BAS1 has a Myb motif and activates HIS4 transcription only in combination with BAS2

Myb and NF-M: combinatorial activators of myeloid genes in heterologous cell types

The c-Myb transcription factor regulates the differentiation of immature erythroid, lymphoid, and myeloid cells, although only the latter cells become transformed by the v-myb oncogene. These are also the only cells that express the Myb-regulated gene mim-1, suggesting that Myb requires tissue-specific, cooperating factors to activate such genes. Here, we investigated the tissue-specific regulation of the mim-1 promoter and found that it not only contains binding sites for Myb but also for NF-M, a myeloid-specific transcription factor that probably corresponds to mammalian C/EBP beta. Both types of binding sites were found to be required for full activity of the promoter. Remarkably, ectopic coexpression of Myb and NF-M proteins in erythroid cells or fibroblasts was sufficient to induce endogenous markers of myeloid differentiation, like the mim-1 and lysozyme genes. Our results indicate that c-Myb and NF-M proteins act as a bipartite, combinatorial signal that regulates the expression of myeloid-specific genes, even in heterologous cell types.

Promoter analysis of the PHO81 gene encoding a 134 kDa protein bearing ankyrin repeats in the phosphatase regulon of Saccharomyces cerevisiae

The PHO81 gene encoding one of the regulators of the phosphatase regulon in Saccharomyces cerevisiae was mapped 9.8 centimorgans distal from the ser2 locus on the right arm of chromosome VII. Determination of the nucleotide sequence of cloned PHO81 DNA revealed a 3537 bp open reading frame encoding a 134 kDa protein. This protein has six repeats of a 33-amino acid sequence homologous to the ankyrin repeat and an asparagine-rich region. Transcription of PHO81 is activated by Pho4 protein in cooperation with Pho2 (i.e., Bas2/Grf10) protein under the influence of the inorganic phosphate (Pi) concentration in the medium, through the PHO regulatory system. Major transcription initiation sites of PHO81, determined by primer extension analysis, are at nucleotide positions -66 and -65 relative to the ATG codon. Deletion analysis showed that a 95 bp region from nucleotide position -385 to -291 is essential for response to the Pi signals. Purified Pho4 protein protected a 19 bp region (positions -350 to -332) in the 95 bp fragment from DNase I digestion in vitro and the protected region includes the core sequence 5'-CACGTG-3', which is also observed in other genes of phosphate metabolism.

A bipartite DNA-binding domain in yeast Reb1p

The REB1 gene encodes a DNA-binding protein (Reb1p) that is essential for growth of the yeast Saccharomyces cerevisiae. Reb1p binds to sites within transcriptional control regions of genes transcribed by either RNA polymerase I or RNA polymerase II. The sequence of REB1 predicts a protein of 809 amino acids. To define the DNA-binding domain of Reb1p, a series of 5' and 3' deletions within the coding region was constructed in a bacterial expression vector. Analysis of the truncated Reb1p proteins revealed that nearly 400 amino acids of the C-terminal portion of the protein are required for maximal DNA-binding activity. To further define the important structural features of Reb1p, the REB1 homolog from a related yeast, Kluyveromyces lactis, was cloned by genetic complementation. The K. lactis REB1 gene supports active growth of an S. cerevisiae strain whose REB1 gene has been deleted. The Reb1p proteins of the two organisms generate almost identical footprints on DNA, yet the K. lactis REB1 gene encodes a polypeptide of only 595 amino acids. Comparison of the two Reb1p sequences revealed that within the region necessary for the binding of Reb1p to DNA were two long regions of nearly perfect identity, separated in the S. cerevisiae Reb1p by nearly 150 amino acids but in the K. lactis Reb1p by only 40 amino acids. The first includes a 105-amino-acid region related to the DNA-binding domain of the myb oncoprotein; the second bears a faint resemblance to myb. The hypothesis that the DNA-binding domain of Reb1p is formed from these two conserved regions was confirmed by deletion of as many as 90 amino acids between them, with little effect on the DNA-binding ability of the resultant protein. We suggest that the DNA-binding domain of Reb1p is made up of two myb-like regions that, unlike myb itself, are separated by as many as 150 amino acids. Since Reb1p protects only 15 to 20 nucleotides in a chemical or enzymatic footprint assay, the protein must fold such that the two components of the binding site are adjacent.

A bipartite DNA-binding domain in yeast Reb1p

The REB1 gene encodes a DNA-binding protein (Reb1p) that is essential for growth of the yeast Saccharomyces cerevisiae. Reb1p binds to sites within transcriptional control regions of genes transcribed by either RNA polymerase I or RNA polymerase II. The sequence of REB1 predicts a protein of 809 amino acids. To define the DNA-binding domain of Reb1p, a series of 5' and 3' deletions within the coding region was constructed in a bacterial expression vector. Analysis of the truncated Reb1p proteins revealed that nearly 400 amino acids of the C-terminal portion of the protein are required for maximal DNA-binding activity. To further define the important structural features of Reb1p, the REB1 homolog from a related yeast, Kluyveromyces lactis, was cloned by genetic complementation. The K. lactis REB1 gene supports active growth of an S. cerevisiae strain whose REB1 gene has been deleted. The Reb1p proteins of the two organisms generate almost identical footprints on DNA, yet the K. lactis REB1 gene encodes a polypeptide of only 595 amino acids. Comparison of the two Reb1p sequences revealed that within the region necessary for the binding of Reb1p to DNA were two long regions of nearly perfect identity, separated in the S. cerevisiae Reb1p by nearly 150 amino acids but in the K. lactis Reb1p by only 40 amino acids. The first includes a 105-amino-acid region related to the DNA-binding domain of the myb oncoprotein; the second bears a faint resemblance to myb. The hypothesis that the DNA-binding domain of Reb1p is formed from these two conserved regions was confirmed by deletion of as many as 90 amino acids between them, with little effect on the DNA-binding ability of the resultant protein. We suggest that the DNA-binding domain of Reb1p is made up of two myb-like regions that, unlike myb itself, are separated by as many as 150 amino acids. Since Reb1p protects only 15 to 20 nucleotides in a chemical or enzymatic footprint assay, the protein must fold such that the two components of the binding site are adjacent.

Expression of myb-related genes in the moss, Physcomitrella patens

Three cDNA clones encoding proteins containing a myb-related DNA binding domain have been isolated from a cDNA library prepared from protonemal tissue of the moss, Physcomitrella patens. The three cDNA clones between them encode two different classes of myb-like proteins, termed Pp1 and Pp2, that, outside of the myb domain, show no regions of significant homology. Acidic domains, capable of forming alpha-helical structures, are present in the carboxy-termini of the derived amino acid sequences from Pp1 and Pp2cDNAs suggesting that, like other myb genes, these proteins probably function as transcriptional activators. In contrast to other plants, where extensive myb-related gene families are present in the genome, a relatively small family is present in P. patens. Analyses of transcript levels during development of P. patens showed that maximum levels of transcription of the two genes occurred in young wild-type protonemal tissue that correlated with the time of maximum mitotic index. A decline in the expression of both genes occurs with increasing age of the wild-type tissue. Aberrant levels of expression of the two genes were observed in developmental mutants of P. patens which, as well as carrying specific morphological mutations, have greatly retarded protonemal growth rates. Transformation of wild-type P. patens with antisense constructs derived from Pp1 and Pp2 cDNA clones led to a dramatically reduced frequency of transformants when the expression of the reporter gene within the constructs was selected. Taken together, the data strongly suggest that expression of Pp1 and Pp2 is essential for cell growth during normal gametophytic development of P. patens.

A REB1-binding site is required for GCN4-independent ILV1 basal level transcription and can be functionally replaced by an ABF1-binding site

The ILV1 gene of Saccharomyces cerevisiae encodes the first committed step in isoleucine biosynthesis and is regulated by general control of amino acid biosynthesis. Deletion analysis of the ILV1 promoter revealed a GC-rich element important for the basal level expression. This cis-acting element, called ILV1BAS, is functional independently of whether GCN4 protein is present. Furthermore, unlike the situation at HIS4, the magnitude of GCN4-mediated derepression is independent of ILV1BAS. The element has homology to the consensus REB1-binding sequence CGGGTARNNR. Gel retardation assays showed that REB1 binds specifically to this element. We show that REB1-binding sites normally situated in the SIN3 promoter and in the 35S rRNA promoter can substitute for the ILV1 REB1 site. Furthermore, a SIN3 REB1 site containing a point mutation that abolishes REB1 binding does not support ILV1 basal level expression, suggesting that binding of REB1 is important for the control of ILV1 basal level expression. Interestingly, an ABF1-binding site can also functionally replace the ILV1 REB1-binding site. A mutated ABF1 site that displays a very low affinity for ABF1 does not functionally replace the ILV1 REB1 site. This suggests that ABF1 and REB1 may have related functions within the cell. Although the REB1-binding site is required for the ILV1 basal level expression, the site on its own stimulates transcription only slightly when combined with the CYC1 downstream promoter elements, indicating that another ILV1 promoter element functions in combination with the REB1 site to control high basal level expression.

A REB1-binding site is required for GCN4-independent ILV1 basal level transcription and can be functionally replaced by an ABF1-binding site

The ILV1 gene of Saccharomyces cerevisiae encodes the first committed step in isoleucine biosynthesis and is regulated by general control of amino acid biosynthesis. Deletion analysis of the ILV1 promoter revealed a GC-rich element important for the basal level expression. This cis-acting element, called ILV1BAS, is functional independently of whether GCN4 protein is present. Furthermore, unlike the situation at HIS4, the magnitude of GCN4-mediated derepression is independent of ILV1BAS. The element has homology to the consensus REB1-binding sequence CGGGTARNNR. Gel retardation assays showed that REB1 binds specifically to this element. We show that REB1-binding sites normally situated in the SIN3 promoter and in the 35S rRNA promoter can substitute for the ILV1 REB1 site. Furthermore, a SIN3 REB1 site containing a point mutation that abolishes REB1 binding does not support ILV1 basal level expression, suggesting that binding of REB1 is important for the control of ILV1 basal level expression. Interestingly, an ABF1-binding site can also functionally replace the ILV1 REB1-binding site. A mutated ABF1 site that displays a very low affinity for ABF1 does not functionally replace the ILV1 REB1 site. This suggests that ABF1 and REB1 may have related functions within the cell. Although the REB1-binding site is required for the ILV1 basal level expression, the site on its own stimulates transcription only slightly when combined with the CYC1 downstream promoter elements, indicating that another ILV1 promoter element functions in combination with the REB1 site to control high basal level expression.

Coregulation of purine and histidine biosynthesis by the transcriptional activators BAS1 and BAS2

We have found cross-pathway regulation between purine and histidine biosynthesis in yeast. The transcription factors BAS1 and BAS2/PHO2, which are also regulators of the histidine pathway, participate in the regulation of the purine biosynthetic pathway. Analysis of four genes of the purine pathway (ADE1, ADE2, ADE5,7, and ADE8) shows that their expression is repressed by adenine. The maximal basal and induced expression of these purine genes requires the presence of both BAS1 and BAS2. The factor BAS1 has been shown to bind at a site containing the TGACTC hexanucleotide motif in the ADE2 and ADE5,7 promoters. This motif is required for both basal and induced activation of the ADE2 gene by BAS1 and BAS2.

Reduction of a conserved Cys is essential for Myb DNA-binding

The human c-Myb gene product is a regulator of transcription with intrinsic DNA-binding activity located in two of three aminoterminal repeats R2R3. Three purified recombinant c-Myb proteins, a 42 kD protein corresponding to the amino-terminal half (HM42), and two proteins representing R2R3 or R1R2R3 (HMR23 and HMR123) have been analyzed either as purified proteins or present in bacterial extracts in gelshift analyses using a high-affinity DNA oligonucleotide. The purified proteins are inactive in DNA-binding unless supplemented with a reducing agent such as dithiothreitol (DTT) in vitro. Alternatively a cellular nuclear extract (Nex) from HeLa cells strongly activates the binding. This effect is dose-dependent and sensitive to heat. The Nex does not lead to changes in the Myb-DNA mobility shift assay excluding a direct association of the complex with a cellular component. Site-directed mutagenesis of the aminoacid residue 130, a single conserved cysteine in HMR23 to serine almost completely abolishes DNA binding. Oxidation by diamide or alkylation by N-ethylmaleimide (NEM) of the Myb-proteins in vitro inhibit their interaction with DNA whereby the diamide effect is reversible by addition of excess of DTT. Nex prepared from COS cells transfected with c-myb leads to Myb-DNA interaction which is not responsive to DTT but sensitive to NEM and diamide. Our data indicate that the reduced cysteine of Myb is essential for its DNA-binding and that Myb function may be regulated by a reduction-oxidation mechanism.

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.

The cellular oncogene c-myb can interact synergistically with the Epstein-Barr virus BZLF1 transactivator in lymphoid cells

Regulation of replicative functions in the Epstein-Barr virus (EBV) genome is mediated through activation of a virally encoded transcription factor, Z (BZLF1). We have shown that the Z gene product, which binds to AP-1 sites as a homodimer and has sequence similarity to c-Fos, can efficiently activate the EBV early promoter, BMRF1, in certain cell types (i.e., HeLa cells) but not others (i.e., Jurkat cells). Here we demonstrate that the c-myb proto-oncogene product, which is itself a DNA-binding protein and transcriptional transactivator, can interact synergistically with Z in activating the BMRF1 promoter in Jurkat cells (a T-cell line) or Raji cells (an EBV-positive B-cell), whereas the c-myb gene product by itself has little effect. The simian virus 40 early promoter is also synergistically activated by the Z/c-myb combination. Synergistic transactivation of the BMRF1 promoter by the Z/c-myb combination appears to involve direct binding by the Z protein but not the c-myb protein. A 30-bp sequence in the BMRF1 promoter which contains a Z binding site (a consensus AP-1 site) is sufficient to transfer high-level lymphoid-specific responsiveness to the Z/c-myb combination to a heterologous promoter. That the c-myb oncogene product can interact synergistically with an EBV-encoded member of the leucine zipper protein family suggests c-myb is likely to engage in similar interactions with cellularly encoded transcription factors.

The cellular oncogene c-myb can interact synergistically with the Epstein-Barr virus BZLF1 transactivator in lymphoid cells

Regulation of replicative functions in the Epstein-Barr virus (EBV) genome is mediated through activation of a virally encoded transcription factor, Z (BZLF1). We have shown that the Z gene product, which binds to AP-1 sites as a homodimer and has sequence similarity to c-Fos, can efficiently activate the EBV early promoter, BMRF1, in certain cell types (i.e., HeLa cells) but not others (i.e., Jurkat cells). Here we demonstrate that the c-myb proto-oncogene product, which is itself a DNA-binding protein and transcriptional transactivator, can interact synergistically with Z in activating the BMRF1 promoter in Jurkat cells (a T-cell line) or Raji cells (an EBV-positive B-cell), whereas the c-myb gene product by itself has little effect. The simian virus 40 early promoter is also synergistically activated by the Z/c-myb combination. Synergistic transactivation of the BMRF1 promoter by the Z/c-myb combination appears to involve direct binding by the Z protein but not the c-myb protein. A 30-bp sequence in the BMRF1 promoter which contains a Z binding site (a consensus AP-1 site) is sufficient to transfer high-level lymphoid-specific responsiveness to the Z/c-myb combination to a heterologous promoter. That the c-myb oncogene product can interact synergistically with an EBV-encoded member of the leucine zipper protein family suggests c-myb is likely to engage in similar interactions with cellularly encoded transcription factors.

Aromatic amino acid biosynthesis in the yeast Saccharomyces cerevisiae: a model system for the regulation of a eukaryotic biosynthetic pathway

This review focuses on the gene-enzyme relationships and the regulation of different levels of the aromatic amino acid biosynthetic pathway in a simple eukaryotic system, the unicellular yeast Saccharomyces cerevisiae. Most reactions of this branched pathway are common to all organisms which are able to synthesize tryptophan, phenylalanine, and tyrosine. The current knowledge about the two main control mechanisms of the yeast aromatic amino acid biosynthesis is reviewed. (i) At the transcriptional level, most structural genes are regulated by the transcriptional activator GCN4, the regulator of the general amino acid control network, which couples transcriptional derepression to amino acid starvation of numerous structural genes in multiple amino acid biosynthetic pathways. (ii) At the enzyme level, the carbon flow is controlled mainly by modulating the enzyme activities at the first step of the pathway and at the branch points by feedback action of the three aromatic amino acid end products. Implications of these findings for the relationship of S. cerevisiae to prokaryotic as well as to higher eukaryotic organisms and for general regulatory mechanisms occurring in a living cell such as initiation of transcription, enzyme regulation, and the regulation of a metabolic branch point are discussed.

Characterization of SIS1, a Saccharomyces cerevisiae homologue of bacterial dnaJ proteins

The Saccharomyces cerevisiae SIS1 gene was identified as a high copy number suppressor of the slow growth phenotype of strains containing mutations in the SIT4 gene, which encodes a predicted serine/threonine protein phosphatase. The SIS1 protein is similar to bacterial dnaJ proteins in the amino-terminal third and carboxyl-terminal third of the proteins. In contrast, the middle third of SIS1 is not similar to dnaJ proteins. This region of SIS1 contains a glycine/methionine-rich region which, along with more amino-terminal sequences, is required for SIS1 to associate with a protein of apparent molecular mass of 40 kD. The SIS1 gene is essential. Strains limited for the SIS1 protein accumulate cells that appear blocked for migration of the nucleus from the mother cell into the daughter cell. In addition, many of the cells become very large and contain a large vacuole. The SIS1 protein is localized throughout the cell but is more concentrated at the nucleus. About one-fourth of the SIS1 protein is released from a nuclear fraction upon treatment with RNase. We also show that overexpression of YDJ1, another yeast protein with similarity to bacterial dnaJ proteins, can not substitute for SIS1.

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.

RAP1 is required for BAS1/BAS2- and GCN4-dependent transcription of the yeast HIS4 gene

The major in vitro binding activity to the Saccharomyces cerevisiae HIS4 promoter is due to the RAP1 protein. In the absence of GCN4, BAS1, and BAS2, the RAP1 protein binds to the HIS4 promoter in vivo but cannot efficiently stimulate HIS4 transcription. RAP1, which binds adjacently to BAS2 on the HIS4 promoter, is required for BAS1/BAS2-dependent activation of HIS4 basal-level transcription. In addition, the RAP1-binding site overlaps with the single high-affinity HIS4 GCN4-binding site. Even though RAP1 and GCN4 bind competitively in vitro, RAP1 is required in vivo for (i) the normal steady-state levels of GCN4-dependent HIS4 transcription under nonstarvation conditions and (ii) the rapid increase in GCN4-dependent steady-state HIS4 mRNA levels following amino acid starvation. The presence of the RAP1-binding site in the HIS4 promoter causes a dramatic increase in the micrococcal nuclease sensitivity of two adjacent regions within HIS4 chromatin: one region contains the high-affinity GCN4-binding site, and the other region contains the BAS1- and BAS2-binding sites. These results suggest that RAP1 functions at HIS4 by increasing the accessibility of GCN4, BAS1, and BAS2 to their respective binding sites when these sites are present within chromatin.

The PHO84 gene of Saccharomyces cerevisiae encodes an inorganic phosphate transporter

The PHO84 gene specifies Pi-transport in Saccharomyces cerevisiae. A DNA fragment bearing the PHO84 gene was cloned by its ability to complement constitutive synthesis of repressible acid phosphatase of pho84 mutant cells. Its nucleotide sequence predicted a protein of 596 amino acids with a sequence homologous to that of a superfamily of sugar transporters. Hydropathy analysis suggested that the secondary structure of the PHO84 protein consists of two blocks of six transmembrane domains separated by 74 amino acid residues. The cloned PH084 DNA restored the Pi transport activity of pho84 mutant cells. The PHO84 transcription was regulated by Pi like those of the PHO5, PHO8, and PHO81 genes. A PHO84-lacZ fusion gene produced beta-galactosidase activity under the regulation of Pi, and the activity was suggested to be bound to a membrane fraction. Gene disruption of PHO84 was not lethal. By comparison of nucleotide sequences and by tetrad analysis with GAL80 as a standard, the PHO84 locus was mapped at a site beside the TUB3 locus on the left arm of chromosome XIII.

RAP1 is required for BAS1/BAS2- and GCN4-dependent transcription of the yeast HIS4 gene

The major in vitro binding activity to the Saccharomyces cerevisiae HIS4 promoter is due to the RAP1 protein. In the absence of GCN4, BAS1, and BAS2, the RAP1 protein binds to the HIS4 promoter in vivo but cannot efficiently stimulate HIS4 transcription. RAP1, which binds adjacently to BAS2 on the HIS4 promoter, is required for BAS1/BAS2-dependent activation of HIS4 basal-level transcription. In addition, the RAP1-binding site overlaps with the single high-affinity HIS4 GCN4-binding site. Even though RAP1 and GCN4 bind competitively in vitro, RAP1 is required in vivo for (i) the normal steady-state levels of GCN4-dependent HIS4 transcription under nonstarvation conditions and (ii) the rapid increase in GCN4-dependent steady-state HIS4 mRNA levels following amino acid starvation. The presence of the RAP1-binding site in the HIS4 promoter causes a dramatic increase in the micrococcal nuclease sensitivity of two adjacent regions within HIS4 chromatin: one region contains the high-affinity GCN4-binding site, and the other region contains the BAS1- and BAS2-binding sites. These results suggest that RAP1 functions at HIS4 by increasing the accessibility of GCN4, BAS1, and BAS2 to their respective binding sites when these sites are present within chromatin.

The PHO84 gene of Saccharomyces cerevisiae encodes an inorganic phosphate transporter

The PHO84 gene specifies Pi-transport in Saccharomyces cerevisiae. A DNA fragment bearing the PHO84 gene was cloned by its ability to complement constitutive synthesis of repressible acid phosphatase of pho84 mutant cells. Its nucleotide sequence predicted a protein of 596 amino acids with a sequence homologous to that of a superfamily of sugar transporters. Hydropathy analysis suggested that the secondary structure of the PHO84 protein consists of two blocks of six transmembrane domains separated by 74 amino acid residues. The cloned PH084 DNA restored the Pi transport activity of pho84 mutant cells. The PHO84 transcription was regulated by Pi like those of the PHO5, PHO8, and PHO81 genes. A PHO84-lacZ fusion gene produced beta-galactosidase activity under the regulation of Pi, and the activity was suggested to be bound to a membrane fraction. Gene disruption of PHO84 was not lethal. By comparison of nucleotide sequences and by tetrad analysis with GAL80 as a standard, the PHO84 locus was mapped at a site beside the TUB3 locus on the left arm of chromosome XIII.

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.

The SIT4 protein phosphatase functions in late G1 for progression into S phase

Saccharomyces cerevisiae strains containing temperature-sensitive mutations in the SIT4 protein phosphatase arrest in late G1 at the nonpermissive temperature. Order-of-function analysis shows that SIT4 is required in late G1 for progression into S phase. While the levels of SIT4 do not change in the cell cycle, SIT4 associates with two high-molecular-weight phosphoproteins in a cell-cycle-dependent fashion. In addition, we have identified a polymorphic gene, SSD1, that in some versions can suppress the lethality due to a deletion of SIT4 and can also partially suppress the phenotypic defects due to a null mutation in BCY1. The SSD1 protein is implicated in G1 control and has a region of similarity to the dis3 protein of Schizosaccharomyces pombe. We have also identified a gene, PPH2alpha, that in high copy number can partially suppress the growth defect of sit4 strains. The PPH2 alpha gene encodes a predicted protein that is 80% identical to the catalytic domain of mammalian type 2A protein phosphatases but also has an acidic amino-terminal extension not present in other phosphatases.

The SIT4 protein phosphatase functions in late G1 for progression into S phase

Saccharomyces cerevisiae strains containing temperature-sensitive mutations in the SIT4 protein phosphatase arrest in late G1 at the nonpermissive temperature. Order-of-function analysis shows that SIT4 is required in late G1 for progression into S phase. While the levels of SIT4 do not change in the cell cycle, SIT4 associates with two high-molecular-weight phosphoproteins in a cell-cycle-dependent fashion. In addition, we have identified a polymorphic gene, SSD1, that in some versions can suppress the lethality due to a deletion of SIT4 and can also partially suppress the phenotypic defects due to a null mutation in BCY1. The SSD1 protein is implicated in G1 control and has a region of similarity to the dis3 protein of Schizosaccharomyces pombe. We have also identified a gene, PPH2alpha, that in high copy number can partially suppress the growth defect of sit4 strains. The PPH2 alpha gene encodes a predicted protein that is 80% identical to the catalytic domain of mammalian type 2A protein phosphatases but also has an acidic amino-terminal extension not present in other phosphatases.

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.

GAL11P: a yeast mutation that potentiates the effect of weak GAL4-derived activators

A mutant yeast in which a weak GAL4-derived activator functions as a strong activator bears a single mis-sense mutation in GAL11 (a.k.a. SPT13). The first 74 amino acids of GAL4, including the zinc-dependent DNA binding region, attached to an acidic activating sequence, are sufficient to respond both to GAL11 and to our mutant GAL11P (potentiator). PPR1, a yeast activator with a similar zinc finger sequence, also responds to GAL11 and to GAL11P, whereas regulators bearing unrelated DNA binding motifs do not. GAL11 itself works as a strong activator when tethered to DNA by fusion to the bacterial LexA protein, and deletion of GAL11 is known to cause a 5- to 10-fold reduction in GAL4 activity. We suggest that a complex of GAL4 and GAL11 constitutes a particularly strong activator; evidence that the putative GAL4-GAL11 complex ordinarily forms preferentially on DNA suggests a biological rationale for GAL11 action.

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.

TATA-dependent and TATA-independent transcription at the HIS4 gene of yeast

Two systems of transcription factors stimulate expression of the HIS4 gene of Saccharomyces cerevisiae. High-level transcription induced by amino-acid starvation is mediated by Gcn4(ref.1) and basal transcription is mediated jointly by the factors Bas1 and Bas2 (Pho2). We now show that wild-type Gcn4 requires the TATA element for correct messenger RNA start-site selection, but Gcn4 derivatives with deletions in the activation domain activate HIS4 transcription at the correct mRNA start site (I) in the absence of the TATA element. Gcn4 derivatives that activate TATA-independent transcription show low levels of activation. Similarly, we find that low levels of transcription by Bas1/Bas2 are TATA-independent, whereas high levels are TATA-dependent. These results show that the HIS4 TATA element is required for mRNA start-site selection only under conditions of high-level transcription.

The mouse c-rel protein has an N-terminal regulatory domain and a C-terminal transcriptional transactivation domain

We have shown that the murine c-rel protein can act as a transcriptional transactivator in both yeast and mammalian cells. Fusion proteins generated by linking rel sequences to the DNA-binding domain of the yeast transcriptional activator GAL4 activate transcription from a reporter gene linked in cis to a GAL4 binding site. The full-length mouse c-rel protein (588 amino acids long) is a poor transactivator; however, the C-terminal portion of the protein between amino acid residues 403 to 568 is a potent transcriptional transactivator. Deletion of the N-terminal half of the c-rel protein augments its transactivation function. We propose that c-rel protein has an N-terminal regulatory domain and a C-terminal transactivation domain which together modulate its function as a transcriptional transactivator.

REB1, a yeast DNA-binding protein with many targets, is essential for growth and bears some resemblance to the oncogene myb

REB1 is a DNA-binding protein that recognizes sites within both the enhancer and the promoter of rRNA transcription as well as upstream of many genes transcribed by RNA polymerase II. We report here the cloning of the gene for REB1 by screening a yeast genomic lambda gt11 library with specific oligonucleotides containing the REB1 binding site consensus sequence. The REB1 gene was sequenced, revealing an open reading frame encoding 809 amino acids. The predicted protein was highly hydrophilic, with numerous OH-containing amino acids and glutamines, features common to many of the general DNA-binding proteins of Saccharomyces cerevisiae, such as ABF1, RAP1, GCN4, and HSF1. There was some homology between a portion of REB1 and the DNA-binding domain of the oncogene myb. REB1 is an essential gene that maps on chromosome II. However, the physiological role that it plays in the cell has yet to be established.

The mouse c-rel protein has an N-terminal regulatory domain and a C-terminal transcriptional transactivation domain

We have shown that the murine c-rel protein can act as a transcriptional transactivator in both yeast and mammalian cells. Fusion proteins generated by linking rel sequences to the DNA-binding domain of the yeast transcriptional activator GAL4 activate transcription from a reporter gene linked in cis to a GAL4 binding site. The full-length mouse c-rel protein (588 amino acids long) is a poor transactivator; however, the C-terminal portion of the protein between amino acid residues 403 to 568 is a potent transcriptional transactivator. Deletion of the N-terminal half of the c-rel protein augments its transactivation function. We propose that c-rel protein has an N-terminal regulatory domain and a C-terminal transactivation domain which together modulate its function as a transcriptional transactivator.

REB1, a yeast DNA-binding protein with many targets, is essential for growth and bears some resemblance to the oncogene myb

REB1 is a DNA-binding protein that recognizes sites within both the enhancer and the promoter of rRNA transcription as well as upstream of many genes transcribed by RNA polymerase II. We report here the cloning of the gene for REB1 by screening a yeast genomic lambda gt11 library with specific oligonucleotides containing the REB1 binding site consensus sequence. The REB1 gene was sequenced, revealing an open reading frame encoding 809 amino acids. The predicted protein was highly hydrophilic, with numerous OH-containing amino acids and glutamines, features common to many of the general DNA-binding proteins of Saccharomyces cerevisiae, such as ABF1, RAP1, GCN4, and HSF1. There was some homology between a portion of REB1 and the DNA-binding domain of the oncogene myb. REB1 is an essential gene that maps on chromosome II. However, the physiological role that it plays in the cell has yet to be established.

Transformation by v-myb correlates with trans-activation of gene expression

The v-myb oncogene of avian myeloblastosis virus causes acute myelomonocytic leukemia in chickens and transforms avian myeloid cells in vitro. Its protein product p48v-myb is a nuclear, sequence-specific, DNA-binding protein which activates gene expression in transient DNA transfection studies. To investigate the relationship between transformation and trans-activation by v-myb, we constructed 15 in-frame linker insertion mutants. The 12 mutants which transformed myeloid cells also trans-activated gene expression, whereas the 3 mutants which did not transform also did not trans-activate. This implies that trans-activation is required for transformation by v-myb. One of the transformation-defective mutants localized to the cell nucleus but failed to bind DNA. The other two transformation-defective mutants localized to the cell nucleus and bound DNA but nevertheless failed to trans-activate. These latter mutants define two distinct domains of p48v-myb which control trans-activation by DNA-bound protein, one within the amino-terminal DNA-binding domain itself and one in a carboxyl-terminal domain which is not required for DNA binding.

Transformation by v-myb correlates with trans-activation of gene expression

The v-myb oncogene of avian myeloblastosis virus causes acute myelomonocytic leukemia in chickens and transforms avian myeloid cells in vitro. Its protein product p48v-myb is a nuclear, sequence-specific, DNA-binding protein which activates gene expression in transient DNA transfection studies. To investigate the relationship between transformation and trans-activation by v-myb, we constructed 15 in-frame linker insertion mutants. The 12 mutants which transformed myeloid cells also trans-activated gene expression, whereas the 3 mutants which did not transform also did not trans-activate. This implies that trans-activation is required for transformation by v-myb. One of the transformation-defective mutants localized to the cell nucleus but failed to bind DNA. The other two transformation-defective mutants localized to the cell nucleus and bound DNA but nevertheless failed to trans-activate. These latter mutants define two distinct domains of p48v-myb which control trans-activation by DNA-bound protein, one within the amino-terminal DNA-binding domain itself and one in a carboxyl-terminal domain which is not required for DNA binding.

trans activation of gene expression by v-myb

The v-myb oncogene causes acute myelomonocytic leukemia in chickens and transforms avian myeloid cells in vitro. Its product, p48v-myb, is a short-lived nuclear protein which binds DNA. We demonstrate that p48v-myb can function as a trans activator of gene expression in transient DNA transfection assays. trans activation requires the highly conserved amino-terminal DNA-binding domain and the less highly conserved carboxyl-terminal domain of p48v-myb, both of which are required for transformation. Multiple copies of a consensus sequence for DNA binding by p48v-myb inserted upstream of a herpes simplex virus thymidine kinase promoter are strongly stimulatory for transcriptional activation by a v-myb-VP16 fusion protein but not by p48v-myb itself, suggesting that the binding of p48v-myb to DNA may not be sufficient for trans activation.

trans activation of gene expression by v-myb

The v-myb oncogene causes acute myelomonocytic leukemia in chickens and transforms avian myeloid cells in vitro. Its product, p48v-myb, is a short-lived nuclear protein which binds DNA. We demonstrate that p48v-myb can function as a trans activator of gene expression in transient DNA transfection assays. trans activation requires the highly conserved amino-terminal DNA-binding domain and the less highly conserved carboxyl-terminal domain of p48v-myb, both of which are required for transformation. Multiple copies of a consensus sequence for DNA binding by p48v-myb inserted upstream of a herpes simplex virus thymidine kinase promoter are strongly stimulatory for transcriptional activation by a v-myb-VP16 fusion protein but not by p48v-myb itself, suggesting that the binding of p48v-myb to DNA may not be sufficient for trans activation.

Characterization of the v-myb DNA binding domain

The transforming protein encoded by the v-myb oncogene is a sequence-specific DNA-binding protein that is thought to be involved in the regulation of gene expression. The N-terminal region of the v-myb protein is composed of two highly conserved tandem repeat sequences of unknown function. It has been speculated that the N-terminal v-myb repeats might be crucial for DNA-binding, since N-terminal deletions destroy the DNA-binding activity of the v-myb protein. Here, we have studied the v-myb DNA-binding domain in more detail. Our results show that the N-terminal region of the v-myb protein is sufficient for specific DNA-binding. Dissection of this region suggests that both repeats are required for DNA-binding, but that both repeats play different roles in v-myb protein DNA interaction. We also show that the myb repeats of a drosophila melanogaster homolog of c-myb function as sequence-specific DNA-binding domain. Our results support the view that specific sequence-recognition, mediated by the conserved myb repeats, is a general feature of myb-related proteins.

Sequence-specific DNA binding of the proto-oncoprotein ets-1 defines a transcriptional activator sequence within the long terminal repeat of the Moloney murine sarcoma virus

The ets proto-oncogene family is a group of sequence-related genes whose normal cellular function is unknown. In a study of cellular proteins involved in the transcriptional regulation of murine retroviruses in T lymphocytes, we have discovered that a member of the ets gene family encodes a sequence-specific DNA-binding protein. A mouse ets-1 cDNA clone was obtained by screening a mouse thymus cDNA expression library with a double-stranded oligonucleotide probe representing 20 bp of the Moloney murine sarcoma virus (MSV) long terminal repeat (LTR). The cDNA sequence has an 813-bp open reading frame (ORF) whose predicted amino acid sequence is 97.6% identical to the 272 carboxy-terminal amino acids of the human ets-1 protein. The ORF was expressed in bacteria, and the 30-kD protein product was shown to bind DNA in a sequence-specific manner by mobility-shift assays, Southwestern blot analysis, and methylation interference. A mutant LTR containing four base pair substitutions in the ets-1 binding site was constructed and was shown to have reduced binding in vitro. Transcriptional efficiency of the MSV LTR promoter containing this disrupted ets-1 binding site was compared to the activity of a wild-type promoter in mouse T lymphocytes in culture, and 15- to 20-fold reduction in expression of a reporter gene was observed. We propose that ets-1 functions as a transcriptional activator of mammalian type-C retroviruses and speculate that ets-related genes constitute a new group of eukaryotic DNA-binding proteins.