A functional analysis of PCNA-binding peptides derived from protein sequence, interaction screening and rational design

Proliferating cell nuclear antigen (PCNA) has no intrinsic enzymatic function, but functions as a sliding platform to mediate protein interactions with the DNA strand. Many proteins interact with PCNA through a small conserved motif with consensus QxxLxxFF. This work uses Schizosaccharomyces pombe and human cells to analyse the function of PCNA-binding peptides. Interacting peptides were identified using two-hybrid screening; one (pep102) binds directly to a physiologically relevant site on PCNA. The EGFP-pep102 overexpression phenotype is consistent with competitive blocking of PCNA-protein interactions. Various PCNA-binding peptides were all shown to inhibit PCNA function by competitive binding in both human and S. pombe cells as EGFP fusion proteins. The action of a p21(WAF1/Cip1)-derived peptide was complicated by the presence of additional functional domains and possible post-translational modification. The activity of pep102 was hampered by low expression in both model systems. The peptide derived from rational design (con1) was stable, highly active in inhibiting PCNA function both S. pombe and human cells and showed a high affinity for PCNA both in vitro and in vivo. These results validate the use of functional screening in yeast to identify peptide aptamers that are functional in mammalian cells; such aptamers provide excellent leads for small molecule antiproliferative therapies.

Use of peptides from p21 (Waf1/Cip1) to investigate PCNA function in Xenopus egg extracts

Cell-free systems derived from unfertilized Xenopus eggs have been particularly informative in the study of the regulation and biochemistry of DNA replication. We have developed a Xenopus-based system to analyze proliferating cell nuclear antigen (PCNA)-specific effects on the functional properties of egg extracts. To do this, we have coupled peptides derived from p21 (Waf1/Cip1) to beads and used these to deplete PCNA from Xenopus egg extracts. The effect on various aspects of DNA replication can be analyzed after the readdition of PCNA and other purified proteins. Using this system, we have shown that replication of single-stranded M13 DNA is entirely dependent upon PCNA. By adding exogenous T7 DNA polymerase to PCNA-depleted extracts, we have uncoupled processive DNA replication from PCNA activity and so created an experimental system to analyze the dependence of postreplicative processes on PCNA function. We have shown that successful chromatin assembly is specifically dependent on PCNA. However, systems for analyzing the far more complex mechanisms required for the replication of nuclear double-stranded DNA have proved so far to be refractory to specific PCNA depletion.

Inhibition of cell proliferation by the PCNA-binding region of p21 expressed as a GFP miniprotein

p21 (WAF1/Cip1) is the only member of the CIP/KIP family which has a well-characterized PCNA-binding domain. p21 is known to have an important function in the coordination of the cellular pathways which are activated in response to DNA damage, though the significance of the p21-PCNA interaction is not completely clear. We have analyzed the effects of expressing a miniprotein containing the PCNA-binding domain of p21 upon the cell cycle and upon the proliferation of various cell types. We have compared this with the effect of expressing a mutant form which is defective in PCNA-binding, but which retains the secondary cyclin-CDK-inhibitory site. No PCNA-dependent effects were seen in the short term upon cell cycle distribution. However, clonogenic assays show that the GFP-peptide miniprotein can significantly suppress proliferation in a PCNA-dependent manner. In some cell types, however, the suppression of proliferation was not PCNA-dependent, suggesting that cellular environment is a contributory factor to the effect of this miniprotein. The capacity of this peptide sequence to suppress cell proliferation in vivo is of interest as the basis for the design of potential antiproliferative therapeutic agents.

The puzzle of PCNA's many partners

The identification of proteins that interact with proliferating cell nuclear antigen (PCNA) has recently been a rapidly expanding field of discovery. PCNA is involved in many aspects of DNA replication and processing, forming a sliding platform that can mediate the interaction of proteins with DNA. It is striking that many proteins bind to PCNA through a small region containing a conserved motif; these include proteins involved in cell cycle regulation as well as those involved in DNA processing. Sequential and regulated binding of motif-containing proteins to PCNA may contribute to the ordering of events during DNA replication and repair. Results from bacteriophages and archaea show that the structural basis for the interaction of this motif with PCNA is extremely ancient. The analysis of how such functional motifs have been recruited to proteins in present day organisms helps us to understand how these complex systems arose from ancestral organisms.

A quantitative study of the in vitro binding of the C-terminal domain of p21 to PCNA: affinity, stoichiometry, and thermodynamics

Proliferating cell nuclear antigen (PCNA) plays an essential role in DNA replication, repair, and control of cell proliferation, and its activity can be modulated by interaction with p21(Waf1/Cip1) [Cox, L. S., (1997) Trends Cell Biol. 7, 493-497]. This protein-protein interaction provides a particularly good model target for designing therapeutic agents to treat proliferative disorders such as cancer. In this study, the formation of complexes between PCNA and peptides derived from the C-terminus of p21 has been investigated at the molecular level and quantified using a competitive PCNA binding assay and isothermal titration calorimetry (ITC). The affinity constant for the interaction between p21 (141-160) peptide and PCNA has been determined to be 1.14 x 10(7) M(-)(1), corresponding to a K(d) of 87.7 nM. Measurement of the interaction of truncation and substitution analogues based on the p21 (141-160) sequence with PCNA revealed that the N-terminal part (residues 141-152) of the above peptide is the minimum recognition motif, required for PCNA binding. Truncation of the C-terminal region p21 (153-160), though, inhibited significantly the ability of the peptides to compete with the full-length p21 (141-160) for binding to PCNA. Alanine mutation of Met 147 or Asp 149 completely abolished or significantly decreased, respectively, the level of the PCNA binding and the inhibition of SV40 DNA replication. Comparison of the data obtained by the competitive PCNA binding assay and the ITC measurements demonstrated the usefulness of this assay for screening for compounds that could modulate the PCNA-p21 interaction. Using this assay, we have screened rationally designed peptides for binding to PCNA and interruption of the PCNA-p21 (141-160) complex. As a result of this screening, we have identified a 16-residue peptide (consensus motif 1 peptide) with the following sequence: SAVLQKKITDYFHPKK. Consensus motif 1 peptide and p21 (141-160) have similar affinities for binding PCNA and abilities to inhibit in vitro replication of DNA originated from SV40. Such peptides could prove useful in assessing p21-mimetic strategies for cancer treatment.

Essential interaction between the fission yeast DNA polymerase delta subunit Cdc27 and Pcn1 (PCNA) mediated through a C-terminal p21(Cip1)-like PCNA binding motif

Direct interaction between DNA polymerase delta and its processivity factor proliferating cell nuclear antigen (PCNA) is essential for effective replication of the eukaryotic genome, yet the precise manner by which this occurs is unclear. We show that the 54 kDa subunit of DNA polymerase delta from Schizosaccharomyces pombe interacts directly with Pcn1 (PCNA) both in vivo and in vitro. Binding is effected via a short sequence at the C-terminus of Cdc27 with significant similarity to the canonical PCNA binding motif first identified in the mammalian p21(Cip1) protein. This motif is both necessary and sufficient for binding of Pcn1 by Cdc27 in vitro and is essential for Cdc27 function in vivo. We also show that the Pcn1 binding motif in Cdc27 is distinct from its binding site for Cdc1, the 55 kDa B-subunit of polymerase delta, and present evidence that Cdc27 can bind to Pcn1 and Cdc1 simultaneously. Finally, we show that Cdc27 performs at least two distinct essential functions, one of which is independent of Pcn1 binding.

Post-replicative base excision repair in replication foci

Base excision repair (BER) is initiated by a DNA glycosylase and is completed by alternative routes, one of which requires proliferating cell nuclear antigen (PCNA) and other proteins also involved in DNA replication. We report that the major nuclear uracil-DNA glycosylase (UNG2) increases in S phase, during which it co-localizes with incorporated BrdUrd in replication foci. Uracil is rapidly removed from replicatively incorporated dUMP residues in isolated nuclei. Neutralizing antibodies to UNG2 inhibit this removal, indicating that UNG2 is the major uracil-DNA glycosylase responsible. PCNA and replication protein A (RPA) co-localize with UNG2 in replication foci, and a direct molecular interaction of UNG2 with PCNA (one binding site) and RPA (two binding sites) was demonstrated using two-hybrid assays, a peptide SPOT assay and enzyme-linked immunosorbent assays. These results demonstrate rapid post-replicative removal of incorporated uracil by UNG2 and indicate the formation of a BER complex that contains UNG2, RPA and PCNA close to the replication fork.

Fen1 expression: a novel marker for cell proliferation

The identification of antigens whose expression is associated with the cell cycle is a particularly attractive method with which to define proliferative populations in histological and cytological preparations. A polyclonal antibody 3220 has been raised which recognizes the structure-specific endonuclease Fen1 and can be used for a wide range of applications including western blotting, immunoprecipitation and immunohistochemical analysis. This antibody has been used to examine Fen1 levels by immunoblotting and its subcellular localization in cultured cells and tissue samples by immunostaining. Although the role Fen1 plays in DNA replication has been well characterized, its function in DNA repair is not so clear. The possible roles of Fen1 in repair have been investigated by examining any changes in level or localization of Fen1 in response to DNA damaging agents. We find that Fen1 is a nuclear antigen, that it is expressed by cycling cells, and that it co-localizes with PCNA and polymerase alpha during S phase. Fen1 expression is topologically regulated in vivo and is associated with proliferative populations. No change has been found in either patterns or levels of Fen1 expression induced by DNA damaging agents, either in vivo or in vitro. This anti-Fen1 antiserum is well suited to the analysis of proliferation in histological material, since (1) the proportion of labelled cells equals the experimentally determined growth fraction in an experimental xenograft system and (2) unlike markers such as PCNA, Fen1 is not induced by DNA damage.

PCNA binding proteins in Drosophila melanogaster : the analysis of a conserved PCNA binding domain

The eukaryotic polymerase processivity factor, PCNA, interacts with cell cycle regulatory proteins such as p21(WAF1/Cip1) and Gadd45, as well as with proteins involved in the mechanics of DNA repair and replication. A conserved PCNA-binding motif is found in a subset of PCNA-interacting proteins, including p21, suggesting that the regulation of these interactions is important for the co-ordination of DNA replication and repair. We have identified several classes of protein which bind to Drosophila PCNA. Two of these proteins contain the consensus PCNA-binding domain: one is the Dacapo protein, a Drosophila homologue of p21(WAF1/Cip1), and the second is the transposase encoded by the Pogo DNA transposon . A conserved PCNA-binding domain is also present in a human relative of Pogo , named Tigger , suggesting that this domain has a functional role in this class of transposable element. This raises interesting possibilities for a novel method of transposition in which the transposase might be targeted to replicating DNA. Finally, we have investigated the use of this conserved PCNA-binding domain as a predictor of PCNA-binding capacity.

The fission yeast mitotic regulator win1+ encodes an MAP kinase kinase kinase that phosphorylates and activates Wis1 MAP kinase kinase in response to high osmolarity

The Schizosaccharomyces pombe win1-1 mutant has a defect in the G2-M transition of the cell cycle. Although the defect is suppressed by wis1+ and wis4+, which are components of a stress-activated MAP kinase pathway that links stress response and cell cycle control, the molecular identity of Win1 has not been known. We show here that win1+ encodes a polypeptide of 1436 residues with an apparent molecular size of 180 kDa and demonstrate that Win1 is a MAP kinase kinase kinase that phosphorylates and activates Wis1. Despite extensive similarities between Win1 and Wis4, the two MAP kinase kinase kinases have distinct functions. Wis4 is able to compensate for loss of Win1 only under unstressed conditions to maintain basal Wis1 activity, but it fails to suppress the osmosignaling defect conferred by win1 mutations. The win1-1 mutation is a spontaneous duplication of 16 nucleotides, which leads to a frameshift and production of a truncated protein lacking the kinase domain. We discuss the cell cycle phenotype of the win1-1 cdc25-22 wee1-50 mutant and its suppression by wis genes.

PCNA binding through a conserved motif

Proliferating cell nuclear antigen (PCNA) has recently been identified as a target for the binding of several proteins. The cell cycle regulatory protein, p21, and the replication endonuclease, Fen1, have already been described as competing for PCNA binding. Two recent reports have identified DNA (cytosine-5)methyltransferase (MCMT) and the DNA repair endonuclease XPG as binding to PCNA. The remarkable thing about these interactions is that they all seem to occur through a conserved motif that is likely to contact the same site on PCNA. This has fascinating implications for a regulatory network linking these diverse protein functions.

PCNA binding through a conserved motif

Proliferating cell nuclear antigen (PCNA) has recently been identified as a target for the binding of several proteins. The cell cycle regulatory protein, p21, and the replication endonuclease, Fen1, have already been described as competing for PCNA binding. Two recent reports have identified DNA (cytosine-5)methyltransferase (MCMT) and the DNA repair endonuclease XPG as binding to PCNA. The remarkable thing about these interactions is that they all seem to occur through a conserved motif that is likely to contact the same site on PCNA. This has fascinating implications for a regulatory network linking these diverse protein functions.

Homologous regions of Fen1 and p21Cip1 compete for binding to the same site on PCNA: a potential mechanism to co-ordinate DNA replication and repair

Following genomic damage, the cessation of DNA replication is co-ordinated with onset of DNA repair; this co-ordination is essential to avoid mutation and genomic instability. To investigate these phenomena, we have analysed proteins that interact with PCNA, which is required for both DNA replication and repair. One such protein is p21Cip1, which inhibits DNA replication through its interaction with PCNA, while allowing repair to continue. We have identified an interaction between PCNA and the structure specific nuclease, Fen1, which is involved in DNA replication. Deletion analysis suggests that p21Cip1 and Fen1 bind to the same region of PCNA. Within Fen1 and its homologues a small region (10 amino acids) is sufficient for PCNA binding, which contains an 8 amino acid conserved PCNA-binding motif. This motif shares critical residues with the PCNA-binding region of p21Cip1. A PCNA binding peptide from p21Cip1 competes with Fen1 peptides for binding to PCNA, disrupts the Fen1-PCNA complex in replicating cell extracts, and concomitantly inhibits DNA synthesis. Competition between homologous regions of Fen1 and p21Cip1 for binding to the same site on PCNA may provide a mechanism to co-ordinate the functions of PCNA in DNA replication and repair.

A new twist to the tale? Apoptosis

The DNA helicases XPB and XPD, components of transcription factor TFIIH, have been implicated in a p53-induced apoptotic pathway. These new findings suggest a role for the core TFIIH complex in the coordination, not only of transcription, the cell cycle and DNA repair, but also of apoptosis.

The wis1 signal transduction pathway is required for expression of cAMP-repressed genes in fission yeast

The wis1 protein kinase of Schizosaccharomyces pombe is a member of the MAP kinase kinase family. Loss of wis1 function has previously been reported to lead to a delay in the G2-mitosis transition, loss of viability in stationary phase, and hypersensitivity to osmotic shock. It acts at least in part by activating the MAP kinase homologue sty1; loss-of-function sty1 mutants share many phenotypes with wis1 deletion mutants. We show here that, in addition, loss of wis1 function leads to defective conjugation, and to suppression of the hyperconjugation phenotype of the pat1-114 mutation. Consistent with this, the induction of the mei2 gene, which is normally induced by nitrogen starvation, is defective in wis1 mutants. In wild-type cells, nitrogen starvation leads to mei2 induction through a fall in intracellular cyclic AMP (cAMP) level and activity of the cAMP-dependent protein kinase. We show here that wis1 function is required for mei2 induction following nitrogen starvation. Expression of the fbp1 gene is negatively regulated by cAMP in response to glucose limitation: induction of fbp1 also requires wis1 and sty1 function. Loss of wis1 is epistatic over increased fbp1 expression brought about by loss of adenylate cyclase (git2/cyr1) or cAMP-dependent protein kinase (pka1) function. These observations can be explained by a model in which the pka1 pathway negatively regulates the wis1 pathway, or the two pathways might act independently on downstream targets. The latter explanation is supported, at least as regards regulation of cell division, by the observation that loss of function of the regulatory subunit of the cAMP-dependent protein kinase (cgs1) brings about a modest increase in cell length at division in both wis1+ and wis1 delta genetic backgrounds.

Gadd45 is a nuclear cell cycle regulated protein which interacts with p21Cip1

GADD45 was originally identified as a cDNA clone induced by growth arrest and DNA damage. We show that Gadd45 is a nuclear protein, widely expressed in normal tissues, particularly in quiescent cellular populations. Using cell synchronisation methods we show that Gadd45 levels are highest in the G1 phase of the cell cycle, and are greatly reduced during S phase. Immunoprecipitation of Gadd45 from mammalian cells reveals that it is tightly associated with a protein which reacts with antibodies to the cyclin dependent kinase inhibitor p21Cip1. Binding of recombinant Gadd45 protein to overlapping p21Cip1 peptides in ELISA assays and use of the yeast two hybrid assay show that Gadd45 directly interacts with this cell cycle inhibitor. These data suggest that Gadd45 may act in the regulation of the cell cycle. It is postulated that the interactions of Gadd45 with both p21Cip1 and PCNA are important for the modulation of cell cycles, and for the inhibition of DNA replication.

Characterisation of the interaction between PCNA and Gadd45

We have examined the interaction between the DNA replication and repair protein PCNA, and the growth arrest and DNA damage induced protein Gadd45. An anti-Gadd45 polyclonal antibody co-immunoprecipitates PCNA but in reciprocal experiments, an anti-C terminal anti-PCNA antibody failed to co-immunoprecipitate Gadd45. We used a yeast two hybrid assay to demonstrate that human Gadd45 interacts with both human and S. pombe PCNA. We have determined that the N-terminal 94 amino acids of Gadd45 bind to PCNA, and using a series of N-terminal and C-terminal deletions of human PCNA we have mapped two potential Gadd45 binding sites. Deletion of the last 6 amino acids of PCNA ablated interaction, suggesting a role in Gadd45 binding. This explains the inability of an anti-C terminal PCNA antibody to co-immunoprecipitate Gadd45. Using a peptide ELISA approach, we showed that Gadd45 protein binds strongly to three regions of PCNA (residues 1-20, 61-80, and 196-215) and weakly to residues 121-170. The crystal structure of PCNA provides insight into our genetic and immunochemical data. Our results confirm an interaction between PCNA and Gadd45, define regions of both molecules involved in this interaction, and are consistent with a potential stoichiometry of 2 Gadd45 molecules to each PCNA monomer. These data provide support for the notion that PCNA-Gadd45 interactions co-ordinate cell cycle and DNA repair.

A small peptide inhibitor of DNA replication defines the site of interaction between the cyclin-dependent kinase inhibitor p21WAF1 and proliferating cell nuclear antigen

BACKGROUND

p21WAF1 is a potent inhibitor of the cell-cycle regulatory cyclin-dependent kinases (Cdks). It acts on Cdks in the G1 and S phases of the cell cycle, and also binds to proliferating cell nuclear antigen (PCNA), blocking DNA replication in vitro. Transcription of p21WAF1 can be induced by the human tumour suppressor protein p53, suggesting that the action of p21WAF1 may be important in cancer prevention. We have investigated the interaction between p21WAF1 and PCNA using a genetic two-hybrid screen and with arrays of synthetic peptides derived from the p21WAF1 protein sequence.

RESULTS

We have established that the carboxy-terminal region of p21WAF1 interacts with PCNA in a yeast two-hybrid screen. Interaction with p21WAF1 involves the central loop of PCNA, which connects the two domains of the PCNA monomer. The interaction was finely mapped using peptides derived from the entire sequence of the p21WAF1 protein, and the critical residues were found to be QTSMTDFY (amino acids 144-151 of p21WAF1). Remarkably, a 20-residue peptide containing this sequence inhibited replication of simian virus 40 (SV40) DNA in vitro and could capture PCNA from whole cell extracts, demonstrating that small molecules can retain the biological activity characteristic of the whole protein. Sequential alanine-scan mutations of the peptide demonstrated that its ability to block replication correlates with its affinity for binding PCNA.

CONCLUSIONS

We have shown that PCNA and the cell-cycle regulator p21WAF1 interact in vivo, and that this interaction requires the central loop of PCNA and an eight amino-acid motif from the carboxyl terminus of p21WAF1.(ABSTRACT TRUNCATED AT 250 WORDS)

A Drosophila melanogaster homolog of the TIS11 family of immediate early genes that can rescue a cdr1 cdc25 mutant strain of fission yeast

A Drosophila melanogaster (Dm) embryonic cDNA library was screened for genes capable of inhibiting wee1+/mik1+ protein kinase (Pk) function. We expected to identify homologs of the Schizosaccharomyces pombe gene nim1+. This gene encodes a Pk capable of phosphorylating and so inhibiting the wee1+ Pk that in turn inhibits p34cdc2. Dm cDNAs capable of complementing the temperature-sensitive phenotype of a nim1/cdr1 cdc25 double mutant strain were identified and found to fall into two classes. One class encodes the Dm Cdc2 protein. The second cDNA class encodes a novel protein containing a central motif consisting of two tandem repeats of a putative Zn(2+)-finger motif. This region is highly conserved in the TIS11 family of immediate early genes, which in mammalian cells are rapidly and transiently induced in response to 12-O-tetradecanoyl phorbol-13-acetate (TPA) and to mitogens such as epidermal growth factor and fibroblast growth factor.

A Drosophila gene encoding a DEAD box RNA helicase can suppress loss of wee1/mik1 function in Schizosaccharomyces pombe

We describe a screen to isolate cDNAs encoding Drosophila mitosis inhibitors capable of suppressing the mitotic catastrophe phenotype resulting in Schizosaccharomyces pombe from the combination of the wee1-50 mutation with either a deletion allele of mik1, or with overexpression of cdc25+. One plasmid was isolated which could suppress the temperature sensitive lethality of both these strains. The cDNA in this plasmid encodes a protein highly homologous to the DEAD-box family of ATP-dependent RNA helicases, rather than to protein kinases as might be expected. It is possible that the RNA helicase described here may regulate entry into mitosis by down regulating the expression of other genes whose activity may be rate-limiting for entry into mitosis.

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.

The wis1 protein kinase is a dosage-dependent regulator of mitosis in Schizosaccharomyces pombe

The wis1+ gene encodes a newly identified mitotic control element in Schizosaccharomyces pombe. It was isolated by virtue of its interaction with the mitotic control genes cdc25, wee1 and win1. The wis1+ gene potentially encodes a 66 kDa protein with homology to the serine/threonine family of protein kinases. wis1+ plays an important role in the regulation of entry into mitosis, as it shares with cdc25+ and nim1+/cdr1+ the property of inducing mitosis in a dosage-dependent manner. Increased levels of wis1+ expression cause mitotic initiation to occur at a reduced cell size. Loss of wis1+ function does not prevent vegetative growth and division, though wis1- cells show an elongated morphology, indicating that their entry into mitosis and cell division is delayed relative to wild type cells. wis1- cells undergo a rapid reduction of viability upon entry into stationary phase, suggesting a role for wis1+ in the integration of nutritional sensing with the control over entry into mitosis.

Controlling cell cycle progress in the fission yeast Schizosaccharomyces pombe

The suitability of fission yeast as a model for understanding the eukaryotic cell cycle has been validated in five years of exciting developments. We review recent advances in understanding the nature of the controls that regulate progression through the cell cycle and the coordination of DNA replication and mitosis.

New elements in the mitotic control of the fission yeast Schizosaccharomyces pombe

The p107wee1 protein kinase plays a central role in regulating the cell cycle of fission yeast. It mediates transmission of signal(s) related to the nutritional status of the cell to the p34cdc2 protein kinase, which is an active component of the MPF complex driving cells into mitosis. p107wee1 is itself subject to control by the products of other genes such as nim1+/cdr1+, win1+, and perhaps wis1+ and other wis+ genes. At present, the relationships between these genes and their possible roles in the mitotic control are unclear and must await further analysis (Fig. 5). It is likely that some of the gene products are concerned with the sensing and/or transmission of nutritional signals. p107wee1 negatively regulates the activity of p34cdc2, probably by direct tyrosine phosphorylation, and also appears to regulate the activities of the cdc1+ and cdc27+ gene products. The effects of nitrogen starvation and of wee1 mutations on conditional lethal mutations at the cdc1, cdc2, and cdc27 loci, taken together, support the largely speculative model shown in Figure 5. During the normal cycle, the balance between phosphorylated and dephosphorylated p34cdc2 changes such that at the appropriate time, p34cdc2 is activated and the cell enters mitosis. We suggest that the cdc1+ and cdc27+ products may be regulated in a similar way. Such a mechanism would ensure coordinated activation of these and perhaps other proteins required for the G2/M transition. There are, of course, many uncertainties, and these must await elucidation by biochemical and genetic analysis.