Characterization of the v-myb DNA binding domain

Oncogenic point mutations in the Myb DNA-binding domain alter the DNA-binding properties of Myb at a physiological target gene

The oncoprotein v-Myb of avian myeloblastosis virus (AMV) transforms myelomonocytic cells by deregulating specific target genes. Previous work has shown that the oncogenic potential of v-Myb was activated by truncation of N- and C-terminal sequences of c-Myb and was further increased by amino acid substitutions in the DNA-binding domain and other parts of the protein. We have analyzed the activation of the chicken lysozyme gene which is strongly activated by c-Myb but not by its oncogenic counterpart v-Myb. We report that Myb acts on two different cis-regulatory elements, the promoter and an enhancer located upstream of the gene. Interestingly, the activation of the enhancer was abolished by the oncogenic amino acid substitutions. We demonstrated that a single Myb-binding site is responsible for the activation of the lysozyme enhancer by Myb and showed that the v-Myb protein of AMV was unable to bind to this site. Our data demonstrate for the first time that oncogenic activation of Myb alters its DNA-binding specificity at a physiological Myb target gene.

The promoter regions of the Myb-regulated Adora2B and Mcm4 genes co-localize with origins of DNA replication

Background

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. We have recently demonstrated that the chicken Gas41 gene, whose promoter co-localizes with an origin of DNA replication, is a bona fide Myb target gene. Because of this finding we have asked whether other Myb-regulated genes are also associated with DNA replication origins.

Results

We show that the promoter region of the chicken adenosine receptor 2B gene (Adora2B), a known Myb-target gene, acts as a DNA replication origin. Furthermore, we have examined known replication origins for the presence of Myb binding sites. We found that the intergenic region between the genes for the minichromosome maintenance 4 protein (Mcm4) and the catalytic subunit of DNA-dependent protein kinase (Prkdc), whose human counterpart has been identified as a replication origin, contains a number of Myb binding sites. Our data show that this region also acts as an origin of replication in chicken cells. Interestingly, we found that the chicken Mcm4 gene is also Myb-regulated.

Conclusion

Our work identifies the chicken Mcm4 gene as a novel Myb target gene and presents evidence for the co-localization of two novel origins of DNA replication with Myb-regulated genes. Our work raises the possibility that a fraction of Myb target gene promoters is associated with DNA replication origins.

v-Myb mediates cooperation of a cell-specific enhancer with the mim-1 promoter

The oncogenic transcription factor v-Myb disrupts myelomonocytic differentiation and transforms myelomonocytic cells by deregulating the expression of specific target genes. One of these genes, the chicken mim-1 gene, is activated by Myb exclusively in myelomonocytic cells and, therefore, has been an interesting model system to study how Myb activates a target in a lineage-specific manner. Previous work has suggested that Myb activates mim-1 by cooperating with CCAAT box/enhancer binding protein beta (C/EBPbeta) or other C/EBP transcription factors at the mim-1 promoter. We have now identified and characterized a powerful Myb-dependent enhancer located 2 kb upstream of the mim-1 promoter. The enhancer is preferentially active in myelomonocytic cells, confers Myb responsiveness onto a heterologous promoter, and dramatically increases Myb responsiveness of the mim-1 promoter. Chromatin immunoprecipitation demonstrates that v-Myb and C/EBPbeta are bound to the enhancer in v-Myb-transformed cells; furthermore, cooperation of the enhancer with the mim-1 promoter is greatly stimulated by C/EBPbeta and p300. Taken together, our results show that the regulation of mim-1 expression by v-Myb is more complex than previously assumed and involves two distinct regions of the mim-1 gene. A major function of v-Myb (in addition to its role at the mim-1 promoter) apparently is to activate the mim-1 enhancer and, together with C/EBPbeta and p300, facilitate its cooperation with the promoter. Interestingly, our work also shows that the v-Myb protein encoded by avian myeloblastosis virus is defective in this function, suggesting an explanation for why primary avian myeloblastosis virus-transformed myeloblasts do not express the mim-1 gene.

The glioma-amplified sequence 41 gene (GAS41) is a direct Myb target gene

The retroviral oncogene v-myb encodes a transcription factor (v-Myb) which 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. The chicken glioma-amplified sequence 41 gene (GAS41) is located immediately downstream of the lysozyme gene, a known Myb-regulated gene. The GAS41 promoter colocalizes with a CpG island which also functions as an origin of replication. Since the GAS41 promoter contains several potential Myb-binding sites (MBSs) we have investigated whether GAS41 is a v-Myb target gene. Our results show that the GAS41 gene is directly activated by a v-Myb/estrogen receptor fusion protein. Furthermore, our studies reveal that the GAS41 promoter is stimulated by v-Myb in co-transfection experiments and that the DNA-binding activity of v-Myb is crucial for transactivation of the promoter. Electrophoretic mobility-shift assays (EMSA) indicate that several Myb-binding sites, residing approximately 250 bp upstream of the transcriptional start site, are bound by Myb in vitro. Furthermore, chromatin immunoprecipitation assays demonstrate that v-Myb is bound to the GAS41 promoter in vivo. Taken together these findings identify the GAS41 gene as a novel v-Myb target gene. We have also analysed the GAS41 replication origin in myelomonocytic cells and have failed to observe significant differences in origin activity in cells expressing or not expressing v-Myb.

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.

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.

Antisense therapeutics in chronic myeloid leukaemia: the promise, the progress and the problems

DNA sequences which are complementary or 'antisense' to a target mRNA can inhibit expression of that mRNA's protein product. Antisense therapeutics has therefore received attention for inhibiting oncogenes in haematological malignancy, in particular in chronic myeloid leukaemia. However, it is now becoming clear that antisense therapeutics is considerably more problematic than was naively initially assumed. In this article, some of these difficulties are discussed, together with the achievements in CML so far. Considerable further research is required in order to define an optimal antisense therapeutics strategy for clinical use.

Solution structure of the B-Myb DNA-binding domain: a possible role for conformational instability of the protein in DNA binding and control of gene expression

Double- and triple-resonance heteronuclear NMR spectroscopy have been used to determine the high-resolution solution structure of the minimal B-Myb DNA-binding domain (B-MybR2R3) and to characterize the specific complex formed with a synthetic DNA fragment corresponding to the Myb target site on the Myb-regulated gene tom-1. B-MybR2R3 is shown to consist of two independent protein domains (R2 and R3) joined by a short linker, which have strikingly different tertiary structures despite significant sequence similarities. In addition, the C-terminal region of B-Myb R2 is confirmed to have a poorly defined structure, reflecting the existence of multiple conformations in slow to intermediate exchange. This contrasts with the tertiary structure reported for c-MybR2R3, in which both R2 and R3 have the same fold and the C-terminal region of R2 forms a stable, well-defined helix [Ogata, K., et al. (1995) Nat. Struct. Biol. 2, 309-320]. The NMR data suggest there are extensive contacts between B-MybR2R3 and its DNA target site in the complex and are consistent with a significant conformational change in the protein on binding to DNA, with one possibility being the formation of a stable helix in the C-terminal region of R2. In addition, conformational heterogeneity identified in R2 of B-MybR2R3 bound to the tom-1-A target site may play an important role in the control of gene expression by Myb proteins.

Biophysical investigations on the myb-DNA system

The oncogene product c-myb is a transcriptional modulator and is known to play important roles in cell growth and differentiation. It binds to DNA in a sequence specific manner and its cognate sequence motifs have been detected in the genes of proteins implying its role in a variety of regulatory functions. The protein has a DNA binding domain consisting of three imperfect repeats with highly conserved tryptophans at regular spacings in each of the repeats. We have carried out a variety of investigations on the structure and interactions of the DNA binding domain of Drosophila c-myb and its cognate DNA target sequences. The domain has been bacterially over-expressed by subcloning a segment of the gene coding for the domain in a pET 11d vector and transforming it into E. coli BL21 (DE3). Circular dichroism of the protein has revealed that the domain is largely helical in nature. Fluorescence investigations indicated that three out of the nine tryptophans are solvent exposed and the others are buried in the interior. The recombinant protein is able to distinguish between specific and non-specific DNA targets in its binding and the interaction is largely electrostatic in nature in both cases. Dynamic fluorescence quenching experiments suggested that the DNA binding sites on the protein for specific and non-specific DNA targets are physically different. Most of the conserved tryptophans are associated with the specific DNA binding site. Simulated annealing and molecular dynamic simulations in a water matrix have been used to predict an energetically favoured conformation for the protein. Calculation of surface accessibilities of the individual residues shows that nearly 60% of the residues are less than 50% accessible to the solvent. Two and three dimensional NMR experiments with isotopically labelled protein have enabled spin system identification for many residue type and the types of residues involved in hydrophobic core formation in the protein. In an attempt to see the DNA surface possibly involved in specific interaction with the protein, a three-dimensional structure of a 12 mer cognate DNA has been determined by NMR in conjunction with restrained energy minimization. The recognition sequence shows interesting structural characteristics that may have important roles in specific interaction.

Interference of Myb transactivation activity by a conditional dominant negative protein: functional interference in a cytotoxic T-cell line results in G1 arrest

The ability to ablate the activity of specific transcription factors in vivo is a potentially important tool to study their roles in cellular processes such as the cell cycle. Previously, production of a dominant interfering c-Myb protein (comprising a fusion of the c-Myb DNA binding domain with the Drosophila Engrailed transrepressor) was found to inhibit the proliferation of immature thymocytes in the developing thymus of transgenic mice. We report here the further development of this stratagem by rendering the c-Myb/Engrailed protein conditionally active by fusion to a modified estrogen receptor hormone binding domain, ER. Co-transfection experiments in NIH 3T3 fibroblasts showed that the resulting chimeric protein, Myb/En/ER, repressed transactivation of a c-Myb-responsive reporter only in the presence of the synthetic steroid, 4-hydroxytamoxifen (OHT). Additionally, we found that Myb/En/ER could counteract transactivation by C/EBP-beta of the mim-1 promoter, which contains juxtaposed Myb and C/EBP binding sites. Cytotoxic T-cells stably producing the inactive Myb/En/ER protein were readily obtained by gene transfection. The addition of OHT to these cells resulted in inhibition of proliferation and arrest in G1. The utility of this experimental system to study Myb and other transcription factors is discussed.

Interaction of C/EBPbeta and v-Myb is required for synergistic activation of the mim-1 gene

The retroviral oncogene v-myb encodes a transcription factor (v-Myb) which activates the myelomonocyte-specific mim-1 gene, a natural myb target gene, by cooperating with members of the C/EBP transcription factor family. The finding that v-Myb, together with C/EBP, is sufficient to activate the mim-1 gene in heterologous cell types has implicated Myb and C/EBP as a bipartite molecular switch, which regulates the expression of myelomonocyte-specific genes. To understand the relationship between v-Myb and C/EBP in more detail, we have examined the molecular basis of the activation of the mim-1 promoter by v-Myb and C/EBPbeta, a member of the C/EBP transcription factor family highly expressed in myelomonocytic cells. We have identified a composite Myb and C/EBP response element which mediates synergistic activation of the mim-1 promoter by both factors and consists of closely spaced Myb- and C/EBP-binding sites. In vitro and in vivo protein-binding studies indicate that v-Myb and C/EBPbeta interact with each other via their DNA-binding domains. We show that this interaction is essential for the synergistic activation of the mim-1 promoter by v-Myb and C/EBPbeta. Our work therefore identifies C/EBPbeta as an interaction partner of v-Myb involved in myelomonocyte gene expression.

Structure of the B-Myb DNA-binding domain in solution and evidence for multiple conformations in the region of repeat-2 involved in DNA binding: implications for sequence-specific DNA binding by Myb proteins

A range of double and triple resonance heteronuclear NMR has been used to obtain nearly complete sequence-specific 15N, 13C and 1H resonance assignments for a 110-residue protein corresponding to the B-Myb DNA-binding domain (B-MybR2R3) and to determine its secondary structure in solution. The protein was found to contain two stable helices in repeat-2 (R2) and three in repeat-3 (R3), involving residues K12-K24 (R2-1), W30-H36 (R2-2), E64-V76 (R3-1), W81-L87 (R3-2) and D93-K105 (R3-3). In addition, the chemical shift and nuclear Overhauser effect data suggest that amino acids Q44-W49 near the C-terminus of R2 form an unstable or nascent helix, which could be stabilised on binding to a specific DNA target site. The two N-terminal helices in R2 and R3 occupy essentially identical positions in the two domains, consistent with the high level of sequence similarity between these regions. In contrast, the C-terminal region forming the third helix in R3 shows low sequence similarity with R2, accounting for the differences in secondary structure. In the case of B-MybR2R3, there is a clear chemical shift and line-broadening evidence for the existence of multiple conformations in the C-terminal region of R2, which is believed to form one half of the DNA-binding site. We propose that conformational instability of part of the DNA-binding motif is a way of increasing the specificity of Myb proteins for a relatively short (6-bp) DNA target site by reducing their affinity for non-specific DNA sequences compared to specific sites.

Oncogenic truncation of the first repeat of c-Myb decreases DNA binding in vitro and in vivo

Oncogenic activation of c-Myb in both avian and murine systems often involves N-terminal truncation. In particular, the first of three DNA-binding repeats in c-Myb has been largely deleted during the genesis of the v-myb oncogenes of avian myeloblastosis virus and E26 avian leukemia virus. This finding suggests that the first DNA-binding repeat may have an important role in cell growth control. We demonstrate that truncation of the first DNA-binding repeat of c-Myb is sufficient for myeloid transformation in culture, but deletion of the N-terminal phosphorylation site and adjacent acidic region is not. Truncation of the first repeat decreases the ability of a Myb-VP16 fusion protein to trans activate the promoter of a Myb-inducible gene (mim-1) involved in differentiation. Moreover, truncation of the first repeat decreases the ability of the Myb protein to bind DNA both in vivo and in vitro. These results suggest that N-terminal mutants of c-Myb may transform by regulating only a subset of those genes normally regulated by c-Myb.

Oncogenic truncation of the first repeat of c-Myb decreases DNA binding in vitro and in vivo

Oncogenic activation of c-Myb in both avian and murine systems often involves N-terminal truncation. In particular, the first of three DNA-binding repeats in c-Myb has been largely deleted during the genesis of the v-myb oncogenes of avian myeloblastosis virus and E26 avian leukemia virus. This finding suggests that the first DNA-binding repeat may have an important role in cell growth control. We demonstrate that truncation of the first DNA-binding repeat of c-Myb is sufficient for myeloid transformation in culture, but deletion of the N-terminal phosphorylation site and adjacent acidic region is not. Truncation of the first repeat decreases the ability of a Myb-VP16 fusion protein to trans activate the promoter of a Myb-inducible gene (mim-1) involved in differentiation. Moreover, truncation of the first repeat decreases the ability of the Myb protein to bind DNA both in vivo and in vitro. These results suggest that N-terminal mutants of c-Myb may transform by regulating only a subset of those genes normally regulated by c-Myb.

Regulation of cell proliferation: late down-regulation of c-myb preceding myelo-monocytic cell differentiation

Expression of the c-myb nuclear oncogene during the cell proliferation and differentiation of HL-60 human promyelocytic leukemia cells was characterized and compared to the expression of c-fos, another nuclear oncogene with transcriptional regulatory activity. During progression through the cell cycle, the amount of c-myb protein increased. The increase was commensurate with total cell size, thus preserving the relative abundance of c-myb protein present at the onset of the cell cycle. In HL-60 cells, the induced metabolic cascade leading to terminal myeloid or monocytic differentiation segregates into two steps occurring over two division cycles. Expression of c-myb did not diverge from the control until late in this metabolic cascade when it declined prior to onset of terminal differentiation. This course of expression was similar for both the retinoic acid induced myeloid or the 1,25-dihydroxy vitamin D2 induced monocytic terminal differentiation of the cells. Bromodeoxyuridine, which induces proliferative arrest but not phenotypic differentiation of these cells, induced the same course of c-myb expression as the inducers of terminal differentiation. The same course of c-myb expression with growth arrest induced by these three different means is consistent with a potential proliferation regulatory role for c-myb in late but not early events leading to terminal differentiation. The dynamics of c-myb expression during this process were qualitatively, but not quantitatively, similar to the course of c-fos expression. Thus, taken with previous results, then amongst the nuclear oncogenes or tumor suppressor genes, c-myc, RB, c-fos, and c-myb, only c-myc and RB expression exhibit early regulation during induced HL-60 cell differentiation.

Extension of the DNA binding consensus of the chicken c-Myb and v-Myb proteins

The chicken c-myb gene and the v-myb oncogene transduced by avian myeloblastosis virus (AMV) encode DNA binding transcription activators. The DNA binding domain of AMV v-Myb displays a number of amino acid changes relative to c-Myb; v-Myb proteins in which one or more of three crucial residues in the DNA binding domain are mutated to resemble the c-Myb sequence display altered transformation phenotypes. In order to establish whether the spectrum of DNA binding sites which AMV v-Myb can recognise is different from that seen by chicken c-Myb, a site selection protocol was used to derive consensus binding sequences for three variant Myb proteins made in vitro, and also using nuclear extract from the v-myb transformed cell line BM2. The results show that the original consensus binding site defined for v-Myb, YAA-CKG, can be extended to YAACKGHH, and that this new consensus holds for both v-Myb and chicken c-Myb.

Functional analysis of the transcriptional activator encoded by the maize B gene: evidence for a direct functional interaction between two classes of regulatory proteins

The B, R, C1, and Pl genes regulating the maize anthocyanin pigment biosynthetic pathway encode tissue-specific transcriptional activators. B and R are functionally duplicate genes that encode proteins with the basic-helix-loop-helix (b-HLH) motif found in Myc proteins. C1 and Pl encode functionally duplicate proteins with homology to the DNA-binding domain of Myb proteins. A member of the b-HLH family (B or R) and a member of the myb family (C1 or Pl) are both required for anthocyanin pigmentation. Transient assays in maize and yeast were used to analyze the functional domains of the B protein and its interaction with C1. The results of these studies demonstrate that the b-HLH domain of B and most of its carboxyl terminus can be deleted with only a partial loss of B-protein function. In contrast, relatively small deletions within the B amino-terminal-coding sequence resulted in no trans-activation. Analysis of fusion constructs encoding the DNA-binding domain of yeast GAL4 and portions of B failed to reveal a transcriptional activation domain in the B protein. However, an amino-terminal domain of B was found to recruit a transcriptional activation domain by an interaction with C1. Formation of this complex resulted in the activation of a synthetic promoter containing GAL4 recognition sites, demonstrating that this interaction does not require the normal target promoters for B and C1. B and C1 fusions with yeast GAL4 DNA-binding and transcriptional activation domains were also found to interact when synthesized and assayed in yeast. The domains responsible for this interaction map to a region that contains the Myb homologous repeats of the C1 protein and to the amino terminus of the B protein, which does not contain the b-HLH motif. These studies suggest that the regulation of the maize anthocyanin pigmentation pathway involves a direct interaction between members of two distinct classes of transcriptional activators.

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.

Specific DNA binding by c-Myb: evidence for a double helix-turn-helix-related motif

The c-Myb protein is a sequence-specific DNA binding protein that activates transcription in hematopoietic cells. Three imperfect repeats (R1, R2, and R3) that contain regularly spaced tryptophan residues form the DNA binding domain of c-Myb. A fragment of c-Myb that contained the R2 and R3 regions bound specifically to a DNA sequence recognized by c-Myb plus ten additional base pairs at the 3' end of the element. The R2R3 fragment was predicted to contain two consecutive helix-turn-helix (HTH) motifs with unconventional turns. Mutagenesis of amino acids in R2R3 at positions that correspond to DNA-contacting amino acids in other HTH-containing proteins abolished specific DNA binding without affecting nonspecific DNA interactions.

Nucleotide preferences in sequence-specific recognition of DNA by c-myb protein

Using a binding site selection procedure, we have found that sequence-specific DNA-binding by the mouse c-myb protein involves recognition of nucleotides outside of the previously identified hexanucleotide motif. Oligonucleotides containing a random nucleotide core were immunoprecipitated in association with c-Myb, amplified by the Polymerase Chain Reaction and cloned in plasmids prior to sequencing. By alignment of sequences it was apparent that additional preferences existed at each of three bases immediately 5' of the hexanucleotide consensus, allowing an extension of the preferred binding site to YGRCVGTTR. The contributions of these 5' nucleotides to binding affinity was established in bandshift analyses with oligonucleotides containing single base substitutions; in particular, it was found that replacement of the preferred guanine at position -2 with any other base greatly reduced c-Myb binding. We found that the protein encoded by the related B-myb gene bound the preferred c-Myb site with similar affinity; however, B-Myb and c-Myb showed distinct preferences for the identity of the nucleotide at position -1 relative to the hexanucleotide consensus. This study demonstrates that the c-Myb DNA-binding site is more extensive than recognised hitherto and points to similar but distinct nucleotide preferences in recognition of DNA by related Myb proteins.

A functional c-myb gene is required for normal murine fetal hepatic hematopoiesis

The c-myb proto-oncogene encodes a sequence-specific DNA-binding protein. To better understand its normal biological function, we have altered the c-myb gene by homologous recombination in mouse embryonic stem cells. Resulting homozygous c-myb mutant mice displayed an interesting phenotype. At day 13 of gestation these mice appeared normal, suggesting that c-myb is not essential for early development. By day 15, however, the mutant mice were severely anemic. Analysis indicated that embryonic erythropoiesis, which occurs in the yolk sac, was not impaired by the c-myb alteration. Adult-type erythropoiesis, which first takes place in the fetal liver, was greatly diminished in c-myb mutants, however. Additional hematopoietic lineages were similarly affected. These results are compatible with a role for c-myb in maintaining the proliferative state of hematopoietic progenitor cells.

Sequence-specific DNA binding by Myc proteins

Myc proteins have a tripartite carboxyl-terminal domain containing specific amino acid sequence motifs: a basic motif, a helix-loop-helix motif, and a leucine heptad repeat. Similar sequence motifs have been identified in several eukaryotic transcription factors and were shown to facilitate protein-DNA and protein-protein interactions. By using recombinant v-Myc proteins obtained by bacterial expression of full-length or partially deleted avian v-myc alleles, the functional relevance of these sequence motifs for Myc protein oligomerization and for DNA binding was investigated. All recombinant v-Myc proteins that have retained the carboxyl-terminal domain dimerize and specifically bind to double-stranded DNA containing the palindromic core sequence CACGTG. This and a closely related DNA sequence element have been defined previously as part of the binding sites for human transcription factors USF and TFE3, which specifically bind to the adenovirus major late promoter or the muE3 motif within the immunoglobulin heavy-chain enhancer, respectively. It is shown that a 61-amino-acid peptide sequence containing only the bipartite basic motif/helix-loop-helix domain of Myc is necessary and sufficient for dimerization and sequence-specific DNA binding of v-Myc recombinant proteins.

The transcriptional control proteins c-Myb and v-Myb contain a basic region DNA binding motif

A class of transcriptional control proteins, typified by c-Myb, are characterised by a highly conserved N-terminal DNA binding domain, which is composed of either 2 or 3 imperfect repeats of about 52 amino acids. A sequence homology search of the SWISSPROT protein sequence data bank with this region from mouse c-Myb has allowed us to identify an area near the C-terminus of repeat 2 that contains a short sequence motif, known as the basic region, which forms the DNA binding site in both leucine zipper and helix-loop-helix transcription factors. We therefore propose that this region of repeat 2 and the homologous part of repeat 3 will form the Myb DNA binding site.

Analysis of the v-myb structural components important for transactivation of gene expression

In order to define the domains of the v-myb protein that are important for transactivation of gene expression, we have studied transactivation by the v-myb gene and a set of v-myb deletion mutants using transient transfection assays in NIH 3T3 cells. Analysis of the set of v-myb deletion products demonstrated that a previously unidentified region in the carboxyl-terminal portion of the protein is required for transactivation. This region lies between amino acids 295-356 with respect to the 5' end of the v-myb gene. Switching the v-myb DNA binding domain with the DNA binding domain of the rat glucocorticoid receptor (rGR) switched the cis-element requirement for v-myb action: only reports containing glucocorticoid response elements were activated by myb-rGR fusion proteins. The carboxyl terminal region essential for transactivation by the intact v-myb gene was also necessary for transactivation by the rGR-fusion gene. Carboxyl-terminal deletion mutations that encompassed the novel transactivation region were able to block wild-type v-myb transactivation when tested in transient co-expression assays. In an unexpected sidelight to our studies, we could demonstrate that the lacZ gene present in the prokaryotic vector sequences contained a DNA element that fortuitously can act as a v-myb-dependent enhancer element, and that v-myb protein can bind to this element in vitro. The lacZ enhancer contains the myb consensus DNA binding site YAAC(G/T)G.

Identification of functional domains in the maize transcriptional activator C1: comparison of wild-type and dominant inhibitor proteins

Genes encoding fusions between the maize regulatory protein C1 and the yeast transcriptional activator GAL4 and mutant C1 proteins were assayed for their ability to trans-activate anthocyanin biosynthetic genes in intact maize tissues. The putative DNA-binding region of C1 fused to the transcriptional activation domain of GAL4 activated transcription of anthocyanin structural gene promoters in c1 aleurones, c1 Rscm2 embryos, and c1 r embryogenic callus. Cells receiving these constructs accumulated purple anthocyanin pigments. The C1 acidic region fused to the GAL4 DNA-binding domain activated transcription of a GAL4-regulated promoter. An internal deletion of C1 also induced pigmentation; however, frameshifts in either the amino-terminal basic or carboxy-terminal acidic region blocked trans-activation, and the latter generated a dominant inhibitory protein. Fusion constructs between the wild-type C1 cDNA and the dominant inhibitor allele C1-I cDNA were used to identify the amino acid changes in C1 responsible for the C1-I inhibitory phenotype. Results from these studies establish that amino acids within the myb-homologous domain are critical for transcriptional activation.

Role of tryptophan repeats and flanking amino acids in Myb-DNA interactions

The c-myb protooncogene codes for a sequence-specific DNA-binding protein that appears to act as a transcriptional regulator and is highly conserved through evolution. The DNA-binding domain of Myb has been shown to contain three imperfectly conserved repeats of 52 amino acids that constitute the amino-terminal end. Within each repeat, there are three tryptophans that are separated by 18 or 19 amino acids and are flanked by basic amino acids. To determine the role of tryptophans and the flanking basic amino acids in the DNA-binding activity of Myb proteins, we have selectively mutagenized individual tryptophans as well as some of the amino acid residues that flank these tryptophans. Replacement of these tryptophans with glycine, proline, or arginine abolished the DNA-binding activity whereas replacement with other aromatic amino acids or leucine or alanine did not appreciably affect this activity. On the other hand the replacement of two amino acids, asparagine and lysine, that flank the last tryptophan with acidic amino acids completely abolished their DNA-binding activity. These results are consistent with a model we present in which the tryptophans form a hydrophobic scaffold that plays a crucial role in maintaining the helix-turn-helix structure of the DNA binding domain. Basic and polar amino acids adjacent to these tryptophans seem to participate directly in DNA binding.