genes.

Intracellular localisation and expression of mammalian CDC2 protein during myogenic differentiation

Myogenic differentiation involves withdrawal of myoblasts from the cell cycle and fusion to form multinucleate myotubes. To examine the role that cell cycle control genes may play in this process, we investigated the steady state levels of CDC2 protein and RNA during myogenesis of L6E9 rat myoblasts. Indirect immunofluorescence using a CDC2 affinity-purified antibody showed that this protein is localised exclusively in the cytoplasm with a higher concentration perinuclearly. Both protein and RNA levels were down-regulated to similar extents early in the differentiation process, as cells became quiescent. There was a further down-regulation of protein after fusion to form myotubes. Autonomous expression of CDC2 protein in L6E9 cells, after stable transfection with a metallothionein: CDC2 gene construct, failed to inhibit the differentiation process. This suggests that, although there is down-regulation in levels of CDC2 RNA and protein during myogenesis, this phenomenon per se does not play a primary role in controlling the differentiation process. If CDC2 is involved in control of differentiation, this must depend on post-translational modification of the protein.

The Xenopus cdc2 protein is a component of MPF, a cytoplasmic regulator of mitosis

In Xenopus, a cytoplasmic agent known as MPF induces entry into mitosis. In fission yeast, genetic studies have shown that the cdc2 kinase regulates mitotic initiation. The 13 kd product of the suc1 gene interacts with the cdc2 kinase in yeast cells. We show that the yeast suc1 gene product (p13) is a potent inhibitor of MPF in cell-free extracts from Xenopus eggs. p13 appears to exert its antagonistic effect by binding directly to MPF. MPF activity is quantitatively depleted by chromatography on a p13 affinity column. Concomitantly, the Xenopus counterpart of the yeast cdc2 protein is adsorbed to the column. A 42 kd protein also binds specifically to the p13 affinity matrix. These findings suggest that the Xenopus cdc2 protein and the 42 kd protein are components of MPF.

Regulated expression and phosphorylation of a possible mammalian cell-cycle control protein

A novel approach to the study of the control of the mammalian cell cycle was opened by the cloning of a human gene by complementation of a fission-yeast cdc2 cell-cycle mutant. We have investigated the behaviour of the RNA and protein products of this human gene, CDC2Hs, and its murine equivalent, CDC2Mm during serum starvation and re-feeding of cultured fibroblasts. In contrast to the pattern of wild-type cdc2+ expression in fission yeast previously described, the mammalian homologue displays variation in both RNA and protein levels during exit from and re-entry into the mitotic cycle. Like its yeast counterpart, however, the mammalian CDC2 protein (p34CDC2) becomes dephosphorylated upon shifting from exponential growth to quiescence, and rephosphorylated late in the G1 phase when cells are stimulated to re-enter the cycle. We propose that phosphorylation of p34CDC2 serves as a regulatory mechanism generally in eukaryotic cells, while transcriptional control of the CDC2 gene in higher eukaryotes may be relevant to long term processes such as senescence and differentiation.

Mitotic induction and maintenance by overexpression of a G2-specific gene that encodes a potential protein kinase

There may be a causal relationship between expression of the G2-specific gene nimA and mitotic regulation in Aspergillus. To test this relationship we have introduced extra inducible copies of nimA into Aspergillus and determined the effect of nimA overproduction on mitotic regulation. The results show that nimA overexpression causes mitotic induction in less than a cell cycle and maintains chromatin in a condensed state. These effects occur even if cells are first blocked in S phase. Sequence analysis shows that the nimA gene encodes a potential protein kinase. These data indicate that there is indeed a causal relationship between expression of nimA and the regulation of mitosis and further implicate protein phosphorylation in mitotic control.

Negative regulation of mitosis by wee1+, a gene encoding a protein kinase homolog

Fission yeast wee1- mutants initiate mitosis at half the cell size of wild type. The wee1+ activity is required to prevent lethal premature mitosis in cells that overproduce the mitotic inducer cdc25+. This lethal phenotype was used to clone wee1+ by complementation. When wee1+ expression is increased, mitosis is delayed until cells grow to a larger size. Thus wee1+ functions as a dose-dependent inhibitor of mitosis, the first such element to be specifically identified and cloned. The carboxy-terminal region of the predicted 112 kd wee1+ protein contains protein kinase consensus sequences, suggesting that negative regulation of mitosis involves protein phosphorylation. Genetic evidence indicates that wee1+ and cdc25+ compete in a control system regulating the cdc2+ protein kinase, which is required for mitotic initiation.