Cell cycle control: many ways to skin a cat

Thirty exponential cell divisions after fertilization would produce the number of cells in a baby mouse, but would not make a mouse. Sophisticated controls govern the cell cycle during development. These controls appear to play a central role in sculpting biological form. Rapid advances in our understanding of the machinery that drives the cell cycle provide a foundation for investigation of the molecular nature of cell cycle control in development. In this article, I emphasize that the design of the cell cycle machinery provides numerous inputs for regulation. I hope that the emphasis I have chosen will avert a tendency towards a narrow perception of cell cycle control.

Triggering the all-or-nothing switch into mitosis

The past decade of cell cycle investigations has identified many roads not taken. The kinase that drives mitosis can be modulated by cyclins, by activating phosphorylation, by inhibitory phosphorylation and by binding of inhibitors, but one of these regulatory options controls the transition from G2 phase to mitosis in most circumstances. A switch-like mechanism integrates signals of cellular status and commits the cell to mitosis by abruptly removing inhibitory phosphate from preformed cyclin:Cdk1 complexes. The pathways that flip this switch alter the balance of modifying reactions to favor dephosphorylation, thereby generating a flood of mitotic kinase.

Cell cycle roles for two 14-3-3 proteins during Drosophila development

Drosophila 14-3-3ε and 14-3-3ζ proteins have been shown to function in RAS/MAP kinase pathways that influence the differentiation of the adult eye and the embryo. Because 14-3-3 proteins have a conserved involvement in cell cycle checkpoints in other systems, we asked (1) whether Drosophila 14-3-3 proteins also function in cell cycle regulation, and (2) whether cell proliferation during Drosophila development has different requirements for the two 14-3-3 proteins. We find that antibody staining for 14-3-3 family members is cytoplasmic in interphase and perichromosomal in mitosis. Using mutants of cyclins, Cdk1 and Cdc25string to manipulate Cdk1 activity, we found that the localization of 14-3-3 proteins is coupled to Cdk1 activity and cell cycle stage. Relocalization of 14-3-3 proteins with cell cycle progression suggested cell-cycle-specific roles. This notion is confirmed by the phenotypes of 14-3-3ε and 14-3-3ζ mutants: 14-3-3ε is required to time mitosis in undisturbed post-blastoderm cell cycles and to delay mitosis following irradiation; 14-3-3ζ is required for normal chromosome separation during syncytial mitoses. We suggest a model in which 14-3-3 proteins act in the undisturbed cell cycle to set a threshold for entry into mitosis by suppressing Cdk1 activity, to block mitosis following radiation damage and to facilitate proper exit from mitosis.

The schedule of destruction of three mitotic cyclins can dictate the timing of events during exit from mitosis

BACKGROUND

Degradation of the mitotic cyclins is a hallmark of the exit from mitosis. Induction of stable versions of each of the three mitotic cyclins of Drosophila, cyclins A, B, and B3, arrests mitosis with different phenotypes. We tested a recent proposal that the destruction of the different cyclins guides progress through mitosis.

RESULTS

Real-time imaging revealed that arrest phenotypes differ because each stable cyclin affects specific mitotic events differently. Stable cyclin A prolonged or blocked chromosome disjunction, leading to metaphase arrest. Stable cyclin B allowed the transition to anaphase, but anaphase A chromosome movements were slowed, anaphase B spindle elongation did not occur, and the monooriented disjoined chromosomes began to oscillate between the spindle poles. Stable cyclin B3 prevented normal spindle maturation and blocked major mitotic exit events such as chromosome decondensation but nonetheless allowed chromosome disjunction, anaphase B, and formation of a cytokinetic furrow, which split the spindle.

CONCLUSIONS

We conclude that degradation of distinct mitotic cyclins is required to transit specific steps of mitosis: cyclin A degradation facilitates chromosome disjunction, cyclin B destruction is required for anaphase B and cytokinesis and for directional stability of univalent chromosome movements, and cyclin B3 degradation is required for proper spindle reorganization and restoration of the interphase nucleus. We suggest that the schedule of degradation of cyclin A, cyclin B, and then cyclin B3 contributes to the temporal coordination of mitotic events.

Drosophila wee1 has an essential role in the nuclear divisions of early embryogenesis

In Drosophila, the maternally expressed mei-41 and grp genes are required for successful execution of the nuclear division cycles of early embryogenesis. In fission yeast, genes encoding similar kinases (rad3 and chk1, respectively) are components of a cell cycle checkpoint that delays mitosis by inhibitory phosphorylation of Cdk1. We have identified mutations in a gene encoding a Cdk1 inhibitory kinase, Drosophila wee1 (Dwee1). Like mei-41 and grp, Dwee1 is zygotically dispensable but is required maternally for completing the embryonic nuclear cycles. The arrest phenotype of Dwee1 mutants, as well as genetic interactions between Dwee1, grp, and mei-41 mutations, suggest that Dwee1 is functioning in the same regulatory pathway as these genes. These findings imply that inhibitory phosphorylation of Cdk1 by Dwee1 is required for proper regulation of the early syncytial cycles of embryogenesis.

Comparative genomics of the eukaryotes

A comparative analysis of the genomes of Drosophila melanogaster, Caenorhabditis elegans, and Saccharomyces cerevisiae-and the proteins they are predicted to encode-was undertaken in the context of cellular, developmental, and evolutionary processes. The nonredundant protein sets of flies and worms are similar in size and are only twice that of yeast, but different gene families are expanded in each genome, and the multidomain proteins and signaling pathways of the fly and worm are far more complex than those of yeast. The fly has orthologs to 177 of the 289 human disease genes examined and provides the foundation for rapid analysis of some of the basic processes involved in human disease.

Cytoskeleton: centrosom-in absentia

Recent results challenge long-held assumptions that centrosomes are essential organizers of mitotic spindles, but suggest that they couple spindle behavior with developmental and cellular events, perhaps by nucleating astral microtubules which mediate interactions with other cytoskeletal components.

Nitric oxide contributes to behavioral, cellular, and developmental responses to low oxygen in Drosophila

A nitric oxide (NO)/cyclic GMP (cGMP) signaling pathway is thought to play an important role in mammalian vasodilation during hypoxia. We show that Drosophila utilizes components of this pathway to respond to hypoxia. Hypoxic exposure rapidly induced exploratory behavior in larvae and arrested the cell cycle. These behavioral and cellular responses were diminished by an inhibitor of NO synthase and by a polymorphism affecting a form of cGMP-dependent protein kinase. Conversely, these responses were induced by ectopic expression of NO synthase. Perturbing components of the NO/cGMP pathway altered both tracheal development and survival during prolonged hypoxia. These results indicate that NO and protein kinase G contribute to Drosophila's ability to respond to oxygen deprivation.

Size control: cell proliferation does not equal growth

Division subdivides mass without increasing it. So one should not expect that an increase in cell division would make an organism bigger. Both classic and recent experiments confirm this simple rationale: altering proliferation produces normally sized body structures with either especially small or exceptionally large cells.

Exit from mitosis in Drosophila syncytial embryos requires proteolysis and cyclin degradation, and is associated with localized dephosphorylation

The cyclin proteolysis that accompanies the exit from mitosis in diverse systems appears to be essential for restoration of interphase. The early syncytial divisions of Drosophila embryos, however, occur without detectable oscillations in the total cyclin level or Cdk1 activity. Nonetheless, we found that injection of an established inhibitor of cyclin proteolysis, a cyclin B amino-terminal peptide, prevents exit from mitosis in syncytial embryos. Similarly, injection of a version of Drosophila cyclin B that is refractory to proteolysis results in mitotic arrest. We infer that proteolysis of cyclins is required for exit from syncytial mitoses. This inference can be reconciled with the failure to observe oscillations in total cyclin levels if only a small pool of cyclins is destroyed in each cycle. We find that antibody detection of histone H3 phosphorylation (PH3) acts as a reporter for Cdk1 activity. A gradient of PH3 along anaphase chromosomes suggests local Cdk1 inactivation near the spindle poles in syncytial embryos. This pattern of Cdk1 inactivation would be consistent with local cyclin destruction at centrosomes or kinetochores. The local loss of PH3 during anaphase is specific to the syncytial divisions and is not observed after cellularization. We suggest that exit from mitosis in syncytial cycles is modified to allow nuclear autonomy within a common cytoplasm.

The cell cycle program in germ cells of the Drosophila embryo

The germ cells of metazoans follow a program of proliferation that is distinct from proliferation programs of somatic cells. Despite their developmental importance, the cell proliferation program in the metazoan primordial germ cells is not well characterized and the regulatory controls are not understood. In Drosophila melanogaster, germ cell precursors (called pole cells) proliferate early in embryogenesis and then enter a prolonged quiescence. We found that polar nuclear divisions are asynchronous and lag behind somatic nuclear divisions during syncytial cycles 9 and 10. Thus, the polar division program deviates from the somatic division program when pole nuclei and somatic nuclei still share a common cytoplasm, earlier than previously thought. The lag in polar nuclear divisions is independent of grapes, which is required for lengthening somatic cell cycles 10-13. Mapping of the last S phase in pole cells and measurement of their DNA content indicate that pole cells become quiescent in G2 phase of the cell cycle. We were able to drive quiescent pole cells into mitosis by induction of either an activator of Cdc2 (Cdc25string phosphatase) or a mutant form of Cdc2 that cannot be inhibited by phosphorylation. In contrast, induction of wild-type Cdc2 with a mitotic cyclin did not induce mitosis in pole cells. We propose that inhibition of Cdc2 by phosphorylation contributes to G2 arrest in pole cells during embryogenesis. Furthermore, pole cells enter G1 following induced mitoses, indicating that entry into both mitosis and S phase is blocked in quiescent pole cells. These studies represent the first molecular characterization of proliferation in embryonic germ cells of Drosophila.

Fluctuations in cyclin E levels are required for multiple rounds of endocycle S phase in Drosophila

The precise cell-cycle alternation of S phase and mitosis is controlled by alternating competence of nuclei to respond to S-phase-inducing factors [1]. Nuclei acquire competence to replicate at the low point in cyclin-dependent kinase (Cdk) activities that follows mitotic destruction of cyclins. The elevation of Cdk activity late in G1 is thought to drive cells into S phase and to block replicated DNA from re-acquiring replication competence [2]. Whereas mitosis is normally required to eliminate the cyclins prior to another cycle of replication, experimental elimination of Cdk activity in G2 can restore competence to replicate [3-6]. Here, we examine the roles of Cdks in the endocycies of Drosophila [7]. In these cycles, rounds of discrete S phases without intervening mitoses result in polyteny. Cyclins A and B are lost in cells as they enter endocycles [8,9], and pulses of Cyclin E expression drive endocycle S phases [10-12]. To address whether oscillations of Cyclin E expression are required for endocycles, we expressed Cyclin E continuously in Drosophila salivary glands. Growth of the cells was severely inhibited, and a period of DNA replication was induced but further replication was inhibited. This replication inhibition could be overcome by the kinase inhibitor 6-dimethylaminopurine (6-DMAP), but not by expression of subunits of the transcription factor E2F. These results indicate that endocycle S phases require oscillations in Cdk activity, but, in contrast to oscillations in mitotic cells, these occur independently of mitosis.

Chromosome association of minichromosome maintenance proteins in Drosophila endoreplication cycles

Minichromosome maintenance (MCM) proteins are essential eukaryotic DNA replication factors. The binding of MCMs to chromatin oscillates in conjunction with progress through the mitotic cell cycle. This oscillation is thought to play an important role in coupling DNA replication to mitosis and limiting chromosome duplication to once per cell cycle. The coupling of DNA replication to mitosis is absent in Drosophila endoreplication cycles (endocycles), during which discrete rounds of chromosome duplication occur without intervening mitoses. We examined the behavior of MCM proteins in endoreplicating larval salivary glands, to determine whether oscillation of MCM-chromosome localization occurs in conjunction with passage through an endocycle S phase. We found that MCMs in polytene nuclei exist in two states: associated with or dissociated from chromosomes. We demonstrate that cyclin E can drive chromosome association of DmMCM2 and that DNA synthesis erases this association. We conclude that mitosis is not required for oscillations in chromosome binding of MCMs and propose that cycles of MCM-chromosome association normally occur in endocycles. These results are discussed in a model in which the cycle of MCM-chromosome associations is uncoupled from mitosis because of the distinctive program of cyclin expression in endocycles.

Mutations of the Drosophila dDP, dE2F, and cyclin E genes reveal distinct roles for the E2F-DP transcription factor and cyclin E during the G1-S transition

Activation of heterodimeric E2F-DP transcription factors can drive the G1-S transition. Mutation of the Drosophila melanogaster dE2F gene eliminates transcriptional activation of several replication factors at the G1-S transition and compromises DNA replication. Here we describe a mutation in the Drosophila dDP gene. As expected for a defect in the dE2F partner, this mutation blocks G1-S transcription of DmRNR2 and cyclin E as previously described for mutations of dE2F. Mutation of dDP also causes an incomplete block of DNA replication. When S phase is compromised by reducing the activity of dE2F-dDP by either a dE2F or dDP mutation, the first phenotype detected is a reduction in the intensity of BrdU incorporation and a prolongation of the labeling. Notably, in many cells, there was no detected delay in entry into this compromised S phase. In contrast, when cyclin E function was reduced by a hypomorphic allele combination, BrdU incorporation was robust but the timing of S-phase entry was delayed. We suggest that dE2F-dDP contributes to the expression of two classes of gene products: replication factors, whose abundance has a graded effect on replication, and cyclin E, which triggers an all-or-nothing transition from G1 to S phase.

Chromosome association of minichromosome maintenance proteins in Drosophila mitotic cycles

Minichromosome maintenance (MCM) proteins are essential DNA replication factors conserved among eukaryotes. MCMs cycle between chromatin bound and dissociated states during each cell cycle. Their absence on chromatin is thought to contribute to the inability of a G2 nucleus to replicate DNA. Passage through mitosis restores the ability of MCMs to bind chromatin and the ability to replicate DNA. In Drosophila early embryonic cell cycles, which lack a G1 phase, MCMs reassociate with condensed chromosomes toward the end of mitosis. To explore the coupling between mitosis and MCM-chromatin interaction, we tested whether this reassociation requires mitotic degradation of cyclins. Arrest of mitosis by induced expression of nondegradable forms of cyclins A and/or B showed that reassociation of MCMs to chromatin requires cyclin A destruction but not cyclin B destruction. In contrast to the earlier mitoses, mitosis 16 (M16) is followed by G1, and MCMs do not reassociate with chromatin at the end of M16. dacapo mutant embryos lack an inhibitor of cyclin E, do not enter G1 quiescence after M16, and show mitotic reassociation of MCM proteins. We propose that cyclin E, inhibited by Dacapo in M16, promotes chromosome binding of MCMs. We suggest that cyclins have both positive and negative roles in controlling MCM-chromatin association.

Limb morphogenesis: connections between patterning and growth

Limb development is a complex process involving precise control of both patterning and growth. Great strides have been made in understanding limb morphogenesis and identifying essential patterning genes in Drosophila. Differential expression of these genes divides the future limb into territories, which will give rise to different regions of the adult appendage. Recent analyses have defined the role of territorial boundaries as organizers of both patterning and growth, highlighting the connection between these two processes. The organizing activity of territorial boundaries seems to be mediated through the activity of two locally produced morphogens: Wingless and Decapentaplegic. We propose a model in which these two molecules, distributed in a graded fashion, act in synergy to promote growth of the entire appendage. We also suggest that existence of growth inhibitors that counteract the action of Wingless and Decapentaplegic; by opposing the gradient of these growth factors, the inhibitors guide the near-uniform proliferation that shapes the imaginal discs from which the adult appendages are formed in Drosophila.

Cdks and the Drosophila cell cycle

Cyclin-dependent kinases play essential roles in driving the cell cycle. Much progress has been made in Drosophila over the past year in identifying the specific requirements for individual cyclins in particular cell cycle events. These studies encompass many aspects of the cell cycle, from the addition of a G1 phase to the cell cycle during embryogenesis to the role of cyclin degradation in progression through anaphase.

E2F-induced S phase requires cyclin E

Both the heterodimeric transcription factor, E2F, and the G1 cyclin, cyclin E, are required for the G1-S transition at the start of the metazoan cell cycle. It has been established that cyclin E can act as an upstream activator of E2F. In addition to this action, we show here that cyclin E has an essential role in DNA replication distinct from activating E2F. We have created transgenic Drosophila capable of inducible, ectopic production of E2F activity. Simultaneous overexpression of both Drosophila E2F subunits, dE2F and dDP, in embryos stimulated the expression of multiple E2F-target genes including cyclin E, and also caused the initiation of S phase. Mutation of cyclin E prevented the initiation of S phase after overexpression of dE2F/dDP without affecting induction of target gene expression. Thus, E2F-directed transcription cannot bypass loss of cyclin E in Drosophila embryos.

Drosophila Cdk8, a kinase partner of cyclin C that interacts with the large subunit of RNA polymerase II

A number of cyclins have been described, most of which act together with their catalytic partners, the cyclin-dependent kinases (Cdks), to regulate events in the eukaryotic cell cycle. Cyclin C was originally identified by a genetic screen for human and Drosophila cDNAs that complement a triple knock-out of the CLN genes in Saccharomyces cerevisiae. Unlike other cyclins identified in this complementation screen, there has been no evidence that cyclin C has a cell-cycle role in the cognate organism. Here we report that cyclin C is a nuclear protein present in a multiprotein complex. It interacts both in vitro and in vivo with Cdk8, a novel protein-kinase of the Cdk family, structurally related to the yeast Srb10 kinase. We also show that Cdk8 can interact in vivo with the large subunit of RNA polymerase II and that a kinase activity that phosphorylates the RNA polymerase II large subunit is present in Cdk8 immunoprecipitates. Based on these observations and sequence similarity to the kinase/cyclin pair Srb10/Srb11 in S. cerevisiae, we suggest that cyclin C and Cdk8 control RNA polymerase II function.

Drosophila MCM protein complexes

MCM genes encode a family of evolutionarily conserved proteins required for DNA replication. In Saccharomyces cerevisiae, where they were first identified, MCM genes interact genetically with each other. Allele specificity in these interactions suggests that MCM proteins physically associate with one another and that this association is essential for function. We describe here an analysis of physical interactions among three Drosophila MCM proteins. Using specific antibodies we detect Drosophila MCMs almost exclusively in 600-kDa protein complexes. Co-immunoprecipitation data demonstrate the existence of at least two distinct types of 600-kDa complexes, one that contains DmCDC46 and one that appears to contain both DmMCM2 and Dpa (a CDC54 homologue). These complexes are stable throughout embryonic division cycles, are resistant to treatments with salt and detergent, and are present during development in tissues undergoing mitotic DNA replication as well as endoreplication. When extracts are prepared under low salt conditions all three MCM proteins co-immunoprecipitate. Consequently, we suggest that the 600-kDa complexes interact in a higher order complex.

Drosophila Wee1 kinase rescues fission yeast from mitotic catastrophe and phosphorylates Drosophila Cdc2 in vitro

Cdc2 kinase activity is required for triggering entry into mitosis in all known eukaryotes. Elaborate mechanisms have evolved for regulating Cdc2 activity so that mitosis occurs in a timely manner, when preparations for its execution are complete. In Schizosaccharomyces pombe, Wee1 and a related Mik1 kinase are Cdc2-inhibitory kinases that are required for preventing premature activation of the mitotic program. To identify Cdc2-inhibitory kinases in Drosophila, we screened for cDNA clones that rescue S. pombe wee1- mik1- mutants from lethal mitotic catastrophe. One of the genes identified in this screen, Drosophila wee1 (Dwee1), encodes a new Wee1 homologue. Dwee1 kinase is closely related to human and Xenopus Wee1 homologues, and can inhibit Cdc2 activity by phosphorylating a critical tyrosine residue. Dwee1 mRNA is maternally provided to embryos, and is zygotically expressed during the postblastoderm divisions of embryogenesis. Expression remains high in the proliferating cells of the central nervous system well after cells in the rest of the embryo have ceased dividing. The loss of zygotically expressed Dwee1 does not lead to mitotic catastrophe during postblastoderm cycles 14 to 16. This result may indicate that maternally provided Dwee1 is sufficient for regulating Cdc2 during embryogenesis, or it may reflect the presence of a redundant Cdc2 inhibitory kinase, as in fission yeast.

A nuclear GFP that marks nuclei in living Drosophila embryos; maternal supply overcomes a delay in the appearance of zygotic fluorescence

The central role of gene expression in regulating development has largely been studied by in situ hybridization and antibody staining techniques in fixed material. However, rapid temporal and spatial changes in gene expression are often difficult to correlate with complex morphogenetic movements. A green fluorescent protein (GFP) from the jellyfish, Aequorea victoria, can be used as a real-time reporter for gene expression and could aid analysis of dynamic events during embryogenesis. Here, we describe a transgenic Drosophila line ubiquitously expressing a nuclear GFP fusion protein that highlights morphogenesis, cell movement, and mitosis in living embryos. The fusion protein is highly fluorescent when maternally supplied, but there is a long delay between its zygotic expression and the appearance of fluorescence. GFP is thus an excellent marker for the expression of stable gene products, but a poor reporter for dynamic zygotic gene expression in early Drosophila embryos.

Cell proliferation and DNA replication defects in a Drosophila MCM2 mutant

The yeast MCM2, MCM3, and MCM5/CDC46 genes are required for DNA replication and have been proposed to act as factors that license the DNA for one and only one round of replication per cell cycle. We have identified a Drosophila gene, DmMCM2, that is highly homologous to MCM2. A P-element insertion into this gene, which prevents its transcription, inhibits proliferation of cells in the imaginal discs and central nervous system (CNS) and causes an apparent prolongation of S phase in the embryonic and larval CNS. DmMCM2 is expressed in the embryo in a pattern corresponding to that of S-phase cells. These results suggest that DmMCM2 plays a role in the regulation of DNA replication analogous to that of its yeast counterpart.

The transcription factor E2F is required for S phase during Drosophila embryogenesis

Overexpression of the E2F-1 cDNA in mammalian cells disrupts normal control of the cell cycle and drives cells into S phase. Whereas eliminating E2F activity would test its inferred involvement in the G1-S transition, elimination is complicated by the existence of gene families encoding mammalian E2F. Here we identify mutations in a single essential Drosophila gene, dE2F, that encodes a homolog of the mammalian E2F gene family. Embryos homozygous for null mutations of dE2F complete early cell cycles, presumably using maternal contributions of gene products, but DNA synthesis falls to virtually undetectable levels in cycle 17. Mutant embryos also lack the pulses of coordinate transcription of genes encoding replication functions that usually accompany each transition from quiescence to S phase. We conclude that in most cells dE2F is essential for a G1-S transcriptional program and for G1-S progression.

Developmental control of the G1 to S transition in Drosophila: cyclin Eis a limiting downstream target of E2F

The E2F transcription factor is required for S phase in Drosophila. While it also triggers expression of replication genes at the G1-S transition, the relevance of this transcription is not clear because many of the induced gene products are sufficiently stable that new expression is not required for S phase. However, one unstable product could couple S phase to E2F activation. Here we show that cyclin E expression at G1-S requires E2F, that activation of E2F without cyclin E is not sufficient for S phase, and that early in G1 ectopic expression of cyclin E alone can bypass E2F and induce S phase. We conclude that cyclin E is the downstream gene that couples E2F activity to G1 control. Not all embryonic cycles are similarly coupled to E2F activation, however. The rapidly proliferating CNS cells, which exhibit no obvious G1, express cyclin E constitutively and independently to E2F. Instead, cyclin E expression activates E2F in the CNS. Thus, this tissue-specific E2F-independent transcription of cyclin E reverses the hierarchical relationship between cyclin E and E2F. Both hierarchies activate expression of the full complement of replication functions controlled by E2F; however, whereas inactivation of E2F can produce a G1 when cyclin E is downstream of E2F, we propose that an E2F-independent source of E eliminates G1.

Transcriptional regulation of string (cdc25): a link between developmental programming and the cell cycle

During postblastoderm embryogenesis in Drosophila, cell cycles progress in an invariant spatiotemporal pattern. Most of these cycles are differentially timed by bursts of transcription of string (cdc25), a gene encoding a phosphatase that triggers mitosis by activating the Cdc2 kinase. An analysis of string expression in 36 pattern-formation mutants shows that known patterning genes act locally to influence string transcription. Embryonic expression of string gene fragments shows that the complete pattern of string transcription requires extensive cis-acting regulatory sequences (> 15.3 kb), but that smaller segments of this regulatory region can drive proper temporal expression in defined spatial domains. We infer that string upstream sequences integrate many local signals to direct string's transcriptional program. Finally, we show that the spatiotemporal progression of string transcription is largely unaffected in mutant embryos specifically arrested in G2 of cycles 14, 15, or 16, or G1 of cycle 17. Thus, there is a regulatory hierarchy in which developmental inputs, not cell cycle inputs, control the timing of string transcription and hence cell cycle progression.

The making of a maggot: patterning the Drosophila embryonic epidermis

Cell fates are instructed by signals emitted from specialized cell populations called organizers. The study of epidermal patterning in Drosophila is contributing novel insights concerning the establishment and action of such organizers. Juxtaposed rows of cells express either the wingless or hedgehog signaling molecules and thereby act as organizers of segment pattern. These signals mediate a mutually re-enforcing interaction between the two rows of cells to sustain organizer function. In a distinct and subsequent phase, wingless and hedgehog act to specify the fates of cells.

A cell-autonomous, ubiquitous marker for the analysis of Drosophila genetic mosaics

Mosaic analysis, the study of animals containing cells of two different genotypes, has been used to address a wealth of questions in developmental biology. Up to now, the cell markers used to distinguish cells of the two genotypes have only been applicable to specific experimental situations (e.g., only in adult wings). We have designed a general purpose cell marker for mosaic analysis. It consists of the bacterial LacZ gene expressed under the control of the constitutive promoter of the Drosophila armadillo gene. Transformants carrying this fusion gene express beta-galactosidase in all tissue and at all stages analyzed. Zygotic expression is detectable as early as gastrulation. In mosaics obtained by nuclear transplantation, cells carrying the transgene are easily distinguished from beta-galactosidase-negative host cells. The marker should also be useful for mosaics generated with the "Flp technique."

Developmental control of a G1-S transcriptional program in Drosophila

We have defined a coordinate program of transcription of S-phase genes (DNA polymerase alpha, PCNA and the two ribonucleotide reductase subunits) that can be induced by the G1 cyclin, cyclin E. In Drosophila embryos, this program drives an intricate spatial and temporal pattern of gene expression that perfectly parallels the embryonic program of S-phase control. This dynamic pattern of expression is not disrupted by a mutation, string, that blocks the cell cycle. Thus, the transcriptional program is not a secondary consequence of cell cycle progression. We suggest that developmental signals control this transcriptional program and that its activation either directly or indirectly drives transition from G1 to S phase in the stereotyped embryonic pattern.

Distinct molecular mechanism regulate cell cycle timing at successive stages of Drosophila embryogenesis

The conserved regulators of cell cycle progression--Cyclins, Cdc2 kinase, and String phosphatase (Cdc25)--accommodate multiple modes of regulation during Drosophila embryogenesis. During cell cycles 2-7, Cdc2/Cyclin complexes are continuously present and show little fluctuation in abundance, phosphomodification, or activity. This suggests that cycling of the mitotic apparatus does not require cytoplasmic oscillations of known regulatory activities. During cycles 8-13 a progressive increase in the degradation of Cyclins at mitosis leads to increasing oscillations of Cdc2 kinase activity. Mutants deficient in cyclin mRNAs suffer cell cycle delays during this period, suggesting that Cyclin accumulation times these cycles. During interphase 14, programmed degradation of maternal String protein leads to inhibitory phosphorylation of Cdc2 and cell cycle arrest. Subsequently, mitoses 14-16 are triggered by pulses of zygotic string transcription.

The use of photoactivatable reagents for the study of cell lineage in Drosophila embryogenesis

Photoactivatable lineage tracers represent a major advance for clonal analysis in the early embryo and the study of cell movements. Any cell in the blastoderm can be marked, and the nuclear localization of the signal allows excellent resolution in identifying the daughters of individual cells. Although the technique is limited by the availability of the water-soluble caged fluorescein and its derivatives for synthesis of the complete tracer, these may become commercially available in the future. The use of caged rhodamine derivatives or antibody amplification of the signal may greatly extend the developmental period over which marked clones can be identified.

A universal target sequence is bound in vitro by diverse homeodomains

To determine the number of DNA binding proteins capable of binding a consensus Engrailed binding site, this consensus sequence was used to screen a library of Drosophila cDNA clones in a bacteriophage expression vector. We retrieved clones encoding 20 distinct DNA binding domains, 17 of which are homeodomains. Binding to a variety of oligonucleotides confirms the related sequence specificity of the retrieved binding domains. Nonetheless, the homeodomains have remarkably diverse amino acid sequences. We conclude that during the evolutionary divergence of homeodomains, the specificity of DNA binding has been much more highly conserved than the amino acid sequence.

Maintenance of the engrailed expression pattern by Polycomb group genes in Drosophila

The stable maintenance of expression patterns of homeotic genes depends on the function of a number of negative trans-regulators, termed the Polycomb (Pc) group of genes. We have examined the pattern of expression of the Drosophila segment polarity gene, engrailed (en), in embryos mutant for several different members of the Pc group. Here we report that embryos mutant for two or more Pc group genes show strong ectopic en expression, while only weak derepression of en occurs in embryos mutant for a single Pc group gene. This derepression is independent of two known activators of en expression: en itself and wingless. Additionally, in contrast to the strong ectopic expression of homeotic genes observed in extra sex combs- (esc-) mutant embryos, the en expression pattern is nearly normal in esc- embryos. This suggests that the esc gene product functions in a pathway independent of the other genes in the group. The data indicate that the same group of genes is required for stable restriction of en expression to a striped pattern and for the restriction of expression of homeotic genes along the anterior-posterior axis, and support a global role for the Pc group genes in stable repression of activity of developmental selector genes.

The state of engrailed expression is not clonally transmitted during early Drosophila development

In Drosophila embryos, boundaries of lineage restriction separate groups of cells, or compartments. Engrailed is essential for specification of the posterior compartment of each segment, and its expression is thought to mark this compartment. Using a new photo-activatable lineage tracer, we followed the progeny of single embryonic cells marked at the blastoderm stage. No clones straddled the anterior edges of engrailed stripes (the parasegment border). However, posterior cells of each stripe lose engrailed expression, producing mixed clones. We suggest that stable expression of engrailed by cells at the anterior edge of the stripe reflects, not cell-intrinsic mechanisms, but proximity to cells that produce Wingless, an extracellular signal needed for maintenance of engrailed expression. If control of posterior cell fate parallels control of engrailed expression, cell fate is initially responsive to cell environment and cell fate determination is a later event.

Progression of the cell cycle through mitosis leads to abortion of nascent transcripts

Nascent transcripts of the Drosophila Ubx gene were detected by in situ hybridization. Following onset of expression, the progress of RNA polymerase (1.4 kb/min) across the gene was visualized as the successive appearance of hybridization signals from different positions within the transcription unit. Nascent transcripts disappeared at mitosis. Hybridization signals reappeared in the next cell cycle first with a 5' probe, and later, following a delay consistent with the transcription rate, with a 3' probe. Nascent transcripts were observed continuously in expressing cells of a mutant embryo in which cells are blocked in interphase. We conclude that progression through mitosis causes abortion of nascent transcripts and suggest that periodic abortion of transcription contributes to regulation of expression of large genes.

An evolutionarily conserved cyclin homolog from Drosophila rescues yeast deficient in G1 cyclins

We have isolated two Drosophila cDNA clones that rescue Saccharomyces cerevisiae deficient in CLN functions. One of these clones is the Drosophila homolog of the cdc2 gene. The second encodes a distant and new member of the cyclin family of proteins, cyclin C. It is highly homologous (72% identity) to a human clone isolated in a similar screen. Yeast cells rescued by a plasmid constitutively expressing this Drosophila cyclin C are unusually small, consistent with an unregulated high level of G1 cyclin function. Sequence comparisons identified regions conserved among the more distantly related cyclins. Based on these conserved elements, we identified homology between cyclins and the ras oncogene.

Multiple modes of engrailed regulation in the progression towards cell fate determination

The engrailed gene product of Drosophila specifies the fate of a subset of cells in each segment. Our studies of engrailed regulation suggest that fate determination is an elaborate, multistep process. At the time in embryogenesis when the engrailed-dependent cell fate is probably determined, four modes of control act in an overlapping progression to govern engrailed expression. After activation by pair-rule genes, both an extracellular signal, wingless, and autoregulation are required for engrailed expression. Autoregulation graduates to wingless independence, but is transient, and is superseded by an engrailed-independent mode of maintenance.

Active repression of transcription by the engrailed homeodomain protein

The Drosophila engrailed gene product (En) is a homeodomain-containing protein that contributes to segmental patterning. In transfection assays it acts as a transcriptional repressor. We show that En is an active repressor, blocking activation by mammalian and yeast activators that bind to sites some distance away from those bound by En. Active repression is distinct from the effects of passive homeodomain-containing proteins, which repress when competing with activators for binding sites and activate when competing with En. Active repression activity maps outside the En homeodomain, and this activity can be transferred to a heterologous DNA binding domain.

Drosophila cdc2 homologs: a functional homolog is coexpressed with a cognate variant

Using probes obtained by PCR amplification, we have cloned Drosophila cDNAs encoding structural homologs of the p34cdc2 cell cycle kinase. Southern blot experiments and in situ hybridization to polytene chromosomes demonstrated that the isolated cDNAs, were derived from two distinct genes, Dm cdc2 (31E) and Dm cdc2c (92F). Northern blot and in situ hybridization experiments revealed that these two genes are coexpressed during embryogenesis and that expression is correlated with cell proliferation. However, despite the similarity in structure and expression, the two gene products differed in functional assays in yeasts. Expression of Dm cdc2 in Schizosaccharomyces pombe and Saccharomyces cerevisiae rescued cell cycle arrest caused by mutations in cdc2+ and CDC28, the genes encoding the p34cdc2 kinase homologs of these yeasts. In contrast, the Dm cdc2c gene product did not restore cell cycle progression. Thus, in addition to the identification of a functional homolog in Drosophila, our results indicate the presence of a closely related cognate of the p34cdc2 cell cycle kinase.

The three postblastoderm cell cycles of Drosophila embryogenesis are regulated in G2 by string

The string (stg) locus of Drosophila encodes a factor that is thought to trigger mitosis by activating the p34cdc2 protein kinase. stg is required for mitosis early in development and is transcribed in a dynamic pattern that anticipates the pattern of embryonic cell divisions. Here we show that differential cell cycle regulation during postblastoderm development (cell cycles 14-16) occurs in G2. We demonstrate that stg mRNA expressed from a heat shock promotor triggers mitosis, and an associated S phase, in G2 cells during these cycles. Hence, differential cell cycle timing at this developmental stage is controlled by stg. Finally, we use heat-induced stg expression to alter the normal pattern of embryonic mitoses. Surprisingly, the complex mitotic pattern evident during normal development is not essential for many features of pattern formation or for viability.

A synthetic homeodomain binding site acts as a cell type specific, promoter specific enhancer in Drosophila embryos

A DNA sequence initially defined as a consensus binding site for the Engrailed protein is also recognized by several other homeodomain proteins and mediates the transcriptional action of these regulators in transfected tissue culture cells. Here we show that these synthetic binding sites have a more restricted and specific ability to enhance transcription when assayed in transformed embryos. Several constructs with the homeodomain binding sites linked to the fushi tarazu or engrailed promoters are silent in transformed embryos. However, when linked to the hsp70 promoter, the sites specifically activate transcription in glial cells. The effect of single base pair mutations in the binding sites suggests that activation is mediated by homeodomain protein(s). We suggest that this specific pattern of expression results from combined action at sequences within the hsp70 promoter fragment and the homeodomain binding sites. Since the tissue culture transfection assay does not show such rigid constraints on promoter activation by homeodomain proteins, it appears that subsidiary phenomena apparent in the transgenic embryos contribute importantly to the specificity of action of functionally homologous homeodomain regulators.

The roles of Drosophila cyclins A and B in mitotic control

We have cloned, sequenced, and characterized the expression of a Drosophila cyclin B gene. The independent evolutionary conservation of A- and B-type cyclins implies that they have distinct roles. Indeed, in mutant embryos deficient in cyclin A, cells that accumulate only cyclin B do not enter mitosis. Thus, in vivo, cyclin B is not sufficient for mitosis. Furthermore, we find that the two cyclins are coexpressed in all proliferating cells throughout development. Though lacking a formal demonstration that cyclin B is essential as it is in other organisms, we propose that each of these proteins fulfills a distinct and essential role in the cell cycle.

Directing cell division during development

Several evolutionarily conserved proteins constitute a universal mitotic trigger that is precisely controlled during the orderly cell divisions of embryogenesis. As development progresses, the mechanisms controlling this trigger change. Early divisions are executed by maternally synthesized gene products, and in Xenopus they are timed by the accumulation and periodic degradation of cyclin, a trigger component. Later, the zygotic genome assumes control, and in Drosophila, zygotic transcription is required for production of another trigger protein, the product of string. After this transition to zygotic control, pulses of string transcription define the timing of highly patterned embryonic cell divisions and cyclin accumulation is not rate limiting.

Evolutionary conservation of homeodomain-binding sites and other sequences upstream and within the major transcription unit of the Drosophila segmentation gene engrailed

The engrailed (en) gene functions throughout Drosophila development and is expressed in a succession of intricate spatial patterns as development proceeds. Normal en function relies on an extremely large cis-acting regulatory region (70 kilobases). We are using evolutionary conservation to help identify en sequences important in regulating patterned expression. Sequence comparison of 2.6 kilobases upstream of the en coding region of D. melanogaster and D. virilis (estimated divergence time, 60 million years) showed that 30% of this DNA occurs in islands of near perfect sequence conservation. One of these conserved islands contains binding sites for homeodomain-containing proteins. It has been shown genetically that homeodomain-containing proteins regulate en expression. Our data suggested that this regulation may be direct. The remaining conserved islands may contain binding sites for other regulatory proteins.