Transduction of c-myb into avian myeloblastosis virus: locating points of recombination within the cellular gene

Identification and in silico structural analysis of Gallus gallus protein arginine methyltransferase 4 (PRMT4)

Protein arginine methyltransferase 4 (PRMT4) is an essential epigenetic regulator of fundamental and conserved processes during vertebrate development, such as pluripotency and differentiation. Surprisingly, PRMT4 homologs have been identified in nearly all vertebrate classes except the avian genome. This raises the possibility that in birds PRMT4 functions are taken over by other PRMT family members. Here, we reveal the existence of a bona fidePRMT4 homolog in the chicken, Gallus gallus. Using a biochemical approach, we initially purified a putative chicken PRMT4 protein and thus provided the first evidence for the presence of an endogenous PRMT4‐specific enzymatic activity toward histone H3 arginine 17 (H3R17) in avian cells. We then isolated a G. gallus PRMT4 (ggPRMT4) transcript encompassing the complete open reading frame. Recombinant ggPRMT4 possesses intrinsic methyltransferase activity toward H3R17. CRISPR/Cas9‐mediated deletion of ggPRMT4 demonstrated that the transcript identified here encodes avian PRMT4. Combining protein–protein docking and homology modeling based on published crystal structures of murine PRMT4, we found a strong structural similarity of the catalytic core domain between chicken and mammalian PRMT4. Strikingly, in silico structural comparison of the N‐terminal Pleckstrin homology (PH) domain of avian and murine PRMT4 identified strictly conserved amino acids that are involved in an interaction interface toward the catalytic core domain, facilitating for the first time a prediction of the relative spatial arrangement of these two domains. Our novel findings are particularly exciting in light of the essential function of the PH domain in substrate recognition and methylation by PRMT4.

A dual activation mechanism for Myb-responsive genes in myelomonocytic cells

The retroviral oncogene v-myb encodes a transcription factor (v-Myb) which is responsible for the transformation of myelomonocytic cells by avian myeloblastosis virus (AMV). v-Myb is thought to exert its biological effects by deregulating the expression of specific target genes. Here we have used DNaseI hypersensitive site mapping and reporter gene assays to study the activation of three Myb target genes--mim-1, the lysozyme gene and the C/EBPbeta gene--all of which are activated by Myb in myelomonocytic cells but not in other hematopoietic lineages. We have found that these genes are activated by Myb via more than one cis-regulatory region. Our data suggest that all three genes are activated by Myb by dual mechanisms involving the promoters as well as enhancers. Using a cell line that expresses an estrogen-inducible v-Myb/estrogen receptor fusion protein we have also determined the effect of Myb on the expression of the C/EBPalpha gene. Our results show that C/EBPalpha expression is down-regulated by v-Myb. Thus, v-Myb affects the expression of two C/EBP family members in opposite directions.

Disruption of B-myb in DT40 cells reveals novel function for B-Myb in the response to DNA-damage

B-Myb is a highly conserved vertebrate member of the Myb transcription factor family, which is expressed in virtually all proliferating cells. A large body of evidence suggests that B-Myb plays an important role in cell cycle regulation; however, the exact nature of its function has not yet been clarified. We have used gene targeting in chicken DT40 cells, a cell line exhibiting very high rates of homologous recombination, to create cells expressing endogenous B-myb in a doxycyclin-dependent manner. We find that the cells proliferate well in the absence of B-Myb, suggesting that B-Myb is not essential for cell proliferation per se. However, cells lacking B-Myb are more sensitive to DNA-damage induced by UV-irradiation and alkylation. Our work provides the first direct evidence for a novel function of B-Myb in the response to DNA-damage. The cells described here should be a useful model to characterize this function in more detail.

Identification and characterization of the Myb-inducible promoter of the chicken adenosine receptor 2B gene

Numerous studies have shown that the retroviral oncogene v-myb encodes a transcription factor (v-Myb) which interferes with the differentiation program of myelomonocytic cells. It is generally thought that v-Myb deregulates the expression of specific target genes and thereby causes transformation of these cells. By using an estrogen-inducible version of v-Myb we have previously identified the gene for the chicken A2B adenosine receptor (A2B-AR), a member of the seven-pass transmembrane receptor superfamily, as a bona fide target gene for v-Myb. The chicken A2B-AR gene is expressed in v-myb transformed myeloblasts as well as in c-myb expressing erythroblasts, offering the opportunity to study how Myb transcription factors activate a target gene in two different hematopoietic lineages. Here, we report the characterization of the promoter of the A2B-AR gene. We show that the A2B-AR promoter region contains an exceptionally large number of Myb binding sites, many of which contribute to the Myb-inducibility of the promoter. The same sites were required for promoter activity in myelomonocytic and erythroid cells. In contrast to the promoters of other Myb target genes the A2B-AR promoter was not activated synergistically by Myb and other lineage-specific transcription factors that have been identified as Myb cooperation partners before. Taken together, our data suggest that the activation of the A2B-AR promoter by Myb depends on the simultaneous binding of a large number of Myb molecules.

Identification of the myb-inducible promoter of the chicken Pdcd4 gene

The retroviral oncogene v-myb encodes a transcription factor (v-Myb) which disrupts the myelomonocytic differentiation program and transforms myelomonocytic cells in vivo and in vitro. It is thought that v-Myb exerts its biological effects by deregulating the expression of specific target genes, most of which are still unknown. c-myb, the cellular progenitor of v-myb, is expressed in all immature hematopoietic cells and is presumed to regulate the expression of genes that are essential for the development of the hematopoietic system. Recently, we have identified the chicken Pdcd4 gene as a novel v-myb target gene. Pdcd4 has originally been identified in a screen for genes upregulated in apoptotic cells and, more recently, has been implicated in tumor progression. As a myb-regulated gene Pdcd4 is of interest because unlike most other myb target genes it is expressed in a broad spectrum of hematopoietic cells. As a first step to study the regulation of Pdcd4 expression in more detail, we here report the identification and preliminary characterization of the myb-inducible promoter of the Pdcd4 gene.

The v-Myb oncoprotein activates C/EBPbeta expression by stimulating an autoregulatory loop at the C/EBPbeta promoter

Previous studies have implicated the CCAAT box/enhancer binding protein beta (C/EBPbeta) in the regulation of cell-type specific gene expression in myelomonocytic cells and in the activation of target genes by the transcription factor v-Myb. To better understand the role of C/EBPbeta in myelomonocytic cells we have cloned the chicken C/EBPbeta gene and studied its regulation. The chicken C/EBPbeta promoter contains a number of C/EBP binding sites and is activated by C/EBPbeta, suggesting that the C/EBPbeta gene is autoregulated by its own protein product. Interestingly, the C/EBPbeta promoter is not activated by C/EBPalpha, another C/EBP family member highly expressed in myelomonocytic cells, indicating that the autoregulation is specific for C/EBPbeta. Comparison of different C/EBP inducible promoters shows that the relative transactivation potential of C/EBPalpha and beta is extremely dependent on the promoter context. By using the promoters of the mim-1 and C/EBPbeta genes and by exchanging the DNA-binding domains between C/EBPalpha and beta we show that the observed promoter preferences of C/EBPalpha and beta are not due to differential DNA-binding but instead depend on the transactivation domains of these proteins. The C/EBPbeta promoter also contains several Myb binding motifs, suggesting that the C/EBPbeta gene is also myb-inducible. We show that the C/EBPbeta promoter is activated synergistically by v-Myb and C/EBPbeta and that transcription of the endogenous C/EBPbeta gene is increased by v-Myb. Thus, our results identify the C/EBPbeta gene as a novel v-Myb target gene. Taken together, our data suggest a model for the regulation of C/EBPbeta expression in which v-Myb stimulates the synthesis of C/EBPbeta by enhancing an autoregulatory loop acting on the C/EBPbeta promoter.

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).

Modulation of c-Myb-induced transcription activation by a phosphorylation site near the negative regulatory domain

The c-myb protooncogene encodes a highly conserved transcription factor that functions as both an activator and a repressor of transcription. The v-myb oncogenes of E26 leukemia virus and avian myeloblastosis virus encode proteins that are truncated at both the amino and the carboxyl terminus, deleting portions of the c-Myb DNA-binding and negative regulatory domains. This has led to speculation that the deleted regions contain important regulatory sequences. We previously reported that the 42-kDa mitogen-activated protein kinase (p42mapk) phosphorylates chicken and murine c-Myb at multiple sites in the negative regulatory domain in vitro, suggesting that phosphorylation might provide a mechanism to regulate c-Myb function. We now report that three tryptic phosphopeptides derived from in vitro phosphorylated c-Myb comigrate with three tryptic phosphopeptides derived from metabolically labeled c-Myb immunoprecipitated from murine erythroleukemia cells. At least two of these peptides are phosphorylated on serine-528. Replacement of serine-528 with alanine results in a 2- to 7-fold increase in the ability of c-Myb to transactivate a Myb-responsive promoter/reporter gene construct. These findings suggest that phosphorylation serves to regulate c-Myb activity and that loss of this phosphorylation site from the v-Myb proteins may contribute to their transforming potential.

Protein truncation is not required for c-myb proto-oncogene activity in neuroretina cells

The v-myb oncogene of avian myeloblastosis virus (AMV) differs from its normal cellular counterpart by a truncation at both its amino and carboxyl termini and by a substitution of 11 amino acid residues. We had previously shown that v-myb-containing AMV, in the presence of basic fibroblast growth factor, transformed chicken neuroretina (CNR) cells. To understand the mechanism of c-myb activation, we have tested whether avian retroviruses that express the full-length c-Myb are also active on CNR cells. We have found that c-Myb, like v-Myb, strongly increases the basic fibroblast growth factor response of CNR cells and that these c-myb-expressing cells are able to grow in soft agar in the presence of the growth factor. We have also found that, in contrast to normal or v-myb-expressing AMV-transformed CNR cells, c-Myb-transformed cells express mim-1, a granulocyte-specific gene. However, normal v-Myb- and c-Myb-expressing CNR cells all express the pax-QNR gene, a newly described paired and homeobox-containing gene specifically expressed in the neuroretina. We conclude that, in contrast to what has been described for hematopoietic cells, overexpression of c-Myb is sufficient to activate gene expression and to induce an abnormal behavior of CNR cells.

Cloning and nucleotide sequence of the 5' part of v-myb cDNA

cDNA of a subgenomic v-myb mRNA from AMV-transformed BM-2 cells was cloned. Sequencing of the 5' end of this cDNA revealed the structure of both AMV leader and the splice junction in v-myb message. The leader is a novel variant of known avian retrovirus leaders. The long open reading frame in the cloned cDNA starts with six gag-derived codons spliced to the myb-specific sequence.

Definition of functional domains in P135gag-myb-ets and p48v-myb proteins required to maintain the response of neuroretina cells to basic fibroblast growth factor

The v-myb- and v-ets-containing E26 retrovirus induces the proliferation of chicken neuroretina (CNR) cells in minimal medium. Proliferation of E26 CNR cells is strongly stimulated by basic fibroblast growth factor (bFGF). The v-myb-containing avian myeloblastosis virus also induces the proliferation of infected CNR cells stimulated by bFGF. Both E26 CNR and avian myeloblastosis virus CNR cells are able to form colonies in soft agar in the presence of bFGF. This suggests that the v-myb product, a nuclear sequence-specific DNA-binding protein which activates gene expression in transient transfection assays, plays a role in the proliferative response of the infected CNR cells. To determine the structure-function relationships of P135gag-myb-ets and p48v-myb, we have used deletion mutants expressed in retroviral vectors and have analyzed their effect on CNR cell proliferation as well as their effect on the CNR cell response to bFGF. We show that v-ets is not required for bFGF stimulation but increases the proliferation of CNR cells in minimal medium. In the v-myb mutants, the gag sequences derived from the helper virus increase the potency of the myb gene. The carboxyl-terminal domain required for the growth and transformation of myeloid cells and needed for maximal trans-activation in transient DNA transfection assays in fibroblasts was not required for the growth and bFGF response of CNR cells. In contrast, the domain encompassing amino acids 240 to 301 (containing part of the transcriptional activation domain of v-myb) was absolutely required for the response of CNR cells to bFGF and could be functionally replaced by the carboxyl-terminal transcriptional activation domain of the VP16 protein of herpes simplex virus.

Retroviral activation of a novel gene encoding a zinc finger protein in IL-3-dependent myeloid leukemia cell lines

Normal hematopoietic stem cells proliferate and differentiate in the presence of growth factors such as interleukin-3 (IL-3). Transformation can alter their growth factor requirements, the ability of the cells to differentiate, or both. To identify genes that are capable of transforming hematopoietic cells, IL-3-dependent cell lines, isolated from retrovirus induced myeloid leukemias, were examined for viral insertions in proto-oncogenes and in common sites of viral integration. Five of 37 cell lines contained proviruses in a common viral integration site termed the ecotropic virus integration 1 site (Evi-1). The integrations were correlated with the activation of transcription from the locus. Sequencing of cDNA clones and genomic clones demonstrated that the integrations had occurred near or in 5' noncoding exons of a novel gene. The sequence of the cDNA clones predicts that the gene product is a 120 kd protein that contains two domains with seven and three repeats of a DNA binding consensus sequence (zinc finger) initially described in the Xenopus transcription factor III A (TFIIIA). This represents the first demonstration of the retroviral activation of a gene encoding a zinc finger protein and the first implication for a member of this gene family in the transformation of hematopoietic cells.

Structural and functional domains of the myb oncogene: requirements for nuclear transport, myeloid transformation, and colony formation

The v-myb oncogene of avian myeloblastosis virus causes acute myelomonocytic leukemia in vivo and transforms only myeloid cells in vitro. Its product, p48v-myb, is a nuclear protein of unknown function. To determine structure-function relationships for this protein, we constructed a series of deletion mutants of v-myb, expressed them in retroviral vectors, and studied their biochemical and biological properties. We used these mutants to identify two separate domains of p48v-myb which had distinct roles in its accumulation in the cell nucleus. We showed that the viral sequences which normally encode both termini of p48v-myb were dispensible for transformation. In contrast, both copies of the highly conserved v-myb amino-terminal repeat were required for transformation. We also identified a carboxyl-terminal domain of p48v-myb which was required for the growth of v-myb-transformed myeloblasts in soft agar but not for morphological transformation.

v-myb dominance over v-myc in doubly transformed chick myelomonocytic cells

Chick myelomonocytic cells transformed by the v-myc oncogene resemble mature macrophages; those transformed by v-myb or v-myb,ets exhibit an immature phenotype. We have analyzed whether these oncogenes are capable of altering the differentiation phenotype of transformed cells by introducing both v-myc plus either v-myb or v-myb,ets into the same cells. Surprisingly, the doubly transformed cells were found to be essentially indistinguishable from cells transformed by v-myb or v-myb,ets alone even when they expressed a high level of v-myc protein. These results demonstrate that v-myb is dominant over v-myc and that, while v-myc induces cell proliferation without affecting differentiation, v-myb induces in the same target cells both proliferation and a block or reversal of differentiation.

The mechanism of transduction of proto-oncogene c-src by avian retroviruses

Chicken c-src sequences have been transduced by avian leukosis viruses (ALV) and by partial src-deletion (td) mutants of Rous sarcoma virus in several independent events. Analyses of the recombination junctions in the genomes of src-containing viruses and the c-src DNA have shed light on the mechanism of transduction, which involves at least two steps of recombination. The initial recombination between a viral genome and the 5' region of c-src appears to occur at the DNA level. This step does not require extensive homology and can be mediated by stretches of sequences with only partial homology. The 5' recombination junction can also be formed by splicing between viral and c-src sequences. The second recombination is presumed to occur between the transducing ALV or td viral RNA and the viral-c-src hybrid RNA molecule generated from the initial recombination. This step involving recombination at the 3' ends of those molecules restores the 3' viral sequences essential for replication to the viral-c-src hybrid molecule. High frequency of c-src transduction by partial td mutants suggests that the second recombination is greatly enhanced when there is sequence homology between the transducing virus and the 3' region of c-src. Incorporation of the c-src sequences into an ALV genome results in greatly elevated expression of the gene. However, increased expression of c-src alone is insufficient to activate its transforming potential. Structural changes in c-src are necessary to convert it into a transforming gene. The changes can be as small as single nucleotide changes resulting in single amino aid substitutions at certain positions. Mutations can occur rapidly during viral replication after c-src is incorporated into the viral genome. Therefore, it is most likely that transduction of c-src by ALV is followed by subsequent mutation and selection for the sarcomagenic virus. In the case of transduction by td viruses that retain certain src sequences, joining of these sequences with the transduced c-src apparently is sufficient to activate its transforming potential.

env-encoded residues are not required for transformation by p48v-myb

The v-myb oncogene of avian myeloblastosis virus induces acute myeloblastic leukemia in chickens and transforms avian myeloid cells in vitro. The protein product of this oncogene, p48v-myb, is partially encoded by the retroviral gag and env genes. We demonstrated that the env-encoded carboxyl terminus of p48v-myb is not required for transformation. Our results showed, in addition, that a coding region of c-myb which is not essential for transformation was transduced by avian myeloblastosis virus.

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

The retroviral oncogene v-myb arose by transduction of the chicken proto-oncogene c-myb. We isolated and sequenced cDNA that represents the entire coding domain of chicken c-myb. By transcribing the cDNA into mRNA in vitro and then translating the RNA, we were able to document the integrity of the cDNA and to identify the codon responsible for initiation of translation from c-myb. Two different alleles of v-myb are extant, one in the genome of avian myeloblastosis virus (AMV) and the other in the genome of erythroblastosis virus 26 (E26V). The proteins encoded by the AMV and E26V alleles of v-myb differ from the product of c-myb in three ways: at their amino termini, they lack 71 and 80 amino acids respectively; at their carboxy termini, they are deficient in 199 and 278 residues; and 11 substitutions of amino acids are scattered throughout the product of AMV allele, whereas the product of the E26V allele contains only a single substitution. The structural origins of tumorigenicity by v-myb and the biological functions of c-myb remain enigmatic. The findings and molecular clones described here should now permit a systematic exploration of these enigmas.

Two modes of c-myb activation in virus-induced mouse myeloid tumors

Two modes of disruption of the protooncogene c-myb by viral insertional mutagenesis in mouse myeloid tumor cells are described. The first mode was found in six tumors in which a Moloney murine leukemia virus component had inserted in the same transcriptional orientation upstream of the 5'-most exon with v-myb homology (vE1). cDNA sequence data indicate the presence of a truncated c-myb mRNA that is initiated in the upstream 5' long terminal repeat of the integrated provirus and processed via a cryptic splice donor sequence in the gag region to the splice acceptor site in vE1 of the c-myb gene, thus removing the remaining downstream viral and myb intronic sequences. Unlike most gag-onc transcripts, the gag and myb sequences in the hybrid transcript were not in the same reading frame. It is presumed that the gag sequence provides a cryptic translation initiation site for the novel amino-truncated c-myb protein. The second mode of disruption was by downstream virus insertion at the 3' side of the c-myb, which results in the synthesis of a small (approximately 2 kilobase) myb transcript. The 5' long terminal repeat of the inserted provirus provides a TGA termination codon that results in the elimination of 240 normal c-myb amino acid residues from the carboxyl terminus of the tumor-specific myb protein. These results suggest that truncated myb proteins play a role in neoplastic transformation of myeloid cells.

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

The retroviral oncogene v-myb arose by transduction of the chicken proto-oncogene c-myb. We isolated and sequenced cDNA that represents the entire coding domain of chicken c-myb. By transcribing the cDNA into mRNA in vitro and then translating the RNA, we were able to document the integrity of the cDNA and to identify the codon responsible for initiation of translation from c-myb. Two different alleles of v-myb are extant, one in the genome of avian myeloblastosis virus (AMV) and the other in the genome of erythroblastosis virus 26 (E26V). The proteins encoded by the AMV and E26V alleles of v-myb differ from the product of c-myb in three ways: at their amino termini, they lack 71 and 80 amino acids respectively; at their carboxy termini, they are deficient in 199 and 278 residues; and 11 substitutions of amino acids are scattered throughout the product of AMV allele, whereas the product of the E26V allele contains only a single substitution. The structural origins of tumorigenicity by v-myb and the biological functions of c-myb remain enigmatic. The findings and molecular clones described here should now permit a systematic exploration of these enigmas.

Nuclear compartmentalization of the v-myb oncogene product

Nuclei obtained from chicken leukemic myeloblasts transformed by avian myeloblastosis virus were fractionated into various subnuclear compartments, which were then analyzed by specific immunoprecipitation for the presence of the leukemogenic product, p48v-myb, of the viral oncogene. In cells labeled for 30 or 60 min with L-[35S]methionine and in unlabeled exponentially dividing leukemic cells analyzed by Western blotting, p48v-myb was detected within the nucleoplasm (29 +/- 9% [standard deviation] of the total), chromatin (7 +/- 4%), and lamina-nuclear matrix (64 +/- 9%). Also, in myeloblasts analyzed by immunofluorescence during mitosis, p48v-myb appeared to be dispersed through the cell like the lamina-nuclear matrix complex. Strong attachment to the nuclear matrix-lamina complex suggests that p48v-myb may be involved in DNA replication or transcription or both.

Transduction of c-src coding and intron sequences by a transformation-defective deletion mutant of Rous sarcoma virus

The mechanism of cellular src (c-src) transduction by a transformation-defective deletion mutant, td109, of Rous sarcoma virus was studied by sequence analysis of the recombinational junctions in three td109-derived recovered sarcoma viruses (rASVs). Our results show that two rASVs have been generated by recombination between td109 and c-src at the region between exons 1 and 2 defined previously. Significant homology between td109 and c-src sequences was present at the sites of recombination. The viral and c-src sequence junction of the third rASV was formed by splicing a cryptic donor site at the 5' region of env of td109 to exon 1 of c-src. Various lengths of c-src internal intron 1 sequences were incorporated into all three rASV genomes, which resulted from activation of potential splice donor and acceptor sites. The incorporated intron 1 sequences were absent in the c-src mRNA, excluding its being the precursor for recombination with td109 and implying that initial recombinations most likely took place at the DNA level. A potential splice acceptor site within the incorporated intron 1 sequences in two rASVs was activated and was used for the src mRNA synthesis in infected cells. The normal env mRNA splice acceptor site was used for src mRNA synthesis for the third rASV.

Expression of molecular clones of v-myb in avian and mammalian cells independently of transformation

We demonstrated that molecular clones of the v-myb oncogene of avian myeloblastosis virus (AMV) can direct the synthesis of p48v-myb both in avian and mammalian cells which are not targets for transformation by AMV. To accomplish this, we constructed dominantly selectable avian leukosis virus derivatives which efficiently coexpress the protein products of the Tn5 neo gene and the v-myb oncogene. The use of chemically transformed QT6 quail cells for proviral DNA transfection or retroviral infection, followed by G418 selection, allowed the generation of cell lines which continuously produce both undeleted infectious neo-myb viral stocks and p48v-myb. The presence of a simian virus 40 origin of replication in the proviral plasmids also permitted high-level transient expression of p48v-myb in simian COS cells without intervening cycles of potentially mutagenic retroviral replication. These experiments establish that the previously reported DNA sequence of v-myb does in fact encode p48v-myb, the transforming protein of AMV.

Transforming potential of the v-myb oncogene from avian myeloblastosis virus: alterations in the oncogene product may reveal a new target specificity

Transfection of brown leghorn chicken embryo fibroblasts by DNA containing v-myb sequences cloned either in a complete AMV proviral DNA or in a retroviral derived vector has led to the isolation of two kinds of transformed cells. A characterization of the proviral sequences retained and expressed in these transformed cells revealed that they contained either new or altered v-myb-related RNA species. The experiments presented in this paper also show that both types of transformants expressed truncated myb-related polypeptides, suggesting that alterations of the v-myb product may result in a new target specificity, leading to the transformation of chicken embryo fibroblasts.

Antibodies to the evolutionarily conserved amino-terminal region of the v-myb-encoded protein detect the c-myb protein in widely divergent metazoan species

Antibodies directed against a bacterial fusion protein that contains the domain encoded by the highly evolutionarily conserved 5' one-third of the v-myb oncogene of avian myeloblastosis virus (AMV) detect the protein products of various members of the myb gene family. Immunoprecipitation or immunoblot analyses with these antibodies yielded the following information. First, the products of the v-myb oncogenes of AMV (p48v-myb) and of E26 virus (p135gag-myb-ets) contain this highly conserved amino acid sequence, as previously hypothesized. Second, p75c-myb, the product of the chicken c-myb protooncogene, also contains this protein domain. Third, these antibodies have identified the products of the human, murine, and Drosophila c-myb genes, which were all found to be nuclear proteins of Mr 75,000-80,000. The human c-myb protein product is present in immature cells of the erythroid, myeloid, and lymphoid lineages.

A cryptic transcription promoter in the myb oncogene of avian myeloblastosis virus

The potential regulatory signals contained in the v-myb oncogene of avian myeloblastosis virus have been inserted upstream to the herpes simplex type 1 thymidine kinase gene in order to test their promoter activity. The isolation of TK+ transformants after transfection of clone 1D(TK-) mouse cells with the resulting recombinant DNAs indicated that the expression of the TK gene was made possible by the myb-derived sequences. Analysis of the TK specific RNA expressed in different TK+ transformants revealed that the regulatory signals contained in v-myb correspond to a weak functional promoter.

Two modes of c-myb activation in virus-induced mouse myeloid tumors

Two modes of disruption of the protooncogene c-myb by viral insertional mutagenesis in mouse myeloid tumor cells are described. The first mode was found in six tumors in which a Moloney murine leukemia virus component had inserted in the same transcriptional orientation upstream of the 5'-most exon with v-myb homology (vE1). cDNA sequence data indicate the presence of a truncated c-myb mRNA that is initiated in the upstream 5' long terminal repeat of the integrated provirus and processed via a cryptic splice donor sequence in the gag region to the splice acceptor site in vE1 of the c-myb gene, thus removing the remaining downstream viral and myb intronic sequences. Unlike most gag-onc transcripts, the gag and myb sequences in the hybrid transcript were not in the same reading frame. It is presumed that the gag sequence provides a cryptic translation initiation site for the novel amino-truncated c-myb protein. The second mode of disruption was by downstream virus insertion at the 3' side of the c-myb, which results in the synthesis of a small (approximately 2 kilobase) myb transcript. The 5' long terminal repeat of the inserted provirus provides a TGA termination codon that results in the elimination of 240 normal c-myb amino acid residues from the carboxyl terminus of the tumor-specific myb protein. These results suggest that truncated myb proteins play a role in neoplastic transformation of myeloid cells.

Nuclear compartmentalization of the v-myb oncogene product

Nuclei obtained from chicken leukemic myeloblasts transformed by avian myeloblastosis virus were fractionated into various subnuclear compartments, which were then analyzed by specific immunoprecipitation for the presence of the leukemogenic product, p48v-myb, of the viral oncogene. In cells labeled for 30 or 60 min with L-[35S]methionine and in unlabeled exponentially dividing leukemic cells analyzed by Western blotting, p48v-myb was detected within the nucleoplasm (29 +/- 9% [standard deviation] of the total), chromatin (7 +/- 4%), and lamina-nuclear matrix (64 +/- 9%). Also, in myeloblasts analyzed by immunofluorescence during mitosis, p48v-myb appeared to be dispersed through the cell like the lamina-nuclear matrix complex. Strong attachment to the nuclear matrix-lamina complex suggests that p48v-myb may be involved in DNA replication or transcription or both.

Sites of recombination between the transforming gene of avian myeloblastosis virus and its helper virus

The sites of recombination between the transforming gene of avian myeloblastosis virus (AMV) and its natural helper myeloblastosis-associated virus (MAV) have been determined. In AMV, the cellular sequence substituting for the viral envelope (env) gene gives rise to a different carboxyl terminus of the DNA polymerase. The 5'-recombination site coincides with the RNA splice acceptor site for the production of env mRNA in MAV-infected cells. The 3'-recombination site reveals that the last 11 amino acids including the termination codon are shared by the env protein and AMV transforming protein. The RNA splice acceptor site for the generation of subgenomic v-myb mRNA is located 84 nucleotides downstream from the 5'-recombination site. The AMV transforming protein consists of helper virus-related sequences at both of its amino and carboxyl termini, and all but 84 nucleotides of the cell-derived v-myb sequence. The comparison of MAV gp85 amino acid sequence with those of subgroups B, C, and E indicates that the MAV present in clone lambda 10A2-1 belongs to subgroup B. The high degree of homology among different avian retroviruses of the same subgroup indicates that the amino acid sequence of gp85 is important in determining the conformation of the envelope glycoprotein.

Transformation of Brown Leghorn chicken embryo fibroblasts by avian myeloblastosis virus proviral DNA

Brown Leghorn chicken embryo fibroblasts were transfected with a mixture of avian myeloblastosis virus (AMV) and myeloblastosis-associated virus type 1 (MAV1) proviral DNA purified from lambda-Charon 4A recombinant clones. A transformed cell line (T1AM) able to grow without anchorage in semisolid medium was obtained. The presence of both proviral AMV and MAV sequences was detected in T1AM DNA by hybridization with v-myb- and MAV1-specific probes. Altered AMV and MAV1 proviral genomes were found in T1AM genome. Characterization of the RNA species expressed in transformed cells showed that in addition to a 2.5-kilobase (kb) putative subgenomic v-myb-specific RNA, three other myb-containing RNAs (9.4, 8.4, and 7.0 kb) were present in T1AM cells. No AMV genomic RNA was detected. Also, a new 5.0-kb MAV1-specific RNA species was expressed in transformed cells in addition to MAV1 genomic RNA species (7.8 kb). No infectious AMV virions are released by T1AM cells. Chicken embryo fibroblasts infected by T1AM-released virions contained and expressed all MAV1 sequences detected in T1AM transformed cells but did not express any transformation parameter. These results indicated that the presence of AMV proviral sequences in T1AM cells is responsible for their transformed phenotype.