Structure of the protein encoded by the chicken proto-oncogene c-myb

MYB function in normal and cancer cells

The transcription factor MYB has a key role as a regulator of stem and progenitor cells in the bone marrow, colonic crypts and a neurogenic region of the adult brain. It is in these compartments that a deficit in MYB activity leads to severe or lethal phenotypes. As was predicted from its leukaemogenicity in several animal species, MYB has now been identified as an oncogene that is involved in some human leukaemias. Moreover, recent evidence has strengthened the case that MYB is activated in colon and breast cancer: a block to MYB expression is overcome by mutation of the regulatory machinery in the former disease and by oestrogen receptor-alpha (ERalpha) in the latter.

Modeling a whole organ using proteomics: the avian bursa of Fabricius

While advances in proteomics have improved proteome coverage and enhanced biological modeling, modeling function in multicellular organisms requires understanding how cells interact. Here we used the chicken bursa of Fabricius, a common experimental system for B cell function, to model organ function from proteomics data. The bursa has two major functional cell types: B cells and the supporting stromal cells. We used differential detergent fractionation-multidimensional protein identification technology (DDF-MudPIT) to identify 5198 proteins from all cellular compartments. Of these, 1753 were B cell specific, 1972 were stroma specific and 1473 were shared between the two. By modeling programmed cell death (PCD), cell differentiation and proliferation, and transcriptional activation, we have improved functional annotation of chicken proteins and placed chicken-specific death receptors into the PCD process using phylogenetics. We have identified 114 transcription factors (TFs); 42 of the bursal B cell TFs have not been reported before in any B cells. We have also improved the structural annotation of a newly sequenced genome by confirming the in vivo expression of 4006 "predicted", and 6623 ab initio, ORFs. Finally, we have developed a novel method for facilitating structural annotation, "expressed peptide sequence tags" (ePSTs) and demonstrate its utility by identifying 521 potential novel proteins from the chicken "unassigned chromosome".

Positive and negative regulation of c-Myb by cyclin D1, cyclin-dependent kinases, and p27 Kip1

The c-Myb transcription factor controls differentiation and proliferation in hematopoietic and other cell types and has latent transforming activity, but little is known about its regulation during the cell cycle. Here, c-Myb was identified as part of a protein complex from human T cells containing the cyclin-dependent kinase (CDK) CDK6. Assays using model reporter constructs as well as endogenous target genes showed that the activity of c-Myb was inhibited by cyclin D1 plus CDK4 or CDK6 but stimulated by expression of the CDK inhibitors p16 Ink4a, p21 Cip1, or p27 Kip1. Mapping experiments identified a highly conserved region in c-Myb which, when transferred to the related A-Myb transcription factor, also rendered it responsive to CDKs and p27. The results suggest that c-Myb activity is directly regulated by cyclin D1 and CDKs and imply that c-Myb activity is regulated during the cell cycle in hematopoietic cells.

Distinct changes in gene expression induced by A-Myb, B-Myb and c-Myb proteins

The c-Myb, A-Myb and B-Myb transcription factors have nearly identical DNA-binding domains, activate the same reporter gene constructs in animal cells, but have different biological roles. The Myb proteins are often coexpressed in the same cells, raising questions about whether they activate similar or distinct gene expression profiles, and whether they cooperate or compete in regulating the same promoters. Here, recombinant adenoviruses were used to express each protein in human mammary cells, and then microarray assays were used to assess global changes in gene expression. Each Myb protein induced a unique and specific set of changes, displaying activities far more complex than revealed by standard reporter gene assays. These results have important implications for the roles of various Myb proteins in normal and transformed human cells, for regulatory pathways that might modify their activities and for the importance of acquired mutations that may qualitatively alter their functions in tumors.

The leucine zipper region of Myb oncoprotein regulates the commitment of hematopoietic progenitors

The development of blood cells proceeds from pluripotent stem cells through multipotent progenitors into mature elements belonging to at least 8 different lineages. The lineage choice process during which stem cells and progenitors commit to a particular lineage is regulated by a coordinated action of extracellular signals and transcription factors. Molecular mechanisms controlling commitment are largely unknown. Here, the transcription factor v-Myb and its leucine zipper region (LZR) are identified as regulators of the commitment of a common myeloid progenitor and progenitors restricted to the myeloid lineage. It is demonstrated that wild-type v-Myb with the intact LZR directs development of progenitors into the macrophage lineage. Mutations in this region compromise commitment toward myeloid cells and cause v-Myb to also support the development of erythroid cells, thrombocytes, and granulocytes, similar to the c-Myb protein. In agreement with that, the wild-type v-Myb induces high expression of myeloid factors C/EBP beta, PU.1, and Egr-1 in its target cells, whereas SCL, GATA-1, and c-Myb are more abundant in cells expressing the v-Myb LZR mutant. It is proposed that Myb LZR can function as a molecular switch, affecting expression of lineage-specifying transcription factors and directing the development of hematopoietic progenitors into either myeloid or erythroid lineages.

Characterization of direct readout contacts of the Myb DNA-binding domain

The Myb protein contacts its recognition sequence by means of direct protein-DNA interactions. We used site-directed mutagenesis in order to substitute amino acids crucial for these contacts and probed the mutant proteins for their DNA-binding and transactiving activities. We could show that amino acids involved in direct readout contacts do not contribute equivalently in the recognition process.

The conserved DNA binding domain mediates similar regulatory interactions for A-Myb, B-Myb, and c-Myb transcription factors

The vertebrate A-Myb, B-Myb, and c-Myb proteins comprise a family of related transcription factors that share a highly conserved DNA binding domain. Although all three proteins are capable of binding the same sites in DNA, they have distinct, but overlapping patterns of expression and are presumed to be regulated independently. Here we show that the transcriptional activity of all three vertebrate Myb proteins can be severely inhibited by coexpression of a dominant-negative allele of p100, a coactivator protein that interacts with Myb DNA binding domains. Thus, the conserved Myb domains mediate interactions with common sites in DNA, as well as common regulators, suggesting that the proteins provide alternative or complementary responses to common upstream signaling pathways.

The role of transcriptional activation in the function of the Drosophila myb gene

Vertebrate myb genes encode DNA-binding proteins that regulate transcription and have been implicated in regulation of cell proliferation, differentiation, and apoptosis. We have demonstrated that the single myb gene in Drosophila melanogaster, Dm myb, is required for the G(2)/M transition of the cell cycle and for suppression of endoreduplication. Recently, it has become apparent that the family of proteins containing Myb-related DNA-binding domains is much larger than originally believed and that the biochemical properties and functions of these proteins are diverse. We undertook studies to characterize the biochemical properties of the Drosophila Myb protein (DMyb). We now provide evidence that in addition to having homology with the vertebrate Myb proteins, the Drosophila Myb protein (DMyb) shares its biochemical properties. DMyb binds to a similar consensus sequence and activates transcription from a reporter construct regulated by vertebrate Myb proteins. We also show that DMyb proteins carrying mutations corresponding to previously isolated mutant alleles of Dm myb are less active as transcriptional activators than wild-type DMyb, indicating that a decrease in transcriptional activation ability is likely to cause the mutant phenotypes.

Transcriptional activation by the myb proteins requires a specific local promoter structure

The biological effects of the cellular c-Myb and the viral v-Myb proteins are strikingly different. While c-Myb is indispensable for normal hematopoiesis, v-Myb induces acute leukemia. The v-Myb DNA-binding domain (DBD) differs from that of c-Myb mainly by deletion of the first of three repeats which correlates with efficient oncogenic transformation and a decrease in DNA-binding activity. To investigate the difference in DNA-binding and transcriptional activation, oligonucleotide selection and electrophoretic mobility shift assays were employed. The v-Myb DBD (R2R3) shows an intrinsic DNA-binding specificity for an AT-rich downstream extension of the Myb recognition element (MRE) PyAAC(T)/(G)G for efficient binding to this site, whereas R1 within the c-Myb DBD allows for more flexibility for this downstream extension. Therefore, due to the presence of repeat R1, c-Myb can bind to a greater number of target sites. The intrinsic DNA-binding specificity of R2R3 is further supported with the results from in vivo transcriptional activation experiments which demonstrated that both the v-Myb and c-Myb DBDs require an extension of the MRE (motif #1) by a downstream T-stretch (motif #2) for full activity. Surprisingly, the T-stretch improves binding when present on either strand, but is required on a specific strand for transcriptional activation.

Transformation by v-Myb

The v-myb oncogene of the avian myeloblastosis virus (AMV) is unique among known oncogenes in that it causes only acute leukemia in animals and transforms only hematopoietic cells in culture. AMV was discovered in the 1930s as a virus that caused a disease in chickens that is similar to acute myelogenous leukemia in humans (Hall et al., 1941). This avian retrovirus played an important role in the history of cancer research for two reasons. First, AMV was used to demonstrate that all oncogenic viruses did not contain a single cancer-causing principle. In particular, although both Rous sarcoma virus (RSV) and AMV could replicate in cultures of either embryonic fibroblasts or hematopoietic cells, RSV could transform only fibroblasts whereas AMV could transform only hematopoietic cells (Baluda, 1963; Durban and Boettiger, 1981a). Second, chickens infected with AMV develop remarkably high white counts and therefore their peripheral blood contains remarkably large quantities of viral particles (Beard, 1963). For this reason AMV was often used as a prototypic retrovirus in order to study viral assembly and later to produce large amounts of reverse transcriptase for both research and commercial purposes. Following the discovery of the v-src oncogene of RSV and the demonstration that it arose from the normal c-src proto-oncogene, a number of acute leukemia viruses were analysed by similar techniques and found to also contain viral oncogenes of cellular origin (Roussel et al., 1979). In the case of AMV, it was shown that almost the entire retroviral env gene had been replaced by a sequence of cellular origin (initially called mab or amv, but later renamed v-myb) (Duesberg et al., 1980; Souza et al., 1980). Remarkably, sequences contained in this myb oncogene were shared between AMV and the avian E26 leukemia virus, but were not contained in any other acutely transforming retroviruses. In addition, the E26 virus contained a second sequence of cellular origin (ets) that was unique. The E26 leukemia virus was first described in the 1960s and causes an acute erythroblastosis in chickens, more reminiscent of the disease caused by avian erythroblastosis virus (AEV) than by AMV (Ivanov et al., 1962).

The late elongated hypocotyl mutation of Arabidopsis disrupts circadian rhythms and the photoperiodic control of flowering

The dominant late elongated hypocotyl (lhy) mutation of Arabidopsis disrupted circadian clock regulation of gene expression and leaf movements and caused flowering to occur independently of photoperiod. LHY was shown to encode a MYB DNA-binding protein. In wild-type plants, the LHY mRNA showed a circadian pattern of expression with a peak around dawn but in the mutant was expressed constantly at high levels. Increased LHY expression from a transgene caused the endogenous gene to be expressed at a constant level, suggesting that LHY was part of a feedback circuit that regulated its own expression. Thus, constant expression of LHY disrupts several distinct circadian rhythms in Arabidopsis, and LHY may be closely associated with the central oscillator of the circadian clock.

Point mutations in v-Myb disrupt a cyclophilin-catalyzed negative regulatory mechanism

The c-Myb protein is controlled by intramolecular interactions, and point mutations can enhance its oncogenic activity. We tested whether conformational changes regulate c-Myb and found that Cyp-40, a widely distributed cyclophilin and peptidyl-prolyl isomerase, could inhibit c-Myb DNA binding activity. Inhibition by Cyp-40 required both its C-terminal protein-interaction domain, which bound specifically to c-Myb, and its N-terminal catalytic domain and was blocked by the competitive inhibitor cyclosporin A. Cyp-40 failed to bind or inhibit the oncogenic derivative v-Myb, which has a mutated Cyp-40 binding site. These results suggest that mutations in v-Myb allow it to evade a negative regulatory mechanism mediated by enzymes such as Cyp-40, and implicate peptidyl-prolyl isomerases in the regulation of transcription, transformation, and differentiation.

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.

Interaction of the co-activator CBP with Myb proteins: effects on Myb-specific transactivation and on the cooperativity with NF-M

The oncoprotein v-Myb is a potent inducer of myeloid leukemias, and its cellular homolog c-Myb plays a crucial role in the regulation of hematopoiesis. Both proteins function as transcriptional regulators. We demonstrate that this function is mediated at least in part by the nuclear co-activator CREB binding protein (CBP). This protein interacts directly with both c-Myb and v-Myb and potentiates Myb-specific transcription as measured on the mim-1 promoter. In contrast, dominant negative mutants of CBP lead to repression, as does E1A, an antagonist of CBP function. Phosphorylation of c-Myb does not appear to be required for interaction with CBP, thus indicating that the binding may be constitutive. Furthermore, the C/EBP family member NF-M, which cooperates with c-Myb in transactivating the mim-1 promoter through an adjacent DNA binding site, is co-activated by CBP in a Ras-dependent manner. Not only the individual activities of c-Myb and NF-M are stimulated by CBP, but also their synergistic transcriptional function, while it is negatively regulated by dominant negative forms of CBP. These data suggest that CBP is recruited by both Myb proteins and NF-M and potentiates their transcriptional activity. We suggest that CBP can bridge between c-Myb and NF-M, thus providing an explanation for the strong synergism between these two proteins.

Two distinct endothelial lineages in ontogeny, one of them related to hemopoiesis

We have shown previously by means of quail/chick transplantations that external and visceral organs, i.e., somatopleural and splanchnopleural derivatives, acquire their endothelial network through different mechanisms, namely immigration (termed angiogenesis) versus in situ emergence of precursors (or vasculogenesis). We have traced the distribution of QH1-positive cells in chick hosts after replacement of the last somites by quail somites (orthotopic grafts) or lateral plate mesoderm (heterotopic grafts). The results lead to the conclusion that the embryo becomes vascularized by endothelial precursors from two distinct regions, splanchnopleural mesoderm and paraxial mesoderm. The territories respectively vascularized are complementary, precursors from the paraxial mesoderm occupy the body wall and kidney, i.e., they settle along with the other paraxial mesoderm derivatives and colonize the somatopleure. The precursors from the two origins have distinct recognition and potentialities properties: endothelial precursors of paraxial origin are barred from vascularizing visceral organs and from integrating into the floor of the aorta, and are never associated with hemopoiesis; splanchnopleural mesoderm grafted in the place of somites, gives off endothelial cells to body wall and kidney but also visceral organs. It gives rise to hemopoietic precursors in addition to endothelial cells.

Myb and Ets proteins cooperate to transactivate an early myeloid gene

The earliest progenitor cell committed to the granulocyte/monocyte developmental pathway can be identified by the appearance of a 150-kDa glycoprotein on the cell surface (CD13/aminopeptidase N (CD13/APN), EC 3.4.11.2). A 455-base pair genomic fragment from the CD13/APN gene containing a Myb consensus-binding site as well as three potential Ets-binding sites was found to regulate tissue-appropriate expression of reporter genes in hematopoietic cell lines. Transactivation experiments with plasmids expressing either a full-length or truncated Myb protein and the full-length Ets-1 or Ets-2 protein demonstrated that these proteins cooperate to positively regulate CD13/APN gene expression. This cooperation is synergistic, as levels of transcriptional activity produced by Myb and Ets in combination were higher than those expected from a purely additive effect. Mutation of the Myb consensus-binding site completely abolished CD13/APN promoter activity in myeloid cells. Introduction of a dominant interfering Myb allele disrupted the ability of endogenous c-Myb in myeloid cells to transactivate the CD13/APN construct. Other myeloid cell-expressed Ets family members (PU.1, Fli-1, and Elf-1) failed to produce a cooperative transactivating effect when combined with the Myb expression construct. These data contrast with previous studies indicating that full-length c-Myb is unable to positively cooperate with Ets proteins in the regulation of myeloid genes. Because intact c-Myb and Ets-2 proteins, both endogenously expressed in myeloid cells, act synergistically to transactivate the CD13/APN promoter, this gene may represent a physiological target for dissection of the roles of these transcription factors in normal and malignant myelopoiesis.

Individual repeats of Drosophila Myb can function in transformation by v-Myb

The v-Myb protein binds to specific DNA sequences and can regulate gene expression. The DNA-binding domain of v-Myb contains the second and third of the three highly conserved tandem repeats found in c-Myb. In general, the ability of mutant forms of v-Myb to transform correlates with their ability to trans activate transcription. Two mutations within the DNA-binding domain of v-Myb which preserve DNA binding in vitro but fail to trans activate or transform have been described. These results suggested that this highly conserved domain might function in specific protein-protein interactions, as well as in DNA binding. We therefore tested the ability of a related protein domain from Drosophila melanogaster to substitute functionally for the homologous region of v-Myb. We found that either the second or third repeat of Drosophila Myb, but not both, could function in trans-activation and transformation by v-Myb. The hybrid containing both the second and third repeats of Drosophila Myb bound to DNA but failed to trans activate transcription either in the context of v-Myb or as a v-Myb-VP16 fusion protein. These results demonstrate that although the protein-DNA contacts made by the Myb repeats have been conserved during the evolution of animals, the protein-protein interactions have diverged.

Chicken "erythroid" cells transformed by the Gag-Myb-Ets-encoding E26 leukemia virus are multipotent

The E26 avian leukemia virus encodes a transcriptional activator-type oncoprotein consisting of Gag, Myb, and Ets domains, and transforms early erythroid cells as well as myeloblasts. Surprisingly, we have found that "early erythroid" transformants obtained in culture are multipotent, since they can be induced to differentiate into myeloblasts and eosinophils after superinfection with retroviruses containing kinase-type or ras oncogenes. In addition, TPA is an efficient inducer that generates predominantly eosinophils at low concentrations and myeloblasts at high concentrations. The determination process involves the complete extinction of erythroid/thrombocytic markers and the subsequent activation of myelomonocytic/eosinophilic properties, including the acquisition of specific growth factor requirements. "Erythroleukemic" cells from virus-infected animals were likewise found to be multipotent, making this a unique system to study the genesis of stem cell leukemias and the molecular basis of lineage commitment during hematopoiesis.

Molecular cloning, expression and in vitro functional characterization of Myb-related proteins in Xenopus

Two cDNAs encoding Myb-related proteins have been cloned from Xenopus laevis and they have been termed Xmyb1 and Xmyb2. The Xmyb1 cDNA clone codes for an open reading frame of 733 amino acids and exhibits a high degree of similarity over the entire predicted protein sequence with the human B-Myb protein. Xmyb2 is a partial cDNA clone encoding three copies of amino-terminal tandem repeat elements typical for the Myb DNA-binding domain. The predicted protein sequence is most closely related to the human A-Myb gene product. In vitro translation of two deletion mutants of Xmyb1, truncated in the 3'-portion of the open reading frame, results in protein products which cross-react with polyvalent as well as monoclonal antibodies directed against the human c-Myb protein. The same two XMyb1 proteins, which both contain the complete set of aminoterminal repeats, specifically bind to the c-Myb-specific DNA binding sequence as evidenced by electrophoretic mobility shift analysis in vitro. RNA expression profiles of Xmyb1 and -2 are very different from each other; Xmyb1 is present throughout oogenesis and early Xenopus embryogenesis; in adult tissue it is primarily detected in blood. In contrast, Xmyb2 is expressed at only very low levels during oogenesis, not detectable in embryonic RNA preparations, and in adult tissue it is predominantly expressed in testis, with only a very low level seen in blood.

Myb protein binds to multiple sites in the human T cell lymphotropic virus type 1 long terminal repeat and transactivates LTR-mediated expression

The members of the c-myb proto-oncogene family encode sequence-specific transcriptional activators. In T cells, expression of c-myb and the related B-myb gene is induced following mitogenic stimulation. Using a purified recombinant protein, we report here that the human T cell lymphotropic virus type 1 (HTLV-1) LTR contains six specific binding sites for Myb. We also show that HTLV-1 LTR chloramphenicol acetyl transferase reporter plasmids are specifically transactivated by c-Myb. These data suggest a role for members of the Myb family as a link between transcriptional activation of the HTLV-1 LTR and T cell activation events.

Acute myeloid leukemia induction by amphotropic murine retrovirus (4070A): clonal integrations involve c-myb in some but not all leukemias

Amphotropic murine retrovirus 4070A was demonstrated to be highly leukemogenic when inoculated intravenously into adult DBA/2 mice that were undergoing an intense chronic inflammatory response, but was nonleukemogenic in the absence of inflammation. The virus-induced promoonocytic leukemias, designated AMPH-ML, are similar morphologically and in cell surface marker expression to monocytic leukemias, called MML and MF-ML, previously shown to be induced by Moloney murine leukemia virus and MF-3 virus (a recombinant between Friend murine leukemia virus and Moloney murine leukemia virus) and resemble certain mature acute monocytic leukemias in humans (AML subtype M5). Approximately two-thirds of the AMPH-MLs (subgroup I) were demonstrated to have alterations in the 5' end of the c-myb locus, an event which occurs in 100% of MML and MF-ML. Data indicate that proviral insertions in AMPH-ML subgroup I resulted in aberrant c-myb mRNA expression and truncation of its translation product at the amino terminus. Approximately one-third of the AMPH-MLs (subgroup II) had not undergone any DNA rearrangements at the c-myb locus. In addition, their transcripts and protein products were of normal size. These latter leukemias also had not undergone DNA rearrangements in c-myc, although retroviruses expressing myc have previously been shown to induce monocyte-macrophage tumors in mice undergoing a chronic inflammation. That subgroup II leukemias had at least one clonal viral insertion suggests that there may be other sites in the cellular genome that can be activated by insertional mutagenesis in these murine acute monocytic leukemias.

Chromosomal reallocation of the chicken c-myb locus and organization of 3'-proximal coding exons

In the course of our studies concerning the tissue-specific expression of the c-myb proto-oncogene, we have established the nucleotide sequence of the chicken c-myb 3'-proximal coding exons. In situ hybridization performed with different genomic DNA probes corresponding to nearly all the c-myb gene allowed us to localize the corresponding locus on the large acrocentric chromosome 3 in chicken. Our sequencing data also indicate that the 3'-proximal noncoding sequences represented in c-myb mRNA species are derived from non-contiguous exons.

Myb DNA binding inhibited by phosphorylation at a site deleted during oncogenic activation

The c-Myb nuclear oncoprotein is phosphorylated in vitro and in vivo at an N-terminal site near its DNA-binding domain by casein kinase II (CK-II) or a CK-II-like activity. This in vitro phosphorylation reversibly inhibits the sequence-specific binding of c-Myb to DNA. The site of this phosphorylation is deleted in nearly all oncogenically activated Myb proteins, resulting in DNA-binding that is independent of CK-II. Because CK-II activity is modulated by growth factors, loss of the site could uncouple c-Myb from its normal physiological regulator.

Effects of 5' and 3' truncations of the myb gene on the transforming ability of avian myeloblastosis virus (AMV)

Proviruses based on the avian myeloblastosis virus (AMV) have been constructed which code for variations of the c-myb and/or v-myb gene product. These proviruses have been used in a soft colony agar assay to assess the contributions of the 5' and 3' deletions of the v-myb oncogene in the cellular transforming activity of the virus. The results indicate that 3' truncations are an integral part of the gene's mechanism of activation and that the truncations on the 5' end of the gene are important either in its mechanism of activation or its expression by viral control elements.

A temperature-sensitive phenotype of avian myeloblastosis virus: determinants that influence the production of viral mRNAs

The oncogene v-myb of avian myeloblastosis virus is expressed from an mRNA that arises by splicing of the viral genome. In previous work, we described a mutant strain of avian myeloblastosis virus (tsAMV) that elicits temperature-sensitive transformation and suggested that the mutation affects production of the mRNA for v-myb. We now report that the principal determinant of the biochemical phenotype of tsAMV is a point mutation located in a crucial region of the splice acceptor site for v-myb mRNA. The mutation reduces v-myb mRNA production but could account for the conditional phenotype only in combination with an independent effect of temperature on the splicing of both wild-type and mutant viral RNAs, which we also describe here. Our findings dramatize the manner in which retroviruses normally control the splicing of their RNAs and implicate the sequence of the splice acceptor site in the control.

Transcriptional activation by the v-myb oncogene and its cellular progenitor, c-myb

The v-myb oncogene, like its cellular progenitor c-myb, encodes a short-lived nuclear protein involved in processes affecting growth and differentiation in a number of cell types. Fusion proteins, in which v-myb sequences are linked to the DNA binding domain of the yeast transcriptional activator GAL4, can activate transcription from a reporter gene linked in cis to a GAL4 binding site. The domain of v-myb responsible for transcriptional activation is located between residues 204 and 254, and is both necessary and sufficient for activation. Intact v-myb and c-myb proteins can also activate transcription, via a myb binding site linked in cis to a reporter gene. A v-myb protein bearing a deletion in the activator domain is no longer capable of stimulating transcription.

Organization of 5'-proximal c-myb exons in chicken DNA. Implications for c-myb tissue-specific transcription

The organization of 5'-proximal c-myb exons in chicken DNA has been established by restriction enzyme mapping and nucleotide sequencing. Hybridization studies performed with cDNA probes revealed that yolk sac and thymic c-myb RNAs differ in their 5'-termini. A comparison of the genomic c-myb sequence with that of cDNAs isolated from normal thymic and lymphoma avian cells suggests that different promoter regions are used to initiate c-myb transcription in hematopoietic cells of different origins.

Generation of altered transcripts by retroviral insertion within the c-myb gene in two murine monocytic leukemias

Two murine monocytic leukemia cell lines, WEHI-265 and WEHI-274, were found to carry a rearranged c-myb gene. The rearrangements are due to insertion of a deleted Moloney murine leukemia virus (Mo-MLV) provirus in the 5' region of the c-myb gene and thus are similar to rearrangements in the ABPL tumors (G. L. C. Shen-Ong, M. Potter, J. F. Mushinski, S. Lavu, and E. P. Reddy, Science 226:1077-1080, 1984). In each cell line, the retroviral insertion has induced high levels of two aberrant RNA species, which, as in the ABPL tumors (G. L. C. Shen-Ong, H. C. Morse, M. Potter, and J. F. Mushinski, Mol. Cell. Biol. 6:380-392, 1986), contain both viral (Mo-MLV) and cellular (myb) sequences. Both species lack the sequences encoding the amino terminus of the c-myb protein and thus could encode a protein which, like the v-myb gene products (and the predicted ABPL myb proteins), is truncated at the amino terminus. We have found that the larger (5.3 kilobase [kb]) and more abundant of the tumor-specific myb RNAs was predominantly nuclear, while the smaller species (3.9 kb) was cytoplasmic. Furthermore, our data imply that the 3.9-kb RNA was derived from the 5.3-kb RNA by an additional splice which utilized a cryptic splice acceptor site within the viral gag sequences. On the basis of subcellular distribution and predicted translational potential, we conclude that the 3.9-kb RNA is probably the mRNA which encodes a truncated myb protein. We also show that, due to different insertion points in W265 and W274, the W274 myb RNAs contained sequences from a c-myb exon upstream of the exons represented in the W265 (and ABPL) RNAs. The significance of our findings with regard to transformation by myb in these tumors is discussed.