The molecular basis of sister-chromatid cohesion.

The replicated copies of each chromosome, the sister chromatids, are attached prior to their segregation in mitosis and meiosis. This association or cohesion is critical for each sister chromatid to bind to microtubules from opposite spindle poles and thus segregate away from each other at anaphase of mitosis or meiosis II. The cohesin protein complex is essential for cohesion in both mitosis and meiosis, and cleavage of one of the subunits is sufficient for loss of cohesion at anaphase. The localization of the cohesin complex and other cohesion proteins permits evaluation of the positions of sister-chromatid associations within the chromosome structure, as well as the relationship between cohesion and condensation. Recently, two key riddles in the mechanism of meiotic chromosome segregation have yielded to molecular answers. First, analysis of the cohesin complex in meiosis provides molecular support for the long-standing hypothesis that sister-chromatid cohesion links homologs in meiosis I by stabilizing chiasmata. Second, the isolation of the monopolin protein that controls kinetochore behavior in meiosis I defines a functional basis by which sister kinetochores are directed toward the same pole in meiosis I.

A genetic screen for suppressors and enhancers of the Drosophila PAN GU cell cycle kinase identifies cyclin B as a target.

The early cell cycles of Drosophila embryogenesis involve rapid oscillations between S phase and mitosis. These unique S-M cycles are driven by maternal stockpiles of components necessary for DNA replication and mitosis. Three genes, pan gu (png), plutonium (plu), and giant nuclei (gnu) are required to control the cell cycle specifically at the onset of Drosophila development by inhibiting DNA replication and promoting mitosis. PNG is a protein kinase that is in a complex with PLU. We employed a sensitized png mutant phenotype to screen for genes that when reduced in dosage would dominantly suppress or enhance png. We screened deficiencies covering over 50% of the autosomes and identified both enhancers and suppressors. Mutations in eIF-5A and PP1 87B dominantly suppress png. Cyclin B was shown to be a key PNG target. Mutations in cyclin B dominantly enhance png, whereas png is suppressed by cyclin B overexpression. Suppression occurs via restoration of Cyclin B protein levels that are decreased in png mutants. The plu and gnu phenotypes are also suppressed by cyclin B overexpression. These studies demonstrate that a crucial function of PNG in controlling the cell cycle is to permit the accumulation of adequate levels of Cyclin B protein.

DNA replication control through interaction of E2F-RB and the origin recognition complex.

The E2F transcription factor and retinoblastoma protein control cell-cycle progression and DNA replication during S phase. Mutations in the Drosophila dE2F1 and dDP genes affect the origin recognition complex (DmORC) and initiation of replication at the chorion gene replication origin. Here we show that mutants of Rbf (an retinoblastoma protein homologue) fail to limit DNA replication. We also show that the dDP, dE2F1 and Rbf proteins are located in a complex with DmORC, and that dE2F1 and DmORC are bound to the chorion origin of replication in vivo. Our results indicate that dE2F1 and Rbf function together at replication origins to limit DNA replication through interactions with DmORC.

PAN GU: a protein kinase that inhibits S phase and promotes mitosis in early Drosophila development.

Following completion of meiosis, DNA replication must be repressed until fertilization. In Drosophila, this replication block requires the products of the pan gu (png), plutonium (plu) and giant nuclei (gnu) genes. These genes also ensure that S phase oscillates with mitosis in the early division cycles of the embryo. We have identified the png gene and shown that it encodes a Ser/Thr protein kinase expressed only in ovaries and early embryos, and that the predicted extent of kinase activity in png mutants inversely correlates with the severity of the mutant phenotypes. The PLU and PNG proteins form a complex that has PNG-dependent kinase activity, and this activity is necessary for normal levels of mitotic cyclins. Our results reveal a novel protein kinase complex that controls S phase at the onset of development apparently by stabilizing mitotic cyclins.

Sister-chromatid cohesion via MEI-S332 and kinetochore assembly are separable functions of the Drosophila centromere.

Attachment, or cohesion, between sister chromatids is essential for their proper segregation in mitosis and meiosis [1,2]. Sister chromatids are tightly apposed at their centromeric regions, but it is not known whether this is due to cohesion at the functional centromere or at flanking centric heterochromatin. The Drosophila MEI-S332 protein maintains sister-chromatid cohesion at the centromeric region [3]. By analyzing MEI-S332's localization requirements at the centromere on a set of minichromosome derivatives [4], we tested the role of heterochromatin and the relationship between cohesion and kinetochore formation in a complex centromere of a higher eukaryote. The frequency of MEI-S332 localization is decreased on minichromosomes with compromised inheritance, despite the consistent presence of two kinetochore proteins. Furthermore, MEI-S332 localization is not coincident with kinetochore outer-plate proteins, suggesting that it is located near the DNA. We conclude that MEI-S332 localization is driven by the functional centromeric chromatin, and binding of MEI-S332 is regulated independently of kinetochore formation. These results suggest that in higher eukaryotes cohesion is controlled by the functional centromere, and that, in contrast to yeast [5], the requirements for cohesion are separable from those for kinetochore assembly.

Separation anxiety at the centromere.

During mitosis, replicated sister-chromatids must maintain cohesion as they attach to the mitotic spindle. At anaphase, cohesion is lost simultaneously along the entire chromosome, releasing sisters from one another and allowing them to segregate to opposite poles. During meiosis, sisters separate in a two-step process. At anaphase of meiosis I, cohesion is lost along the chromosome arms but is maintained at centromeric regions. Not until meiosis II are sister chromatids able to break the connection at the centromere and separate away from one another. Recent studies suggest that the centromere exhibits dynamics that are very different compared with those of the chromatid arms during both mitosis and meiosis. This review discusses the nature of the specialized chromatid cohesion seen at the centromere.

The putative Drosophila methyltransferase gene dDnmt2 is contained in a transposon-like element and is expressed specifically in ovaries.

Several organisms, including Drosophila melanogaster, are apparently devoid of DNA methylation. This might reflect a highly restricted activity of DNA methyltransferases, a loss of methyltransferase activity during evolution or the dispensability of DNA methylation due to an efficient substitute mechanism. Vestiges of a Drosophila DNA methylation system have been identified recently. We show here that the putative DNA methyltransferase gene, dDnmt2, is the component of a transposon-like element. This element also contains a second, novel open reading frame with homologies to a yeast protein involved in RNA processing. Both open reading frames are coordinately expressed and transcripts are present specifically in ovarian nurse cells as well as during early stages of embryonic development.

Drosophila double parked: a conserved, essential replication protein that colocalizes with the origin recognition complex and links DNA replication with mitosis and the down-regulation of S phase transcripts.

We identified a Drosophila gene, double parked (dup), that is essential for DNA replication and belongs to a new family of replication proteins conserved from Schizosaccharomyces pombe to humans. Strong mutations in dup cause embryonic lethality, preceded by a failure to undergo S phase during the postblastoderm divisions. dup is required also for DNA replication in the adult ovary, establishing that dup is needed for DNA replication at multiple stages of development. Strikingly, DUP protein colocalizes with the origin recognition complex to specific sites in the ovarian follicle cells. This suggests that DUP plays a direct role in DNA replication. The dup transcript is cell cycle regulated and is under the control of E2F and Cyclin E. Interestingly, dup mutant embryos fail both to downregulate S phase genes and to engage a checkpoint preventing mitosis until completion of S phase. This could be either because these events depend on progression of S phase beyond the point blocked in the dup mutants or because DUP is needed directly for these feedback mechanisms.

Mammalian (cytosine-5) methyltransferases cause genomic DNA methylation and lethality in Drosophila.

CpG methylation is essential for mouse development as well as gene regulation and genome stability. Many features of mammalian DNA methylation are consistent with the action of a de novo methyltransferase that establishes methylation patterns during early development and the post-replicative maintenance of these patterns by a maintenance methyltransferase. The mouse methyltransferase Dnmt1 (encoded by Dnmt) shows a preference for hemimethylated substrates in vitro, making the enzyme a candidate for a maintenance methyltransferase. Dnmt1 also has de novo methylation activity in vitro, but the significance of this finding is unclear, because mouse embryonic stem (ES) cells contain a de novo methylating activity unrelated to Dnmt1 (ref. 10). Recently, the Dnmt3 family of methyltransferases has been identified and shown in vitro to catalyse de novo methylation. To analyse the function of these enzymes, we expressed Dnmt and Dnmt3a in transgenic Drosophila melanogaster. The absence of endogenous methylation in Drosophila facilitates detection of experimentally induced methylation changes. In this system, Dnmt3a functioned as a de novo methyltransferase, whereas Dnmt1 had no detectable de novo methylation activity. When co-expressed, Dnmt1 and Dnmt3a cooperated to establish and maintain methylation patterns. Genomic DNA methylation impaired the viability of transgenic flies, suggesting that cytosine methylation has functional consequences for Drosophila development.

Drosophila ORC specifically binds to ACE3, an origin of DNA replication control element.

In the yeast Saccharomyces cerevisiae, sequence-specific DNA binding by the origin recognition complex (ORC) is responsible for selecting origins of DNA replication. In metazoans, origin selection is poorly understood and it is unknown whether specific DNA binding by metazoan ORC controls replication. To address this problem, we used in vivo and in vitro approaches to demonstrate that Drosophila ORC (DmORC) binds to replication elements that direct repeated initiation of replication to amplify the Drosophila chorion gene loci in the follicle cells of egg chambers. Using immunolocalization, we observe that ACE3, a 440-bp chorion element that contains information sufficient to drive amplification, directs DmORC localization in follicle cells. Similarly, in vivo cross-linking and chromatin immunoprecipitation assays demonstrate association of DmORC with both ACE3 and two other amplification control elements, AER-d and ACE1. To demonstrate that the in vivo localization of DmORC is related to its DNA-binding properties, we find that purified DmORC binds to ACE3 and AER-d in vitro, and like its S. cerevisiae counterpart, this binding is dependent on ATP. Our findings suggest that sequence-specific DNA binding by ORC regulates initiation of metazoan DNA replication. Furthermore, adaptation of this experimental approach will allow for the identification of additional metazoan ORC DNA-binding sites and potentially origins of replication.

ORC localization in Drosophila follicle cells and the effects of mutations in dE2F and dDP.

We isolated mutations in Drosophila E2F and DP that affect chorion gene amplification and ORC2 localization in the follicle cells. In the follicle cells of the ovary, the ORC2 protein is localized throughout the follicle cell nuclei when they are undergoing polyploid genomic replication, and its levels appear constant in both S and G phases. In contrast, when genomic replication ceases and specific regions amplify, ORC2 is present solely at the amplifying loci. Mutations in the DNA-binding domains of dE2F or dDP reduce amplification, and in these mutants specific localization of ORC2 to amplification loci is lost. Interestingly, a dE2F mutant predicted to lack the carboxy-terminal transcriptional activation and RB-binding domain does not abolish ORC2 localization and shows premature chorion amplification. The effect of the mutations in the heterodimer subunits suggests that E2F controls not only the onset of S phase but also origin activity within S phase.

The mitotic centromeric protein MEI-S332 and its role in sister-chromatid cohesion.

Faithful segregation of sister chromatids during cell division requires properly regulated cohesion between the sister centromeres. The sister chromatids are attached along their lengths, but particularly tightly in the centromeric regions. Therefore specific cohesion proteins may be needed at the centromere. Here we show that Drosophila MEI-S332 protein localizes to mitotic metaphase centromeres. Both overexpression and mutation of MEI-S332 increase the number of apoptotic cells. In mei-S332 mutants the ratio of metaphase to anaphase figures is lower than wild type, but it is higher if MEI-S332 is overexpressed. In chromosomal squashes centromeric attachments appear weaker in mei-S332 mutants than wild type and tighter when MEI-S332 is overexpressed. These results are consistent with MEI-S332 contributing to centromeric sister-chromatid cohesion in a dose-dependent manner. MEI-S332 is the first member identified of a predicted class of centromeric proteins that maintain centromeric cohesion.

Maintenance of sister-chromatid cohesion at the centromere by the Drosophila MEI-S332 protein.

Sister-chromatid cohesion is essential for the faithful segregation of chromosomes during cell division. Recently biochemical analysis with Xenopus extracts suggests that cohesion is established during S phase by a cohesion complex but that other proteins must maintain it in mitosis. The Drosophila melanogaster MEI-S332 protein is present on centromeres in mitosis and meiosis and is essential for cohesion at the centromeres in meiosis II. Here, we analyze the timing of MEI-S332 assembly onto centromeres and the functional domains of the MEI-S332 protein. We find that MEI-S332 is first detectable on chromosomes during prometaphase, and this localization is independent of microtubules. MEI-S332 contains two separable functional domains, as mutations within these domains show intragenic complementation. The carboxy-terminal basic region is required for chromosomal localization. The amino-terminal coiled-coil domain may facilitate protein-protein interactions between MEI-S332 and male meiotic proteins. MEI-S332 interacts with itself in the yeast two-hybrid assay and in immunoprecipitates from Drosophila oocyte and embryo extracts. Thus it appears that MEI-S332 assembles into a multimeric protein complex that localizes to centromeric regions during prometaphase and is required for the maintenance of sister-chromatid cohesion until anaphase, rather than its establishment in S phase.

Genetic interactions between mei-S332 and ord in the control of sister-chromatid cohesion.

The Drosophila mei-S332 and ord gene products are essential for proper sister-chromatid cohesion during meiosis in both males and females. We have constructed flies that contain null mutations for both genes. Double-mutant flies are viable and fertile. Therefore, the lack of an essential role for either gene in mitotic cohesion cannot be explained by compensatory activity of the two proteins during mitotic divisions. Analysis of sex chromosome segregation in the double mutant indicates that ord is epistatic to mei-S332. We demonstrate that ord is not required for MEI-S332 protein to localize to meiotic centromeres. Although overexpression of either protein in a wild-type background does not interfere with normal meiotic chromosome segregation, extra ORD+ protein in mei-S332 mutant males enhances nondisjunction at meiosis II. Our results suggest that a balance between the activity of mei-S332 and ord is required for proper regulation of meiotic cohesion and demonstrate that additional proteins must be functioning to ensure mitotic sister-chromatid cohesion.

The cohesion protein MEI-S332 localizes to condensed meiotic and mitotic centromeres until sister chromatids separate.

The Drosophila MEI-S332 protein has been shown to be required for the maintenance of sister-chromatid cohesion in male and female meiosis. The protein localizes to the centromeres during male meiosis when the sister chromatids are attached, and it is no longer detectable after they separate. Drosophila melanogaster male meiosis is atypical in several respects, making it important to define MEI-S332 behavior during female meiosis, which better typifies meiosis in eukaryotes. We find that MEI-S332 localizes to the centromeres of prometaphase I chromosomes in oocytes, remaining there until it is delocalized at anaphase II. By using oocytes we were able to obtain sufficient material to investigate the fate of MEI-S332 after the metaphase II-anaphase II transition. The levels of MEI-S332 protein are unchanged after the completion of meiosis, even when translation is blocked, suggesting that the protein dissociates from the centromeres but is not degraded at the onset of anaphase II. Unexpectedly, MEI-S332 is present during embryogenesis, localizes onto the centromeres of mitotic chromosomes, and is delocalized from anaphase chromosomes. Thus, MEI-S332 associates with the centromeres of both meiotic and mitotic chromosomes and dissociates from them at anaphase.

S phase and differential DNA replication during Drosophila oogenesis.

A modified cell cycle, the endo cycle, produces the polyploid or polytene cells that are present in some tissues of most organisms. In the endo cycle, the S phase alternates with a gap phase, but mitosis does not occur. Genes needed to inhibit mitosis during the endo cycle and to promote the onset of S phase have been identified in Drosophila. Genomic intervals are differentially replicated during the endo cycle S phase such that some regions are under-replicated, while others can be amplified. Cyclin E and E2F are needed for this differential DNA replication during Drosophila oogenesis.