Myb: an old oncoprotein with new roles

Diverse developing mouse lineages exhibit high-level c-Myb expression in immature cells and loss of expression upon differentiation

The c-myb gene encodes a sequence specific transactivator that is required for fetal hematopoiesis, but its potential role in other tissues is less clear because of the early fetal demise of mice with targeted deletions of the c-myb gene and incomplete of knowledge about c-myb's expression pattern. In the hematopoietic system, c-Myb protein acts on target genes whose expression is restricted to individual lineages, despite Myb's presence and role in multiple immature lineages. This suggests that c-Myb actions within different cell type-specific contexts are strongly affected by combinatorial interactions. To consider the possibility of similar c-Myb actions could extend into non-hematopoietic systems in other cell and tissue compartments, we characterized c-myb expression in developing and adult mice using in situ hybridization and correlated this with stage-specific differentiation and mitotic activity. Diverse tissues exhibited strong c-myb expression during development, notably tooth buds, the thyroid primordium, developing trachea and proximal branching airway epithelium, hair follicles, hematopoietic cells, and gastrointestinal crypt epithelial cells. The latter three of these all maintained high expression into adulthood, but with characteristic restriction to immature cell lineages prior to their terminal differentiation. In all sites, during fetal and adult stages, loss of c-Myb expression correlated strikingly with the initiation of terminal differentiation, but not the loss of mitotic activity. Based on these data, we hypothesize that c-Myb's function during cellular differentiation is both an activator of immature gene expression and a suppressor of terminal differentiation in diverse lineages.

Induction of antisense Pax-3 expression leads to the rapid morphological differentiation of neuronal cells and an altered response to the mitogenic growth factor bFGF

Mutations within the Pax-3 gene lead to a range of developmental abnormalities in both humans and mice. In this report, we have investigated the role that Pax-3 plays in neuronal cell development by specifically downregulating Pax-3 expression within a neuronal cell line. This was achieved by stably transfecting the neuronal cell line ND7 with an expression vector in which antisense Pax-3 RNA was produced under the control of the inducible MMTV promoter. In the stable transfectants, we found that the addition of dexamethasone led to the induction of antisense Pax-3 RNA and a rapid downregulation in endogenous Pax-3 protein expression. The decrease in endogenous Pax-3 protein expression corresponded with a dramatic change in the morphology of the cell: the normally rounded ND7 cells exhibited increased cell to substrate adhesion, extended long neurite processes and expressed genes such as snap-25 that are characteristic of a mature neuron. The morphological differentiation induced by a reduction in Pax-3 expression was followed 24–48 hours later by a cessation in cell proliferation. Interestingly the morphological differentiation and cessation in cell proliferation inducted in the cell lines lacking Pax-3 could be reversed by the addition of the mitogenic growth factor EGF but not by bFGF, whose receptor was downregulated in these cells. These results suggest that the expression of Pax-3 is essential to maintain the undifferentiated phenotype of these immature neuronal cells, and in its absence the cells acquire many of the characteristics of a mature neuronal cell. The slow onset of cell cycle arrest in the cells lacking Pax-3 argues against this transcription factor playing a direct role in the regulation of neuronal cell proliferation.

Regulation of the CD13/aminopeptidase N gene by DMP1, a transcription factor antagonized by D-type cyclins

The binding of the Myb-like DMP1 transcription factor to DNA consensus sequences [CCCG(G/T)ATGT] in artificial promoters is antagonized by D-type cyclins with no requirement for their catalytic partners, cyclin-dependent kinase (CDK) 4 and CDK6. The subset of DMP1 binding sites containing the GGA core can bind Ets family transcription factors Ets-1 and Ets-2. Screening of a series of natural promoters revealed that the CD13/aminopeptidase N (APN; EC 3.4.11.2) promoter could bind and be activated by DMP1. Activation of CD13/APN required both the intact DNA binding and transactivation domains of DMP1 and was inhibited by D-type cyclins, but not by cyclins A, B, C, or H, in a CDK-independent manner. CD13/APN is transactivated by a cooperative interaction between c-Myb bound to its cognate site and Ets-1 tethered to one of three GGA core-containing sites located 30-50 base pairs downstream. DMP1 binds to one of the Ets binding sites (designated Ets C) and synergizes with c-Myb in activating CD13/APN expression. Analysis of nuclear lysates from KG1a early myeloid cells using an oligonucleotide probe containing only the DMP1/Ets C binding site indicated that endogenous DMP1 and a putative Ets family member bind this element in vivo. DMP1-DNA complexes were significantly more stable than those containing the Ets factor. These data indicate that two different Myb family proteins collaborate in regulating APN gene expression and point to a role for DMP1 in normal myeloid cell development.

GRIFIN, a novel lens-specific protein related to the galectin family

The vertebrate lens is a relatively simple cellular structure that has evolved to refract light. The ability of the lens to focus light on the retina derives from a number of properties including the expression at high levels of a selection of soluble proteins referred to as the crystallins. In the present study, we have used differential cDNA display techniques to identify a novel, highly abundant and soluble lens protein. Though related to the family of soluble lectins called galectins, it does not bind beta-galactoside sugars and has atypical sequences at normally conserved regions of the carbohydrate-binding domain. Like some galectin family members, it can form a stable dimer. It is expressed only in the lens and is located at the interface between lens fiber cells despite the apparent lack of any membrane-targeting motifs. This protein is designated GRIFIN (galectin-related inter-fiber protein) to reflect its exclusion from the galectin family given the lack of affinity for beta-galactosides. Although the abundance, solubility, and lens-specific expression of GRIFIN would argue that it represents a new crystallin, its location at the fiber cell interface might suggest that its primary function is executed at the membrane.

Physical interaction between retinoic acid receptor and the oncoprotein myb inhibits retinoic acid-dependent transactivation

The c-myb protooncogene is predominantly expressed in hematopoietic cells and plays a vital role in hematopoiesis. Retinoic acid (RA) is able to induce differentiation of several hematopoietic cells. This differentiation is linked to decreased c-myb expression, suggesting that retinoid receptors (RAR/RXR) may down-regulate c-myb gene expression. Furthermore, recent data indicate that RAR inhibits the function of the Myb protein itself. In addition, the Myb-Ets oncogenic fusion protein has been shown to inhibit transcriptional activation by RAR and thyroid hormone receptor. Myb-Ets also antagonizes the biological response of erythrocytic progenitor cells to RA and thyroid hormone. This prompted us to investigate a possible cross talk between RAR and Myb. Here, we demonstrate that RA inhibits the expression of the endogenous Myb target gene tom-1. Conversely, Myb functions as a potent inhibitor of RA-induced biological responses. Functional analysis of Myb mutants in transfection studies revealed that the Myb DNA-binding domain (DBD) is necessary for repression whereas the transactivation domain is dispensable. Furthermore, we show that v-Myb and RAR interact in vitro and in vivo. This interaction requires the DBD of RAR. In contrast, glutathione S-transferase-pulldown assays with v-Myb mutants indicate that the DBD and the C terminus of Myb directly interact with RAR. Our results suggest that the physical interaction between Myb and RAR may play a role in the regulation of hematopoietic gene expression.

The c-Myb oncoprotein

The c-myb gene is the cellular homologue of the v-myb oncogenes carried by the avian leukaemia viruses AMV and E26. It encodes a transcription factor (c-Myb), as does each of the viral oncogenes, which recognises the core DNA sequence C/T-A-A-C-G/T-G via a repeated helix-turn-helix-like motif. c-myb is expressed in immature haemopoietic cells, as well as immature cells of the gastro-intestinal epithelium and is down-regulated with differentiation. Enforced expression of activated or even normal forms of Myb can transform haemopoietic cells, most often of the myeloid lineage, in vitro and in vivo. Although many genes have been identified which are likely to be regulated by c-Myb, the critical target genes involved in Myb's transforming activity are not known. Together with data showing increased c-myb expression in colonic tumours, these observations raise the possibility that c-myb may play a role in human malignant disease.

More than 80R2R3-MYB regulatory genes in the genome of Arabidopsis thaliana

Transcription factors belonging to the R2R3-MYB family contain the related helix-turn-helix repeats R2 and R3. The authors isolated partial cDNA and/or genomic clones of 78 R2R3-MYB genes from Arabidopsis thaliana and found accessions corresponding to 31 Arabidopsis genes of this class in databanks, seven of which were not represented in the authors' collection. Therefore, there are at least 85, and probably more than 100, R2R3-MYB genes present in the Arabidopsis thaliana genome, representing the largest regulatory gene family currently known in plants. In contrast, no more than three R2R3-MYB genes have been reported in any organism from other phyla. DNA-binding studies showed that there are differences but also frequent overlaps in binding specificity among plant R2R3-MYB proteins, in line with the distinct but often related functions that are beginning to be recognized for these proteins. This large-sized gene family may contribute to the regulatory flexibility underlying the developmental and metabolic plasticity displayed by plants.

Rearrangement of the human CDC5L gene by a t(6;19)(p21;q13.1) in a patient with multicystic renal dysplasia

Genetic studies have implicated the short arm of chromosome 6 in congenital hydronephrosis. In previous studies, we described a fetus carrying a t(6;19)(p21;q13.1) as the sole cytogenetic anomaly and suffering from bilateral multicystic renal dysplasia caused by a bilateral complete pelviureteric junction obstruction, resulting in a massive hydronephrosis. Characterization of the chromosome 19 breakpoint region revealed that the transcription factor-encoding USF2 gene is affected. In this report, we show that the CDC5L gene on chromosome 6p is rearranged in the cells of the fetus. CDC5L encodes a protein that is related to the product of the Schizosaccharomyces pombe Cdc5 gene, which exerts its effects at the G2/M transition during cell cycle progression. We have established the genomic organization of the CDC5L gene and found that it consists of at least 16 exons spanning approximately 50 kb of chromosome segment 6p21. Northern blot analysis indicated that the gene is ubiquitously expressed as a single mRNA of about 3.4 kb in both fetal and adult tissues. The translation product of the CDC5L gene has an electrophoretic mobility of about 100 kDa and is predicted to be a nuclear protein, since it contains a Myb-related DNA binding domain and potential nuclear localization signals in its aminoterminal region. Immunocytochemical analysis confirmed the nuclear localization of the CDC5L protein. CDC5L was also predicted to contain a hydrophilic, proline-rich region in its central part, which might function as a transcriptional activating domain. The chromosome 6 breakpoint was found in the intron between exons 9 and 10, indicating that, as a direct result of the 6;19 translocation, the Myb-related DNA binding domains and the nuclear localization signals are separated from the putative transactivating domain. Northern blot and RT-PCR experiments revealed that the other CDC5L allele is unaffected, and in Western blot experiments, expression of the 100-kDa protein was detected in fibroblasts of the fetus. Expression of a truncated or hybrid CDC5L transcript resulting from the CDC5L rearrangement could not be demonstrated.

Drosophila myb is required for the G2/M transition and maintenance of diploidy

The myb proto-oncogenes are thought to have a role in the cell division cycle. We have examined this possibility by genetic analysis in Drosophila melanogaster, which possesses a single myb gene. We have described previously two temperature-sensitive, recessive lethal mutants in Drosophila myb (Dm myb). The phenotypes of these mutants revealed a requirement for myb in diverse cellular lineages throughout the course of Drosophila development. We now report a cellular explanation for these findings by showing that Dm myb is required for both mitosis and prevention of endoreduplication in wing cells. Myb apparently acts at or near the time of the G2/M transition. The two mutant alleles of Dm myb produce the same cellular phenotype, although the responsible mutations are located in different functional domains of the gene product. The mutant phenotype can be partially suppressed by ectopic expression of either cdc2 or string, two genes that are known to promote the transition from G2 to M. We conclude that Dm myb is required for completion of cell division and may serve two independent functions: promotion of mitosis, on the one hand, and prevention of endoreduplication when cells are arrested in G2, on the other.

Myb proteins in life, death and differentiation

Myb transcription factors are crucial to the control of proliferation and differentiation in a number of cell types but their mechanism of action is unclear. Regulation of Myb proteins by phosphorylation and intermolecular cooperation has recently been demonstrated, together with a new role for the proteins, in the control of apoptosis.

Myc represses transcription of the growth arrest gene gas1

Cell proliferation is regulated by the induction of growth promoting genes and the suppression of growth inhibitory genes. Malignant growth can result from the altered balance of expression of these genes in favor of cell proliferation. Induction of the transcription factor, c-Myc, promotes cell proliferation and transformation by activating growth promoting genes, including the ODC and cdc25A genes. We show that c-Myc transcriptionally represses the expression of a growth arrest gene, gas1. A conserved Myc structure, Myc box 2, is required for repression of gas1, and for Myc induction of proliferation and transformation, but not for activation of ODC. Activation of a Myc-estrogen receptor fusion protein by 4-hydroxytamoxifen was sufficient to repress gas1 gene transcription. These findings suggest that transcriptional repression of growth arrest genes, including gas1, is one step in promotion of cell growth by Myc.

Minimal truncation of the c-myb gene product in rapid-onset B-cell lymphoma

Oncogenic activation of c-myb by insertional mutagenesis has been implicated in rapid-onset B-cell lymphomas induced by the nonacute avian leukosis virus EU-8. In these tumors, proviruses are integrated either upstream of the c-myb coding region or within the first intron of c-myb. Tumors with either type of integration contained identical chimeric mRNAs in which the viral 5' splice site was juxtaposed to the 3' splice site of c-myb exon 2 and myb exon 1 was eliminated. Both classes of integrations generated truncated Myb proteins that were indistinguishable by Western analysis. In contrast to most other examples of c-myb activation, the truncation consisted of only 20 N-terminal amino acids and did not disrupt either the DNA binding domain near the N terminus or the negative regulatory domain near the C terminus of Myb. The significance of the 20-amino-acid Myb truncation to tumorigenesis was tested by infection of chicken embryos with retroviral vectors expressing different myb gene products. While virus expressing either wild-type c-myb or c-myb mutated at the N-terminal casein kinase II sites was only weakly oncogenic at 10 weeks, the minimally truncated myb virus induced a high incidence of rapid-onset tumors, including B-cell lymphomas, sarcomas, and adenocarcinomas.

c-Myb trans-activates the human DNA topoisomerase IIalpha gene promoter

DNA topoisomerase IIalpha (topo IIalpha) is an essential proliferation-dependent nuclear enzyme which has been exploited as an anti-tumor drug target. Since the proliferative status of human leukemia cells is associated with expression of the c-myb proto-oncogene, c-Myb was investigated as a trans-activator of the topo IIalpha gene. Using topo IIalpha promoter-luciferase reporter plasmids, c-myb expression caused trans-activation of the topo IIalpha promoter a maximum of approximately 4.5-fold over basal levels in HL-60 human promyelocytic leukemia cells. Trans-activation was submaximal with higher levels of c-myb expression plasmid but a Myb protein lacking its negative regulatory domain resulted in approximately 19-fold trans-activation. Mutagenesis and 5'-deletion studies revealed that Myb trans-activation was mediated via a Myb-binding site at positions -16 to -11 and that this region governed the bulk of basal topo IIalpha promoter activity in human leukemia cells. Trans-activation of topo IIalpha by c-Myb was lymphoid- or myeloid-dependent. However, B-Myb, a more widely-expressed Myb family member, caused topo IIalpha trans-activation in both HL-60 cells and HeLa epithelial cervical carcinoma cells. These data provide evidence for a new Myb-responsive gene which is directly linked to and required for cellular proliferation.

Pombe Cdc5-related protein. A putative human transcription factor implicated in mitogen-activated signaling

The Schizosaccharomyces pombe cdc5 gene product is a cell cycle regulator that exerts its effects at the G2/M transition in fission yeast. We describe the cloning of a putative human transcription factor, pombe Cdc5-related protein (PCDC5RP), which bears significant homology to S. pombe Cdc5 and to expressed sequences in mouse, nematode, and budding yeast. PCDC5RP is expressed widely in normal adult human tissues and thus may have an important general function that has been preserved evolutionarily. PCDC5RP contains two tandem repeats of a helix-turn-helix DNA binding motif, four consensus nuclear localization signals, and a hydrophilic, proline-rich central region similar to the transcriptional activating domain in Myb family members. Remarkably, PCDC5RP moved rapidly from cytoplasm to nucleus upon serum stimulation of cultured cells. This movement correlated temporally with an increase in PCDC5RP phosphorylation. Thus, PCDC5RP is a presumed transcription factor that appears to transduce cytoplasmic signals to the nucleus upon serum stimulation.

Oncogenic point mutations induce altered conformation, redox sensitivity, and DNA binding in the minimal DNA binding domain of avian myeloblastosis virus v-Myb

c-Myb is the founder member of a class of transcription factors with tryptophan-rich repeats responsible for DNA binding. Activated oncogenic forms of Myb are encoded by the avian retroviruses, avian myeloblastosis virus (AMV) and E26. AMV v-Myb encodes a truncated protein with 11 point mutations relative to c-Myb. The mutations in the DNA binding domain (DBD) were reported to impose distinct phenotypes of differentiation on transformed myeloid cells (Introna, M., Golay, J., Frampton, J., Nakano, T., Ness, S. A., and Graf, T. (1990) Cell 63, 1287-1297). The molecular mechanism operating has remained elusive since no change in sequence specificity has been found. We introduced AMV-specific point mutations in the minimal DBD of chicken c-Myb and studied their effect on structure and function of the purified protein. Fluorescence emission spectra and fluorescence quenching experiments showed that the AMV-specific point mutations had a significant effect on the conformation of the DBD, giving rise to a more compact structure, a change that was accompanied by a reduced sensitivity toward cysteine-specific alkylation and oxidation. The DNA binding properties were also altered by the AMV-specific point mutations, leading to protein-DNA complexes with highly reduced stability. This reduction in stability was, however, more severe with certain subtypes of binding sequences than with others. This differential behavior was also observed in an in vivo model system where DBD-VP16 fusions were coexpressed with various reporters. These findings imply that different subsets of Myb-responsive promoters may react differentially toward the AMV-specific mutations, a phenomenon that could contribute to the altered patterns of gene expression induced by the AMV v-Myb relative to wild type c-Myb.

MYB transcription factors in plants

The cloning of the first transcription factor from plants, the C1 gene of maize, indicated that plants use transcription factors that are structurally related to those of animals in their control of gene expression, because C1 showed significant structural homology to the vertebrate cellular proto-oncogene c-MYB. Since 1987, the catalogue of MYB-related transcription factors has increased considerably in size due, primarily, to the ever-expanding number of MYB genes identified in higher plants (Arabidopsis thaliana is estimated to contain more than a hundred MYB genes). In vertebrates, the MYB-related proto-oncogenes comprise a small family with a central role in controlling cellular proliferation and commitment to development. However, while the functions of some plant MYB genes are relatively well understood they are, at present, quite distinct from their animal counterparts.

A single residue substitution causes a switch from the dual DNA binding specificity of plant transcription factor MYB.Ph3 to the animal c-MYB specificity

Transcription factor MYB.Ph3 from Petunia binds to two types of sequences, MBSI and MBSII, whereas murine c-MYB only binds to MBSI, and Am305 from Antirrhinum only binds to MBSII. DNA binding studies with hybrids of these proteins pointed to the N-terminal repeat (R2) as the most involved in determining binding to MBSI and/or MBSII, although some influence of the C-terminal repeat (R3) was also evident. Furthermore, a single residue substitution (Leu71 --> Glu) in MYB.Ph3 changed its specificity to that of c-MYB, and c-MYB with the reciprocal substitution (Glu132 --> Leu) essentially gained the MYB.Ph3 specificity. Molecular modeling and DNA binding studies with site-specific MYB.Ph3 mutants strongly supported the notion that the drastic changes in DNA binding specificity caused by the Leu --> Glu substitution reflect the fact that certain residues influence this property both directly, through base contacts, and indirectly, through interactions with other base-contacting residues, and that a single residue may establish alternative base contacts in different targets. Additionally, differential effects of mutations at non-base-contacting residues in MYB.Ph3 and c-MYB were observed, reflecting the importance of protein context on DNA binding properties of MYB proteins.

The glucocorticoid receptor is a key regulator of the decision between self-renewal and differentiation in erythroid progenitors

During development and in regenerating tissues such as the bone marrow, progenitor cells constantly need to make decisions between proliferation and differentiation. We have used a model system, normal erythroid progenitors of the chicken, to determine the molecular players involved in making this decision. The molecules identified comprised receptor tyrosine kinases (c-Kit and c-ErbB) and members of the nuclear hormone receptor superfamily (thyroid hormone receptor and estrogen receptor). Here we identify the glucocorticoid receptor (GR) as a key regulator of erythroid progenitor self-renewal (i.e. continuous proliferation in the absence of differentiation). In media lacking a GR ligand or containing a GR antagonist, erythroid progenitors failed to self-renew, even if c-Kit, c-ErbB and the estrogen receptor were activated simultaneously. To induce self-renewal, the GR required the continuous presence of an activated receptor tyrosine kinase and had to cooperate with the estrogen receptor for full activity. Mutant analysis showed that DNA binding and a functional AF-2 transactivation domain are required for proliferation stimulation and differentiation arrest. c-myb was identified as a potential target gene of the GR in erythroblasts. It could be demonstrated that delta c-Myb, an activated c-Myb protein, can functionally replace the GR.

Differences between plant and animal Myb domains are fundamental for DNA binding activity, and chimeric Myb domains have novel DNA binding specificities

Several Myb domain proteins have been identified in plants, in which they play important regulatory roles in specific cellular processes. Plant and animal Myb domains have significant differences, but how these differences are important for function is not yet understood. The P gene encodes a Myb domain protein that activates a subset of flavonoid biosynthetic genes in maize floral organs. P and v-Myb bind different DNA sequences in vitro. Here we show that the Myb domain is solely responsible for the sequence-specific DNA binding activity of P, which binds DNA only in the reduced state. Differences in the DNA binding domains of v-Myb and P, which are conserved among animal and plant Myb domains, are fundamental for the high affinity DNA binding activity of these proteins to the corresponding binding sites but are not sufficient for the distinct DNA binding specificities of P and v-Myb. We conclude that significant structural differences distinguish plant from animal Myb domains. A chimeric Myb domain with a novel DNA binding specificity was created by combining Myb repeats of P and v-Myb. This approach could be used to artificially create novel Myb domains and to target transcription factors to genes containing specific promoters or to modify Myb-mediated interactions with other cellular factors.

Overexpression of HOXA10 in murine hematopoietic cells perturbs both myeloid and lymphoid differentiation and leads to acute myeloid leukemia

Multiple members of the A, B, and C clusters of Hox genes are expressed in hematopoietic cells. Several of these Hox genes have been found to display distinctive expression patterns, with genes located at the 3' side of the clusters being expressed at their highest levels in the most primitive subpopulation of human CD34+ bone marrow cells and genes located at the 5' end having a broader range of expression, with downregulation at later stages of hematopoietic differentiation. To explore if these patterns reflect different functional activities, we have retrovirally engineered the overexpression of a 5'-located gene, HOXA10, in murine bone marrow cells and demonstrate effects strikingly different from those induced by overexpression of a 3'-located gene, HOXB4. In contrast to HOXB4, which causes selective expansion of primitive hematopoietic cells without altering their differentiation, overexpression of HOXA10 profoundly perturbed myeloid and B-lymphoid differentiation. The bone marrow of mice reconstituted with HOXA10-transduced bone marrow cells contained in high frequency a unique progenitor cell with megakaryocytic colony-forming ability and was virtually devoid of unilineage macrophage and pre-B-lymphoid progenitor cells derived from the transduced cells. Moreover, and again in contrast to HOXB4, a significant proportion of HOXA10 mice developed a transplantable acute myeloid leukemia with a latency of 19 to 50 weeks. These results thus add to recognition of Hox genes as important regulators of hematopoiesis and provide important new evidence of Hox gene-specific functions that may correlate with their normal expression pattern.

Isolation and characterization of a tobacco mosaic virus-inducible myb oncogene homolog from tobacco

Salicylic acid (SA) plays an important role in signaling the activation of plant defense responses against pathogen attack including induction of pathogenesis-related (PR) proteins. To gain further insight into the SA-mediated signal transduction pathway, we have isolated and characterized a tobacco mosaic virus (TMV)-inducible myb oncogene homolog (myb1) from tobacco. The myb1 gene was induced upon TMV infection during both the hypersensitive response and development of systemic acquired resistance in the resistant tobacco cultivar following the rise of endogenous SA, but was not activated in the susceptible cultivar that fails to accumulate SA. The myb1 gene was also induced by incompatible bacterial pathogen Pseudomonas syringae pv. syringae during the hypersensitive response. Exogenous SA treatment rapidly (within 15 min) activated the expression of myb1 in both resistant and susceptible tobacco cultivars with the subsequent induction of PR genes occurring several hours later. Biologically active analogs of SA and 2,6-dichloroisonicotinic acid (a synthetic functional analog of SA), which induce PR genes and enhanced resistance, also activated the myb1 gene. In contrast, biologically inactive analogs were poor inducers of myb1 gene expression. Furthermore, the recombinant Myb1 protein was shown to specifically bind to a Myb-binding consensus sequence found in the promoter of the PR-1a gene. Taken together, these results suggest that the tobacco myb1 gene encodes a signaling component down-stream of SA that may participate in transcriptional activation of PR genes and plant disease resistance.

myb provides an essential function during Drosophila development

Vertebrate c-myb encodes a transcription factor thought to play an important role in the cell cycle. To gain further insight into myb function, we have been studying a related gene in Drosophila. We found that Drosophila myb is abundantly expressed throughout development in all mitotically active tissues, at lower levels in some postmitotic tissues, but not at detectable levels in polyploid larval tissues. We performed genetic screens to isolate recessive lethal mutations in the chromosomal region that includes the myb gene. We obtained two temperature-sensitive alleles of myb, demonstrating that the gene provides an essential function. Examination of the mutant phenotype revealed that Drosophila myb is important for both embryonic and imaginal development and that myb serves a role in the development of many tissues and during oogenesis.

A dominant interfering Myb mutant causes apoptosis in T cells

The c-Myb transcription factor is required for the production of most hemopoietic lineages, but information is sparse about its mode of action and the key genes it regulates. We have made an inducible dominant interfering Myb protein, by creating a chimera comprising the DNA binding domain of c-Myb, the Drosophila Engrailed repressor domain, and a modified estrogen receptor hormone binding domain. When expressed in the murine thymoma cell line EL4, activation of this mutant results in a significant proportion of the cell population undergoing apoptosis, as assessed by nuclear breakdown and DNA fragmentation, but has no apparent effect on cell-cycle progression. The apoptotic phenotype is mirrored during thymopoiesis in transgenic mice expressing dominant interfering Myb mutants; their T cells are fragile both in vivo and in vitro. Induction of the Myb dominant interfering mutant in EL4 cells correlates with down-regulation of bcl-2, but does not affect transcription of other bcl-2 family members; conversely, overexpression of bcl-2 in the transgenic mouse model rescues thymocytes from death. Analysis of the bcl-2 promoter by run-on transcription, bandshifting, and transient expression assays shows that it is a direct target of Myb. These data suggest a new and important role for Myb proteins as regulators of cell survival during hemopoiesis.

The importance of the linker connecting the repeats of the c-Myb oncoprotein may be due to a positioning function

The DNA-binding domain of the oncoprotein c-Myb consists of three imperfect tryptophan-rich repeats, R1, R2 and R3. Each repeat forms an independent mini-domain with a helix-turn-helix related motif and they are connected by linkers containing highly conserved residues. The location of the linker between two DNA-binding units suggests a function analogous to a dimerisation motif with a critical role in positioning the recognition helices of each mini-domain. Mutational analysis of the minimal DNA-binding domain of chicken c-Myb (R2 and R3), revealed that besides the recognition helices of each repeat, the linker connecting them was of critical importance in maintaining specific DNA-binding. A comparison of several linker sequences from different Myb proteins revealed a highly conserved motif of four amino acids in the first half of the linker: LNPE (L138 to E141 in chicken c-Myb R2R3). Substitution of residues within this sequence led to reduced stability of protein-DNA complexes and even loss of DNA-binding. The two most affected mutants showed increased accessibility to proteases, and fluorescence emission spectra and quenching experiments revealed greater average exposure of tryptophans which suggests changes in conformation of the proteins. From the structure of R2R3 we propose that the LNPE motif provides two functions: anchorage to the first repeat (through L) and determination of the direction of the bridge to the next repeat (through P).