Myb controls G(2)/M progression by inducing cyclin B expression in the Drosophila eye imaginal disc

Rice Big Grain1 enhances biomass and plant growth-promoting traits in rhizospheric yeast Candida tropicalis

The Big Grain1 (BG1) gene of rice (Oryza sativa L.) is reported to increase the yield of rice crops; however, its molecular mechanism is largely concealed. To explore its functional prospects, we have taken a structure-function-based approach. In silico analyses suggest OsBG1 is a DNA- and phytohormone-binding protein. Heterologous expression of OsBG1 with galactose-inducible promoter GAL1p in the rhizospheric yeast Candida tropicalis SY005 revealed 7.9- and 1.5-fold higher expression of the gene at 12 and 24 h, respectively, compared to the expression at 36 h post-galactose induction. Functional activity of the induced OsBG1 in engineered yeast increased cell density, specific growth rate, and biomass by 28.5%, 29.8%, and 14.1%, respectively, and decreased the generation time by 21.25%. Flow cytometry-based cell cycle analysis of OsBG1-expressing yeast cells exhibited an increase in the cells of the G2/M population by 15.8% after 12 h of post-galactose induction. The gene expression study of yeast transformants disclosed that OsBG1 regulates cell division by upregulating the expression of the endogenous gene cyclin B1 (CtCYB1) by 1.3- and 1.9-folds at 10 and 12 h, respectively, compared to the control, and is positively influenced by the phytohormone indole acetic acid (IAA). Further, the study revealed that OsBG1 significantly increases biofilm formation, stress tolerance, and IAA production in C. tropicalis SY005, implying its prospective role in enhancing plant growth-promoting traits in microbes. OsBG1-expressing rhizospheric yeast cells significantly improved the germination and growth parameters of the bio-inoculated rice seeds. Altogether, this study suggests OsBG1 can be employed to genetically improve suitable bio-inoculants for their plant growth-promoting traits to augment crop productivity. KEY POINTS: • In silico analyses suggested OsBG1 is a phytohormone-binding transcription factor. • OsBG1 enhanced growth in rhizospheric Candida tropicalis by upregulating CtCYB1. • OsBG1 improved plant growth-promoting traits of the rhizospheric yeast C. tropicalis.

NetREX-CF integrates incomplete transcription factor data with gene expression to reconstruct gene regulatory networks

The inference of Gene Regulatory Networks (GRNs) is one of the key challenges in systems biology. Leading algorithms utilize, in addition to gene expression, prior knowledge such as Transcription Factor (TF) DNA binding motifs or results of TF binding experiments. However, such prior knowledge is typically incomplete, therefore, integrating it with gene expression to infer GRNs remains difficult. To address this challenge, we introduce NetREX-CF-Regulatory Network Reconstruction using EXpression and Collaborative Filtering-a GRN reconstruction approach that brings together Collaborative Filtering to address the incompleteness of the prior knowledge and a biologically justified model of gene expression (sparse Network Component Analysis based model). We validated the NetREX-CF using Yeast data and then used it to construct the GRN for Drosophila Schneider 2 (S2) cells. To corroborate the GRN, we performed a large-scale RNA-Seq analysis followed by a high-throughput RNAi treatment against all 465 expressed TFs in the cell line. Our knockdown result has not only extensively validated the GRN we built, but also provides a benchmark that our community can use for evaluating GRNs. Finally, we demonstrate that NetREX-CF can infer GRNs using single-cell RNA-Seq, and outperforms other methods, by using previously published human data.

MYB oncoproteins: emerging players and potential therapeutic targets in human cancer

MYB transcription factors are highly conserved from plants to vertebrates, indicating that their functions embrace fundamental mechanisms in the biology of cells and organisms. In humans, the MYB gene family is composed of three members: MYB, MYBL1 and MYBL2, encoding the transcription factors MYB, MYBL1, and MYBL2 (also known as c-MYB, A-MYB, and B-MYB), respectively. A truncated version of MYB, the prototype member of the MYB family, was originally identified as the product of the retroviral oncogene v-myb, which causes leukaemia in birds. This led to the hypothesis that aberrant activation of vertebrate MYB could also cause cancer. Despite more than three decades have elapsed since the isolation of v-myb, only recently investigators were able to detect MYB genes rearrangements and mutations, smoking gun evidence of the involvement of MYB family members in human cancer. In this review, we will highlight studies linking the activity of MYB family members to human malignancies and experimental therapeutic interventions tailored for MYB-expressing cancers.

Adaptation of gene loci to heterochromatin in the course of Drosophila evolution is associated with insulator proteins

Pericentromeric heterochromatin is generally composed of repetitive DNA forming a transcriptionally repressive environment. Dozens of genes were embedded into pericentromeric heterochromatin during evolution of Drosophilidae lineage while retaining activity. However, factors that contribute to insusceptibility of gene loci to transcriptional silencing remain unknown. Here, we find that the promoter region of genes that can be embedded in both euchromatin and heterochromatin exhibits a conserved structure throughout the Drosophila phylogeny and carries motifs for binding of certain chromatin remodeling factors, including insulator proteins. Using ChIP-seq data, we demonstrate that evolutionary gene relocation between euchromatin and pericentric heterochromatin occurred with preservation of sites of insulation of BEAF-32 in evolutionarily distant species, i.e. D. melanogaster and D. virilis. Moreover, promoters of virtually all protein-coding genes located in heterochromatin in D. melanogaster are enriched with insulator proteins BEAF-32, GAF and dCTCF. Applying RNA-seq of a BEAF-32 mutant, we show that the impairment of BEAF-32 function has a complex effect on gene expression in D. melanogaster, affecting even those genes that lack BEAF-32 association in their promoters. We propose that conserved intrinsic properties of genes, such as sites of insulation near the promoter regions, may contribute to adaptation of genes to the heterochromatic environment and, hence, facilitate the evolutionary relocation of genes loci between euchromatin and heterochromatin.

A long lost key opens an ancient lock: Drosophila Myb causes a synthetic multivulval phenotype in nematodes

The five-protein MuvB core complex is highly conserved in animals. This nuclear complex interacts with RB-family tumor suppressor proteins and E2F-DP transcription factors to form DREAM complexes that repress genes that regulate cell cycle progression and cell fate. The MuvB core complex also interacts with Myb family oncoproteins to form the Myb-MuvB complexes that activate many of the same genes. We show that animal-type Myb genes are present in Bilateria, Cnidaria and Placozoa, the latter including the simplest known animal species. However, bilaterian nematode worms lost their animal-type Myb genes hundreds of millions of years ago. Nevertheless, amino acids in the LIN9 and LIN52 proteins that directly interact with the MuvB-binding domains of human B-Myb and Drosophila Myb are conserved in Caenorhabditiselegans Here, we show that, despite greater than 500 million years since their last common ancestor, the Drosophila melanogaster Myb protein can bind to the nematode LIN9-LIN52 proteins in vitro and can cause a synthetic multivulval (synMuv) phenotype in vivo This phenotype is similar to that caused by loss-of-function mutations in C. elegans synMuvB-class genes including those that encode homologs of the MuvB core, RB, E2F and DP. Furthermore, amino acid substitutions in the MuvB-binding domain of Drosophila Myb that disrupt its functions in vitro and in vivo also disrupt these activities in C. elegans We speculate that nematodes and other animals may contain another protein that can bind to LIN9 and LIN52 in order to activate transcription of genes repressed by DREAM complexes.

Radial or Bilateral? The Molecular Basis of Floral Symmetry

In the plant kingdom, the flower is one of the most relevant evolutionary novelties. Floral symmetry has evolved multiple times from the ancestral condition of radial to bilateral symmetry. During evolution, several transcription factors have been recruited by the different developmental pathways in relation to the increase of plant complexity. The MYB proteins are among the most ancient plant transcription factor families and are implicated in different metabolic and developmental processes. In the model plant Antirrhinum majus, three MYB transcription factors (DIVARICATA, DRIF, and RADIALIS) have a pivotal function in the establishment of floral dorsoventral asymmetry. Here, we present an updated report of the role of the DIV, DRIF, and RAD transcription factors in both eudicots and monocots, pointing out their functional changes during plant evolution. In addition, we discuss the molecular models of the establishment of flower symmetry in different flowering plants.

A Cyclin A-Myb-MuvB-Aurora B network regulates the choice between mitotic cycles and polyploid endoreplication cycles

Endoreplication is a cell cycle variant that entails cell growth and periodic genome duplication without cell division, and results in large, polyploid cells. Cells switch from mitotic cycles to endoreplication cycles during development, and also in response to conditional stimuli during wound healing, regeneration, aging, and cancer. In this study, we use integrated approaches in Drosophila to determine how mitotic cycles are remodeled into endoreplication cycles, and how similar this remodeling is between induced and developmental endoreplicating cells (iECs and devECs). Our evidence suggests that Cyclin A / CDK directly activates the Myb-MuvB (MMB) complex to induce transcription of a battery of genes required for mitosis, and that repression of CDK activity dampens this MMB mitotic transcriptome to promote endoreplication in both iECs and devECs. iECs and devECs differed, however, in that devECs had reduced expression of E2F1-dependent genes that function in S phase, whereas repression of the MMB transcriptome in iECs was sufficient to induce endoreplication without a reduction in S phase gene expression. Among the MMB regulated genes, knockdown of AurB protein and other subunits of the chromosomal passenger complex (CPC) induced endoreplication, as did knockdown of CPC-regulated cytokinetic, but not kinetochore, proteins. Together, our results indicate that the status of a CycA-Myb-MuvB-AurB network determines the decision to commit to mitosis or switch to endoreplication in both iECs and devECs, and suggest that regulation of different steps of this network may explain the known diversity of polyploid cycle types in development and disease.

Lack of Cyclin B1 in zebrafish causes lengthening of G2 and M phases

An essential part of the Mitosis Promoting Factor, Cyclin B1 is indispensable for cells to enter mitosis. We report here that the zebrafish early arrest mutant specter is a loss-of-function mutation in the сyclin B1 gene. cyclin B1 is maternally transcribed in zebrafish, and the zygotic phenotype is apparent by early segmentation. Lack of zygotic Cyclin B1 does not stop cells from dividing, rather it causes an abnormal and elongated progression through the G2 and M phases of the cell cycle. Many mutant cells show signs of chromosomal instability or enter apoptosis. Using CRISPR-mediated gene editing, we produced a more severe gain-of-function mutation confirming that specter is the result of nonfunctional Cyclin B1. Although also a recessive phenotype, this new mutation produces an alternative splice-form of cyclin B1 mRNA, whose product lacks several key components for Cyclin B1, but not the Cdk1-binding domain. This mutant form of Cyclin B1 completely prevents cell division. We conclude that, although Cyclin B1 is critical for cells to enter mitosis, another cell cycle protein may be cooperating with Cdk1 at the G2/M checkpoint to sustain a partly functional Mitosis Promoting Factor.

Assessment of citalopram and escitalopram on neuroblastoma cell lines. Cell toxicity and gene modulation

Selective serotonin reuptake inhibitors (SSRI) are common antidepressants which cytotoxicity has been assessed in cancers notably colorectal carcinomas and glioma cell lines. We assessed and compared the cytotoxicity of 2 SSRI, citalopram and escitalopram, on neuroblastoma cell lines. The study was performed on 2 non-MYCN amplified cell lines (rat B104 and human SH-SY5Y) and 2 human MYCN amplified cell lines (IMR32 and Kelly). Citalopram and escitalopram showed concentration-dependent cytotoxicity on all cell lines. Citalopram was more cytotoxic than escitalopram. IMR32 was the most sensitive cell line. The absence of toxicity on human primary Schwann cells demonstrated the safety of both molecules for myelin. The mechanisms of cytotoxicity were explored using gene-expression profiles and quantitative real-time PCR (qPCR). Citalopram modulated 1 502 genes and escitalopram 1 164 genes with a fold change ≥ 2. 1 021 genes were modulated by both citalopram and escitalopram; 481 genes were regulated only by citalopram while 143 genes were regulated only by escitalopram. Citalopram modulated 69 pathways (KEGG) and escitalopram 42. Ten pathways were differently modulated by citalopram and escitalopram. Citalopram drastically decreased the expression of MYBL2, BIRC5 and BARD1 poor prognosis factors of neuroblastoma with fold-changes of -107 (p<2.26 10⁻⁷), -24.1 (p<5.6 10⁻⁹) and -17.7 (p<1.2 10⁻⁷). CCNE1, AURKA, IGF2, MYCN and ERBB2 were more moderately down-regulated by both molecules. Glioma markers E2F1, DAPK1 and CCND1 were down-regulated. Citalopram displayed more powerful action with broader and distinct spectrum of action than escitalopram.

MYBL2 (B-Myb): a central regulator of cell proliferation, cell survival and differentiation involved in tumorigenesis

Limitless cell proliferation, evasion from apoptosis, dedifferentiation, metastatic spread and therapy resistance: all these properties of a cancer cell contribute to its malignant phenotype and affect patient outcome. MYBL2 (alias B-Myb) is a transcription factor of the MYB transcription factor family and a physiological regulator of cell cycle progression, cell survival and cell differentiation. When deregulated in cancer cells, MYBL2 mediates the deregulation of these properties. In fact, MYBL2 is overexpressed and associated with poor patient outcome in numerous cancer entities. MYBL2 and players of its downstream transcriptional network can be used as prognostic and/or predictive biomarkers as well as potential therapeutic targets to offer less toxic and more specific anti-cancer therapies in future. In this review, we summarize current knowledge on the physiological roles of MYBL2 and highlight the impact of its deregulation on cancer initiation and progression.

Parallel Regulation of von Hippel-Lindau Disease by pVHL-Mediated Degradation of B-Myb and Hypoxia-Inducible Factor α

pVHL, the protein product of the von Hippel-Lindau (VHL) tumor suppressor gene, is a ubiquitin ligase that targets hypoxia-inducible factor α (HIF-α) for proteasomal degradation. Although HIF-α activation is necessary for VHL disease pathogenesis, constitutive activation of HIF-α alone did not induce renal clear cell carcinomas and pheochromocytomas in mice, suggesting the involvement of an HIF-α-independent pathway in VHL pathogenesis. Here, we show that the transcription factor B-Myb is a pVHL substrate that is degraded via the ubiquitin-proteasome pathway and that vascular endothelial growth factor (VEGF)- and/or platelet-derived growth factor (PDGF)-dependent tyrosine 15 phosphorylation of B-Myb prevents its degradation. Mice injected with B-Myb knockdown 786-O cells developed dramatically larger tumors than those bearing control cell tumors. Microarray screening of B-Myb-regulated genes showed that the expression of HIF-α-dependent genes was not affected by B-Myb knockdown, indicating that B-Myb prevents HIF-α-dependent tumorigenesis through an HIF-α-independent pathway. These data indicate that the regulation of B-Myb by pVHL plays a critical role in VHL disease.

High-throughput sequencing and degradome analysis identify miRNAs and their targets involved in fruit senescence of Fragaria ananassa

In non-climacteric fruits, the respiratory increase is absent and no phytohormone is appearing to be critical for their ripening process. They must remain on the parent plant to enable full ripening and be picked at or near the fully ripe stage to obtain the best eating quality. However, huge losses often occur for their quick post-harvest senescence. To understanding the complex mechanism of non-climacteric fruits post-harvest senescence, we constructed two small RNA libraries and one degradome from strawberry fruit stored at 20°C for 0 and 24 h. A total of 88 known and 1224 new candidate miRNAs, and 103 targets cleaved by 19 known miRNAs families and 55 new candidatemiRNAs were obtained. These targets were associated with development, metabolism, defense response, signaling transduction and transcriptional regulation. Among them, 14 targets, including NAC transcription factor, Auxin response factors (ARF) and Myb transcription factors, cleaved by 6 known miRNA families and 6 predicted candidates, were found to be involved in regulating fruit senescence. The present study provided valuable information for understanding the quick senescence of strawberry fruit, and offered a foundation for studying the miRNA-mediated senescence of non-climacteric fruits.

The DREAM complex: master coordinator of cell cycle-dependent gene expression

The dimerization partner, RB-like, E2F and multi-vulval class B (DREAM) complex provides a previously unsuspected unifying role in the cell cycle by directly linking p130, p107, E2F, BMYB and forkhead box protein M1. DREAM mediates gene repression during the G0 phase and coordinates periodic gene expression with peaks during the G1/S and G2/M phases. Perturbations in DREAM complex regulation shift the balance from quiescence towards proliferation and contribute to the increased mitotic gene expression levels that are frequently observed in cancers with a poor prognosis.

The DREAM complex: master coordinator of cell cycle-dependent gene expression

The dimerization partner, RB-like, E2F and multi-vulval class B (DREAM) complex provides a previously unsuspected unifying role in the cell cycle by directly linking p130, p107, E2F, BMYB and forkhead box protein M1. DREAM mediates gene repression during the G0 phase and coordinates periodic gene expression with peaks during the G1/S and G2/M phases. Perturbations in DREAM complex regulation shift the balance from quiescence towards proliferation and contribute to the increased mitotic gene expression levels that are frequently observed in cancers with a poor prognosis.

The complex containing Drosophila Myb and RB/E2F2 regulates cytokinesis in a histone H2Av-dependent manner

In Drosophila, mutation of the oncogene Myb reduced the expression of mitotic genes, such as polo and ial, and caused multiple mitotic defects, including disrupted chromosome condensation and abnormal spindles. We now show that binucleate cells, the hallmark phenotype of cytokinesis failure, accumulate in Myb-null ovarian follicle cell and wing disc epithelia. Myb functions as an activator in the generally repressive Drosophila RBF, E2F2, and Myb (dREAM)/Myb-MuvB complex. Absence of the dREAM subunit Mip130 or E2F2 suppressed the Myb-null cytokinesis defect. Therefore, we used Myb-null binucleate cells as a quantitative phenotypic readout of transcriptional repression by the dREAM complex. In the absence of Myb, the complex was sensitive to the dose of the subunits E2F2, Mip120, Caf1, and Lin-52 but not Mip130 or Mip40. Surprisingly, reduction of the dose of His2Av/H2A.z also suppressed the Myb-null binucleate cell phenotype, suggesting a novel role for this variant histone in transcriptional repression by the dREAM complex.

Duplication and maintenance of the Myb genes of vertebrate animals

Gene duplication is an important means of generating new genes. The major mechanisms by which duplicated genes are preserved in the face of purifying selection are thought to be neofunctionalization, subfunctionalization, and increased gene dosage. However, very few duplicated gene families in vertebrate species have been analyzed by functional tests in vivo. We have therefore examined the three vertebrate Myb genes (c-Myb, A-Myb, and B-Myb) by cytogenetic map analysis, by sequence analysis, and by ectopic expression in Drosophila. We provide evidence that the vertebrate Myb genes arose by two rounds of regional genomic duplication. We found that ubiquitous expression of c-Myb and A-Myb, but not of B-Myb or Drosophila Myb, was lethal in Drosophila. Expression of any of these genes during early larval eye development was well tolerated. However, expression of c-Myb and A-Myb, but not of B-Myb or Drosophila Myb, during late larval eye development caused drastic alterations in adult eye morphology. Mosaic analysis implied that this eye phenotype was cell-autonomous. Interestingly, some of the eye phenotypes caused by the retroviral v-Myb oncogene and the normal c-Myb proto-oncogene from which v-Myb arose were quite distinct. Finally, we found that post-translational modifications of c-Myb by the GSK-3 protein kinase and by the Ubc9 SUMO-conjugating enzyme that normally occur in vertebrate cells can modify the eye phenotype caused by c-Myb in Drosophila. These results support a model in which the three Myb genes of vertebrates arose by two sequential duplications. The first duplication was followed by a subfunctionalization of gene expression, then neofunctionalization of protein function to yield a c/A-Myb progenitor. The duplication of this progenitor was followed by subfunctionalization of gene expression to give rise to tissue-specific c-Myb and A-Myb genes.

The dREAM/Myb-MuvB complex and Grim are key regulators of the programmed death of neural precursor cells at the Drosophila posterior wing margin

Successful development of a multicellular organism depends on the finely tuned orchestration of cell proliferation, differentiation and apoptosis from embryogenesis through adulthood. The MYB-gene family encodes sequence-specific DNA-binding transcription factors that have been implicated in the regulation of both normal and neoplastic growth. The Drosophila Myb protein, DMyb (and vertebrate B-Myb protein), has been shown to be part of the dREAM/MMB complex, a large multi-subunit complex, which in addition to four Myb-interacting proteins including Mip130, contains repressive E2F and pRB proteins. This complex has been implicated in the regulation of DNA replication within the context of chorion gene amplification and transcriptional regulation of a wide array of genes. Detailed phenotypic analysis of mutations in the Drosophila myb gene, Dm myb, has revealed a previously undiscovered function for the dREAM/MMB complex in regulating programmed cell death (PCD). In cooperation with the pro-apoptotic protein Grim and dREAM/MMB, DMyb promotes the PCD of specified sensory organ precursor daughter cells in at least two different settings in the peripheral nervous system: the pIIIb precursor of the neuron and sheath cells in the posterior wing margin and the glial cell in the thoracic microchaete lineage. Unlike previously analyzed settings, in which the main role of DMyb has been to antagonize the activities of other dREAM/MMB complex members, it appears to be the critical effector in promoting PCD. The finding that Dm myb and grim are both involved in regulating PCD in two distinct settings suggests that these two genes may often work together to mediate PCD.

Drosophila FMRP participates in the DNA damage response by regulating G2/M cell cycle checkpoint and apoptosis

Fragile X syndrome, the most common form of inherited mental retardation, is caused by the loss of the fragile X mental retardation protein (FMRP). FMRP is a ubiquitously expressed, multi-domain RNA-binding protein, but its in vivo function remains poorly understood. Recent studies have shown that FMRP participates in cell cycle control during development. Here, we used Drosophila mutants to test if FMRP plays a role in DNA damage response under genotoxic stress. We found significantly fewer dfmr1 mutants survived to adulthood than wild-types following irradiation or exposure to chemical mutagens, demonstrating that the loss of drosophila FMRP (dFMRP) results in hypersensitivity to genotoxic stress. Genotoxic stress significantly reduced mitotic cells in wild-type brains, indicating the activation of a DNA damage-induced G2/M checkpoint, while mitosis was only moderately suppressed in dfmr1 mutants. Elevated expression of cyclin B, a protein critical for the G2 to M transition, was observed in the larval brains of dfmr1 mutants. CycB mRNA transcripts were enriched in the dFMRP-containing complex, suggesting that dFMRP regulates DNA damage-induced G2/M checkpoint by repressing CycB mRNA translation. Reducing CycB dose by half in dfmr1 mutants rescued the defective G2/M checkpoint and reversed hypersensitivity to genotoxic stress. In addition, dfmr1 mutants exhibited more DNA breaks and elevated p53-dependent apoptosis following irradiation. Moreover, a loss-of-heterozygosity assay showed decreased irradiation-induced genome stability in dfmr1 mutants. Thus, dFMRP maintains genome stability under genotoxic stress and regulates the G2/M DNA damage checkpoint by suppressing CycB expression.

Genome-wide analysis of the MYB transcription factor superfamily in soybean

BACKGROUND

The MYB superfamily constitutes one of the most abundant groups of transcription factors described in plants. Nevertheless, their functions appear to be highly diverse and remain rather unclear. To date, no genome-wide characterization of this gene family has been conducted in a legume species. Here we report the first genome-wide analysis of the whole MYB superfamily in a legume species, soybean (Glycine max), including the gene structures, phylogeny, chromosome locations, conserved motifs, and expression patterns, as well as a comparative genomic analysis with Arabidopsis.

RESULTS

A total of 244 R2R3-MYB genes were identified and further classified into 48 subfamilies based on a phylogenetic comparative analysis with their putative orthologs, showed both gene loss and duplication events. The phylogenetic analysis showed that most characterized MYB genes with similar functions are clustered in the same subfamily, together with the identification of orthologs by synteny analysis, functional conservation among subgroups of MYB genes was strongly indicated. The phylogenetic relationships of each subgroup of MYB genes were well supported by the highly conserved intron/exon structures and motifs outside the MYB domain. Synonymous nucleotide substitution (dN/dS) analysis showed that the soybean MYB DNA-binding domain is under strong negative selection. The chromosome distribution pattern strongly indicated that genome-wide segmental and tandem duplication contribute to the expansion of soybean MYB genes. In addition, we found that ~ 4% of soybean R2R3-MYB genes had undergone alternative splicing events, producing a variety of transcripts from a single gene, which illustrated the extremely high complexity of transcriptome regulation. Comparative expression profile analysis of R2R3-MYB genes in soybean and Arabidopsis revealed that MYB genes play conserved and various roles in plants, which is indicative of a divergence in function.

CONCLUSIONS

In this study we identified the largest MYB gene family in plants known to date. Our findings indicate that members of this large gene family may be involved in different plant biological processes, some of which may be potentially involved in legume-specific nodulation. Our comparative genomics analysis provides a solid foundation for future functional dissection of this family gene.

Drosophila lin-52 acts in opposition to repressive components of the Myb-MuvB/dREAM complex

The Drosophila melanogaster Myb-MuvB/dREAM complex (MMB/dREAM) participates in both the activation and repression of developmentally regulated genes and origins of DNA replication. Mutants in MMB subunits exhibit diverse phenotypes, including lethality, eye defects, reduced fecundity, and sterility. Here, we used P-element excision to generate mutations in lin-52, which encodes the smallest subunit of the MMB/dREAM complex. lin-52 is required for viability, as null mutants die prior to pupariation. The generation of somatic and germ line mutant clones indicates that lin-52 is required for adult eye development and for early embryogenesis via maternal effects. Interestingly, the maternal-effect embryonic lethality, larval lethality, and adult eye defects could be suppressed by mutations in other subunits of the MMB/dREAM complex. These results suggest that a partial MMB/dREAM complex is responsible for the lethality and eye defects of lin-52 mutants. Furthermore, these findings support a model in which the Lin-52 and Myb proteins counteract the repressive activities of the other members of the MMB/dREAM complex at specific genomic loci in a developmentally controlled manner.

Animal-specific C-terminal domain links myeloblastosis oncoprotein (Myb) to an ancient repressor complex

Members of the Myb oncoprotein and E2F-Rb tumor suppressor protein families are present within the same highly conserved multiprotein transcriptional repressor complex, named either as Myb and synthetic multivuval class B (Myb-MuvB) or as Drosophila Rb E2F and Myb-interacting proteins (dREAM). We now report that the animal-specific C terminus of Drosophila Myb but not the more highly conserved N-terminal DNA-binding domain is necessary and sufficient for (i) adult viability, (ii) proper localization to chromosomes in vivo, (iii) regulation of gene expression in vivo, and (iv) interaction with the highly conserved core of the MuvB/dREAM transcriptional repressor complex. In addition, we have identified a conserved peptide motif that is required for this interaction. Our results imply that an ancient function of Myb in regulating G2/M genes in both plants and animals appears to have been transferred from the DNA-binding domain to the animal-specific C-terminal domain. Increased expression of B-MYB/MYBL2, the human ortholog of Drosophila Myb, correlates with poor prognosis in human patients with breast cancer. Therefore, our results imply that the specific interaction of the C terminus of Myb with the MuvB/dREAM core complex may provide an attractive target for the development of cancer therapeutics.

Addiction of MYCN amplified tumours to B-MYB underscores a reciprocal regulatory loop

MYCN is a member of the MYC family of oncoproteins frequently amplified or overexpressed in aggressive, paediatric tumours of the nervous system. In this study we have identified the gene B-MYB, encoding the transcription factor also known as MYBL2, as a downstream target of MYCN. Using multiple in silico databases we show that expression of B-MYB significantly correlates with that of MYCN in neuroblastoma patients. MYCN binds to and activates the B-MYB gene in vivo and in vitro. Blunting B-MYB expression by RNA interference causes reduced proliferation of MYCN amplified, but not MYCN-non amplified, neuroblastoma cell lines, indicating that tumour cells are addicted to B-MYB in a MYCN dependent manner. Notably, B-MYB binds in vivo to the MYCN amplicon and is required for its expression. We conclude that MYCN and B-MYB are engaged in a reciprocal regulatory loop whose pharmacological targeting could be beneficial to patients with the aggressive forms of cancer in which MYCN is amplified.

Alterations to proteome and tissue recovery responses in fish liver caused by a short-term combination treatment with cadmium and benzo[a]pyrene

The livers of soles (Solea senegalensis) injected with subacute doses of cadmium (Cd), benzo[a]pyrene (B[a]P), or their combination, were screened for alterations to cytosolic protein expression patterns, complemented by cytological and histological analyses. Cadmium and B[a]P, but not combined, induced hepatocyte apoptosis and Kupfer cell hyperplasia. Proteomics, however, suggested that apoptosis was triggered through distinct pathways. Cadmium and B[a]P caused upregulation of different anti-oxidative enzymes (peroxiredoxin and glutathione peroxidase, respectively) although co-exposure impaired induction. Similarly, apoptosis was inhibited by co-exposure, to which may have contributed a synergistic upregulation of tissue metalloproteinase inhibitor, beta-actin and a lipid transport protein. The regulation factors of nine out of eleven identified proteins of different types revealed antagonistic or synergistic effects between Cd and B[a]P at the prospected doses after 24 h of exposure. The results indicate that co-exposure to Cd and B[a]P may enhance toxicity by impairing specific responses and not through cumulative damage.

Addiction of MYCN amplified tumours to B-MYB underscores a reciprocal regulatory loop

MYCN is a member of the MYC family of oncoproteins frequently amplified or overexpressed in aggressive, paediatric tumours of the nervous system. In this study we have identified the gene B-MYB, encoding the transcription factor also known as MYBL2, as a downstream target of MYCN. Using multiple in silico databases we show that expression of B-MYB significantly correlates with that of MYCN in neuroblastoma patients. MYCN binds to and activates the B-MYB gene in vivo and in vitro. Blunting B-MYB expression by RNA interference causes reduced proliferation of MYCN amplified, but not MYCN-non amplified, neuroblastoma cell lines, indicating that tumour cells are addicted to B-MYB in a MYCN dependent manner. Notably, B-MYB binds in vivo to the MYCN amplicon and is required for its expression. We conclude that MYCN and B-MYB are engaged in a reciprocal regulatory loop whose pharmacological targeting could be beneficial to patients with the aggressive forms of cancer in which MYCN is amplified.

Expansion and diversification of the Populus R2R3-MYB family of transcription factors

The R2R3-MYB proteins comprise one of the largest families of transcription factors in plants. R2R3-MYB family members regulate plant-specific processes, such as the elaboration of specialized cell types, including xylem, guard cells, trichomes, and root hairs, and the biosynthesis of specialized branches of metabolism, including phenylpropanoid biosynthesis. As such, R2R3-MYB family members are hypothesized to contribute to the emergence of evolutionary innovations that have arisen in specific plant lineages. As a first step in determining the role played by R2R3-MYB family members in the emergence of lineage-specific innovations in the genus Populus, the entire Populus trichocarpa R2R3-MYB family was characterized. The Populus R2R3-MYB complement is much larger than that found in other angiosperms with fully sequenced genomes. Phylogenetic analyses, together with chromosome placement, showed that the expansion of the Populus R2R3-MYB family was not only attributable to whole genome duplication but also involved selective expansion of specific R2R3-MYB clades. Expansion of the Populus R2R3-MYB family prominently involved members with expression patterns that suggested a role in specific components of Populus life history, including wood formation and reproductive development. An expandable compendium of microarray-based expression data (PopGenExpress) and associated Web-based tools were developed to better enable within- and between-species comparisons of Populus R2R3-MYB gene expression. This resource, which includes intuitive graphic visualization of gene expression data across multiple tissues, organs, and treatments, is freely available to, and expandable by, scientists wishing to better understand the genome biology of Populus, an ecologically dominant and economically important forest tree genus.

A B-Myb complex containing clathrin and filamin is required for mitotic spindle function

B-Myb is one member of the vertebrate Myb family of transcription factors and is ubiquitously expressed. B-Myb activates transcription of a group of genes required for the G2/M cell cycle transition by forming the dREAM/Myb-MuvB-like complex, which was originally identified in Drosophila. Mutants of zebrafish B-myb and Drosophila myb exhibit defects in cell cycle progression and genome instability. Although the genome instability caused by a loss of B-Myb has been speculated to be due to abnormal cell cycle progression, the precise mechanism remains unknown. Here, we have purified a B-Myb complex containing clathrin and filamin (Myb-Clafi complex). This complex is required for normal localization of clathrin at the mitotic spindle, which was previously reported to stabilize kinetochore fibres. The Myb-Clafi complex is not tightly associated with the mitotic spindles, suggesting that this complex ferries clathrin to the mitotic spindles. Thus, identification of the Myb-Clafi complex reveals a previously unrecognized function of B-Myb that may contribute to its role in chromosome stability, possibly, tumour suppression.

Epigenetic regulation of gene expression by Drosophila Myb and E2F2-RBF via the Myb-MuvB/dREAM complex

The Drosophila Myb oncoprotein, the E2F2 transcriptional repressor, and the RBF and Mip130/LIN-9 tumor suppressor proteins reside in a conserved Myb-MuvB (MMB)/dREAM complex. We now show that Myb is required in vivo for the expression of Polo kinase and components of the spindle assembly checkpoint (SAC). Surprisingly, the highly conserved DNA-binding domain was not essential for assembly of Myb into MMB/dREAM, for transcriptional regulation in vivo, or for rescue of Myb-null mutants to adult viability. E2F2, RBF, and Mip130/LIN-9 acted in opposition to Myb by repressing the expression of Polo and SAC genes in vivo. Remarkably, the absence of both Myb and Mip130, or of both Myb and E2F2, caused variegated expression in which high or low levels of Polo were stably inherited through successive cell divisions in imaginal wing discs. Restoration of Myb resulted in a uniformly high level of Polo expression similar to that seen in wild-type tissue, whereas restoration of Mip130 or E2F2 extinguished Polo expression. Our results demonstrate epigenetic regulation of gene expression by Myb, Mip130/LIN-9, and E2F2-RBF in vivo, and also provide an explanation for the ability of Mip130-null mutants to rescue the lethality of Myb-null mutants in vivo.

Two components of the Myb complex, DMyb and Mip130, are specifically associated with euchromatin and degraded during prometaphase throughout development

The Drosophila Myb protein, DMyb, is a transcription factor important for cell proliferation and development. Unlike the mRNAs produced by mammalian myb genes, Drosophila myb transcripts do not fluctuate substantially during the cell cycle. A comprehensive analysis of the localization and degradation of the DMyb protein has now revealed that DMyb is present in nuclei during S phase of all mitotically active tissues throughout embryogenesis and larval development. However, DMyb and Mip130, another member of the Myb complex, are not uniformly distributed throughout the nucleus. Instead, both proteins, which colocalize, appear to be specifically excluded from heterochromatic regions of chromosomes. Furthermore, DMyb and Mip130 are unstable proteins that are degraded during prometaphase of mitosis. The timing of their degradation is reminiscent of Cyclin A, but at least for DMyb, the mechanism differs; although DMyb degradation is dependent on core APC/C components, it does not depend on the Fizzy or Fizzy-related adaptor proteins. DMyb levels are also high in actively endoreplicating polyploid cells, but there is no indication of cyclical degradation. We conclude that cell cycle specific degradation of DMyb and Mip130 is likely to be utilized as a key regulatory mechanism in down-regulating their levels and the activity of the Myb complex.

Genomic profiling and expression studies reveal both positive and negative activities for the Drosophila Myb MuvB/dREAM complex in proliferating cells

Myb-MuvB (MMB)/dREAM is a nine-subunit complex first described in Drosophila as a repressor of transcription, dependent on E2F2 and the RBFs. Myb, an integral member of MMB, curiously plays no role in the silencing of the test genes previously analyzed. Moreover, Myb plays an activating role in DNA replication in Drosophila egg chamber follicle cells. The essential functions for Myb are executed as part of MMB. This duality of function lead to the hypothesis that MMB, which contains both known activator and repressor proteins, might function as part of a switching mechanism that is dependent on DNA sites and developmental context. Here, we used proliferating Drosophila Kc tissue culture cells to explore both the network of genes regulated by MMB (employing RNA interference and microarray expression analysis) and the genomic locations of MMB following chromatin immunoprecipitation (ChIP) and tiling array analysis. MMB occupied 3538 chromosomal sites and was promoter-proximal to 32% of Drosophila genes. MMB contains multiple DNA-binding factors, and the data highlighted the combinatorial way by which the complex was targeted and utilized for regulation. Interestingly, only a subset of chromatin-bound complexes repressed genes normally expressed in a wide range of developmental pathways. At many of these sites, E2F2 was critical for repression, whereas at other nonoverlapping sites, Myb was critical for repression. We also found sites where MMB was a positive regulator of transcript levels that included genes required for mitotic functions (G2/M), which may explain some of the chromosome instability phenotypes attributed to loss of Myb function in myb mutants.

B-MYB is hypophosphorylated and resistant to degradation in neuroblastoma: implications for cell survival

B-MYB is an oncoprotein highly expressed and frequently amplified in human neoplasia. B-MYB is more expressed in neuroblastoma patients with adverse prognostic indicators, corroborating the hypothesis that it plays an important role in this pediatric malignancy. While attempting targeting strategies for therapeutic purposes, we found that the B-MYB protein was difficult to downregulate in neuroblastoma cells using siRNA approaches. This lead us to discover that the B-MYB protein half-life is increased in neuroblastoma compared to other normal or transformed human cell lines. The B-MYB protein is quickly destroyed and apoptosis is induced in Ewing sarcoma cells exposed to UV irradiation. In contrast, neuroblastoma cells are resistant to UV-induced apoptosis and B-MYB protein levels do not change in UV-treated cells. In further experiments, we show that the B-MYB protein extracted from neuroblastoma cells is hypophosphorylated. It was previously shown that B-MYB phosphorylation activates its transcriptional activity but also promotes its destruction. Overexpression of a non-phosphorylatable B-MYB mutant protects cells from UV-induced apoptosis, suggesting that its reduced phosphorylation, rather than causing its inactivation, facilitates B-MYB pro-survival activity. Thus, expression of stable, hypophosphorylated B-MYB in neuroblastoma may promote cell survival and induce aggressive tumour growth.

Loss of Drosophila Myb interrupts the progression of chromosome condensation

Completion of chromosome condensation is required before segregation during the mitotic cell cycle to ensure the transmission of genetic material with high fidelity in a timely fashion. In many eukaryotes this condensation is regulated by phosphorylation of histone H3 on Ser 10 (H3S10). This phosphorylation normally begins in the late-replicating pericentric heterochromatin and then spreads to the earlier replicating euchromatin. Here, we show that these phases of condensation are genetically separable in that the absence of Drosophila Myb causes cells to arrest with H3S10 phosphorylation of heterochromatin but not euchromatin. In addition, we used mosaic analysis to demonstrate that although the Myb protein can be removed in a single cell cycle, the failure of chromosome condensation occurs only after many cell divisions in the absence of Myb protein. The Myb protein is normally located in euchromatic but not heterochromatic regions of the nucleus, suggesting that Myb may be essential for a modification of euchromatin that is required for the efficient spread of chromosome condensation.

R1R2R3-Myb proteins positively regulate cytokinesis through activation of KNOLLE transcription in Arabidopsis thaliana

G2/M phase-specific gene transcription in tobacco cells is mediated by R1R2R3-Myb transcriptional activators, NtmybA1 and NtmybA2, which bind to mitosis-specific activator (MSA) elements. We show here that two structurally related genes, MYB3R1 and MYB3R4, which encode homologs of NtmybA1 and NtmybA2, play a partially redundant role in positively regulating cytokinesis in Arabidopsis thaliana. The myb3r1 myb3r4 double mutant often fails to complete cytokinesis, resulting in multinucleate cells with gapped walls and cell wall stubs in diverse tissues. These defects correlate with the selective reduction of transcript levels of several G2/M phase-specific genes, which include B2-type cyclin (CYCB2), CDC20.1 and KNOLLE (KN). These genes contain MSA-like motifs in their promoters and were activated by MYB3R4 in transient expression assays in tobacco cells. The KN gene encodes a cytokinesis-specific syntaxin that is essential for cell plate formation. The cytokinesis defects of myb3r1 myb3r4 double mutants were partially rescued by KN gene expression from heterologous promoters. In addition, a kn heterozygous mutation enhanced cytokinesis defects resulting from heterozygous or homozygous mutations in the MYB3R1 and MYB3R4 genes. Our results suggest that a pair of structurally related R1R2R3-Myb transcription factors may positively regulate cytokinesis mainly through transcriptional activation of the KN gene.

c-Myb contributes to G2/M cell cycle transition in human hematopoietic cells by direct regulation of cyclin B1 expression

Myb family proteins are ubiquitously expressed transcription factors. In mammalian cells, they play a critical role in regulating the G(1)/S cell cycle transition but their role in regulating other cell cycle checkpoints is incompletely defined. Herein, we report experiments which demonstrate that c-Myb upregulates cyclin B1 expression in normal and malignant human hematopoietic cells. As a result, it contributes directly to G(2)/M cell cycle progression. In cell lines and primary cells, cyclin B1 levels varied directly with c-Myb expression. Chromatin immunoprecipitation assays, mutation analysis, and luciferase reporter assays revealed that c-Myb bound the cyclin B1 promoter preferentially at a site just downstream of the transcriptional start site. The biological significance of c-Myb, versus B-Myb, binding the cyclin B1 promoter was demonstrated by the fact that expression of inducible dominant negative c-Myb in K562 cells accelerated their exit from M phase. In addition, expression of c-Myb in HCT116 cells rescued cyclin B1 expression after B-myb expression was silenced with small interfering RNA. These results suggest that c-Myb protein plays a previously unappreciated role in the G(2)/M cell cycle transition of normal and malignant human hematopoietic cells and expands the known repertoire of c-myb functions in regulating human hematopoiesis.

The human synMuv-like protein LIN-9 is required for transcription of G2/M genes and for entry into mitosis

Regulated gene expression is critical for the proper timing of cell cycle transitions. Here we report that human LIN-9 has an important function in transcriptional regulation of G2/M genes. Depletion of LIN-9 by RNAi in human fibroblasts strongly impairs proliferation and delays progression from G2 to M. We identify a cluster of G2/M genes as direct targets of LIN-9. Activation of these genes is linked to an association between LIN-9 and B-MYB. Chromatin immunoprecipitation assays revealed binding of both LIN-9 and B-MYB to the promoters of G2/M regulated genes. Depletion of B-MYB recapitulated the biological outcome of LIN-9 knockdown, including impaired proliferation and reduced expression of G2/M genes. These data suggest a critical role for human LIN-9, together with B-MYB, in the activation of genes that are essential for progression into mitosis.

The transcription factor B-Myb is essential for S-phase progression and genomic stability in diploid and polyploid megakaryocytes

The cell-cycle-regulated Myb-family transcription factor B-Myb is crucial during S phase in many diploid cell types. We have examined the expression and function of B-Myb in megakaryocytic differentiation, during which cells progress from a diploid to a polyploid state. In contrast to terminal differentiation of most haematopoietic cells, during which B-myb is rapidly downregulated, differentiation of megakaryocytes is accompanied by continued B-myb RNA and protein expression. Overexpression of B-Myb in a megakaryoblastic cell line resulted in an increase in the number of cells entering S phase and, upon induction of differentiation, the fraction of cells actively endoreplicating increased. By contrast, reduction of B-Myb levels using short interfering (si)RNA resulted in a decline in S-phase progression during both normal and endoreplicative DNA synthesis. This effect correlated with aberrant localisation of initiation of DNA replication within the nucleus and an increased fraction of cells in mitosis. Chromosomal fragmentation and other aberrations, including shorter, thicker chromatids, end-to-end fusion, and loss of a chromatid, suggest that reduced B-Myb activity is also associated with structural chromosomal instability.

Generation of a conditional allele of the B-myb gene

B-Myb is an essential transcription factor involved in control of the cell cycle and the regulation of tissue-specific gene expression in a wide range of cell types. Loss of both alleles results in early embryonic lethality at E4.5-6.5. To address the function of B-Myb in later stages of embryogenesis and in specific adult tissues, a floxed B-myb allele (B-mybF) was generated. Cre-mediated deletion in vivo was demonstrated by breeding with a transgenic GATA-Cre mouse line. An intermediate allele produced in the creation of the floxed allele, in which the PGK-neo(R) cassette is present in intron 3 (B-myb(loxneo)), was deduced to be a weak hypomorph based on the later embryonic death of homozygotes compared to B-myb(-/-) embryos. To demonstrate the efficiency and possible consequences of B-myb inactivation, we performed conditional deletion in cultured MEFs and observed decreased growth that correlated with aberrant nuclear DNA replication.

Small molecules that delay S phase suppress a zebrafish bmyb mutant

Bmyb is a ubiquitously expressed transcription factor involved in cellular proliferation and cancer. Loss of bmyb function in the zebrafish mutant crash&burn (crb) results in decreased cyclin B1 expression, mitotic arrest and genome instability. These phenotypic observations in crb mutants could be attributed to the decreased expression of cyclin B1, a cell-cycle regulatory protein that is responsible for driving cell progression from G2 through mitosis. To identify small molecules that interact with the bmyb pathway, we developed an embryo-based suppressor screening strategy. In 16 weeks we screened a diverse approximately 16,000 compound library, and discovered one previously unknown compound, persynthamide (psy, 1), that suppressed bmyb-dependent mitotic defects. Psy-treated embryos showed an S-phase delay, and knockdown of the cell-cycle checkpoint regulator ataxia telangiectasia--and Rad-related kinase (ATR) abrogated the suppression of crb. The DNA synthesis inhibitors aphidicolin (2) and hydroxyurea (3) also suppressed crb. S-phase inhibition upregulated cyclin B1 mRNA, promoting the progression of cells through mitosis. Our study demonstrates that chemical suppressor screening in zebrafish can identify compounds with cell-cycle activity and can be used to identify pathways that interact with specific cell-cycle phenotypes.

B-MYB, a transcription factor implicated in regulating cell cycle, apoptosis and cancer

B-MYB belongs to the MYB family of transcription factors that include A-MYB and c-MYB. While A-MYB and c-MYB are tissue-specific, B-MYB is broadly expressed in rapidly dividing cells of developing or adult mammals. B-MYBs liaisons with important players of the cell cycle and transcription machinery, such as E2F and retinoblastoma proteins, suggest that its essential function in stem cell formation and mammalian development could be related to its ability to directly or indirectly impinge on gene expression. Besides its role in the cell cycle, B-MYB has been shown to promote cell survival by activating antiapoptotic genes such as ApoJ/clusterin and BCL2. Here, we discuss how B-MYB could be implicated in tumourigenesis by regulating gene expression.

A zebrafish bmyb mutation causes genome instability and increased cancer susceptibility

A major goal of cancer research has been to identify genes that contribute to cancer formation. The similar pathology between zebrafish and human tumors, as well as the past success of large-scale genetic screens in uncovering human disease genes, makes zebrafish an ideal system in which to find such new genes. Here, we show that a zebrafish forward genetic screen uncovered multiple cell proliferation mutants including one mutant, crash&burn (crb), that represents a loss-of-function mutation in bmyb, a transcriptional regulator and member of a putative proto-oncogene family. crb mutant embryos have defects in mitotic progression and spindle formation, and exhibit genome instability. Regulation of cyclin B levels by bmyb appears to be the mechanism of mitotic accumulation in crb. Carcinogenesis studies reveal increased cancer susceptibility in adult crb heterozygotes. Gene-expression signatures associated with loss of bmyb in zebrafish are also correlated with conserved signatures in human tumor samples, and down-regulation of the B-myb signature genes is associated with retention of p53 function. Our findings show that zebrafish screens can uncover cancer pathways, and demonstrate that loss of function of bmyb is associated with cancer.

Conservation and diversification of three-repeat Myb transcription factors in plants

The Myb family of transcription factors is characterized by the presence of a conserved DNA-binding domain called the Myb domain, which typically contains two or three imperfect repeat sequences. Within this family, Myb proteins containing three repeat motifs are evolutionarily conserved and have important roles in the cell cycle. Vertebrates have three Myb proteins, c-Myb, A-Myb, and B-Myb, all of which contain three repeats and are proposed to have a role at the G1/S transition. In plants, Myb proteins with three repeats are encoded by genes in a small subfamily within the large Myb gene family, most of which encode for Myb proteins with only two repeats. We have shown that Myb proteins with three repeats have an important role at the G2/M in tobacco, by regulating transcription of cyclin B genes and many other genes that are expressed at a similar time in the cell cycle. In this review, we summarize current knowledge on structure, function, and regulation of the plant Myb factors with three repeats, and discuss their conserved and divergent features in comparison with animal counterparts.

Functional evolution of the vertebrate Myb gene family: B-Myb, but neither A-Myb nor c-Myb, complements Drosophila Myb in hemocytes

The duplication of genes and genomes is believed to be a major force in the evolution of eukaryotic organisms. However, different models have been presented about how duplicated genes are preserved from elimination by purifying selection. Preservation of one of the gene copies due to rare mutational events that result in a new gene function (neofunctionalization) necessitates that the other gene copy retain its ancestral function. Alternatively, preservation of both gene copies due to rapid divergence of coding and noncoding regions such that neither retains the complete function of the ancestral gene (subfunctionalization) may result in a requirement for both gene copies for organismal survival. The duplication and divergence of the tandemly arrayed homeotic clusters have been studied in considerable detail and have provided evidence in support of the subfunctionalization model. However, the vast majority of duplicated genes are not clustered tandemly, but instead are dispersed in syntenic regions on different chromosomes, most likely as a result of genome-wide duplications and rearrangements. The Myb oncogene family provides an interesting opportunity to study a dispersed multigene family because invertebrates possess a single Myb gene, whereas all vertebrate genomes examined thus far contain three different Myb genes (A-Myb, B-Myb, and c-Myb). A-Myb and c-Myb appear to have arisen by a second round of gene duplication, which was preceded by the acquisition of a transcriptional activation domain in the ancestral A-Myb/c-Myb gene generated from the initial duplication of an ancestral B-Myb-like gene. B-Myb appears to be essential in all dividing cells, whereas A-Myb and c-Myb display tissue-specific requirements during spermatogenesis and hematopoiesis, respectively. We now report that the absence of Drosophila Myb (Dm-Myb) causes a failure of larval hemocyte proliferation and lymph gland development, while Dm-Myb(-/-) hemocytes from mosaic larvae reveal a phagocytosis defect. In addition, we show that vertebrate B-Myb, but neither vertebrate A-Myb nor c-Myb, can complement these hemocyte proliferation defects in Drosophila. Indeed, vertebrate A-Myb and c-Myb cause lethality in the presence or absence of endogenous Dm-Myb. These results are consistent with a neomorphic origin of an ancestral A-Myb/c-Myb gene from a duplicated B-Myb-like gene. In addition, our results suggest that B-Myb and Dm-Myb share essential conserved functions that are required for cell proliferation. Finally, these experiments demonstrate the utility of genetic complementation in Drosophila to explore the functional evolution of duplicated genes in vertebrates.

Temperature-dependent modification and activation of B-MYB: implications for cell survival

B-MYB is a ubiquitous transcription factor with an essential role in mouse development. Because cells with a disrupted B-MYB gene cannot be obtained, it is still unknown what is the critical function(s) exerted by B-MYB in mammalian cells. In this study we have observed that reducing B-MYB expression in primary human fibroblasts by using RNA interference results in a partial block of the cells in the G(2) phase of the cell cycle and cell death. Surprisingly, suppressing B-MYB transcriptional activity with a dominant-negative molecule is without effect, suggesting that its transactivating function is not essential. Only human or murine fibroblasts exposed to high temperature are sensitized to cell death in the presence of dominant-negative B-MYB. This correlates with temperature-dependent binding of endogenous B-MYB to transcriptional regulatory elements of the stress-related gene ApoJ/clusterin. We find that regulation of ApoJ/clusterin by B-MYB is a pro-survival response to thermal stress. Thus, B-MYB is regulated by temperature to activate genes required for cell survival.

Identification of a Drosophila Myb-E2F2/RBF transcriptional repressor complex

The Drosophila Myb complex has roles in both activating and repressing developmentally regulated DNA replication. To further understand biochemically the functions of the Myb complex, we fractionated Drosophila embryo extracts relying upon affinity chromatography. We found that E2F2, DP, RBF1, RBF2, and the Drosophila homolog of LIN-52, a class B synthetic multivulva (synMuv) protein, copurify with the Myb complex components to form the Myb-MuvB complex. In addition, we found that the transcriptional repressor protein, lethal (3) malignant brain tumor protein, L(3)MBT, and the histone deacetylase, Rpd3, associated with the Myb-MuvB complex. Members of the Myb-MuvB complex were localized to promoters and were shown to corepress transcription of developmentally regulated genes. These and other data now link together the Myb and E2F2 complexes in higher-order assembly to specific chromosomal sites for the regulation of transcription.

E2Fs link the control of G1/S and G2/M transcription

Previous work has provided evidence for E2F-dependent transcription control of both G1/S- and G2/M-regulated genes. Analysis of the G2-regulated cdc2 and cyclin B1 genes reveals the presence of both positive- and negative-acting E2F promoter elements. Additional elements provide both positive (CCAAT and Myb) and negative (CHR) control. Chromatin immunoprecipitation assays identify multiple interactions of E2F proteins that include those previously shown to activate and repress transcription. We find that E2F1, E2F2, and E2F3 bind to the positive-acting E2F site in the cdc2 promoter, whereas E2F4 binds to the negative-acting site. We also find that binding of an activator E2F is dependent on an adjacent CCAAT site that is bound by the NF-Y transcription factor and binding of a repressor E2F is dependent on an adjacent CHR element, suggesting a role for cooperative interactions in determining both activation and repression. Finally, the kinetics of B-Myb interaction with the G2-regulated promoters coincides with the activation of the genes, and RNAi-mediated reduction of B-Myb inhibits expression of cyclin B1 and cdc2. The ability of B-Myb to interact with the cdc2 promoter is dependent on an intact E2F binding site. These results thus point to a role for E2Fs, together with B-Myb, which is an E2F-regulated gene expressed at G1/S, in linking the regulation of genes at G1/S and G2/M.