Dual mutations in the AML1 and FLT3 genes are associated with leukemogenesis in acute myeloblastic leukemia of the M0 subtype

Point mutations of the transcription factor AML1 are associated with leukemogenesis in acute myeloblastic leukemia (AML). Internal tandem duplications (ITDs) in the juxtamembrane domain and mutations in the second tyrosine kinase domain of the Fms-like tyrosine kinase 3 (FLT3) gene represent the most frequent genetic alterations in AML. However, such mutations per se appear to be insufficient for leukemic transformation. To evaluate whether both AML1 and FLT3 mutations contribute to leukemogenesis, we analyzed mutations of these genes in AML M0 subtype in whom AML1 mutations were predominantly observed. Of 51 patients, eight showed a mutation in the Runt domain of the AML1 gene: one heterozygous missense mutation with normal function, five heterozygous frameshift mutations and two biallelic nonsense or frameshift mutations, resulting in haploinsufficiency or complete loss of the AML1 activities. On the other hand, a total of 10 of 49 patients examined had the FLT3 mutation. We detected the FLT3 mutation in five of eight (63%) patients with AML1 mutation, whereas five of 41 (12%) without AML1 mutation showed the FLT3 mutation (P=0.0055). These observations suggest that reduced AML1 activities predispose cells to the acquisition of the activating FLT3 mutation as a secondary event leading to full transformation in AML M0.

Point mutations of the RUNx1/AML1 gene in sporadic and familial myeloid leukemias

The RUNX1/AML1 gene is known to be the most frequent target for chromosomal translocation in leukemia. In addition, recent studies have demonstrated point mutations in the RUNX1 gene as an another mode of genetic lesion resulting in leukemia. Of particular interest, sporadic point mutations of biallelic type are found in a tight association with either the acute myelogenous leukemia (AML) MO subtype or trisomy 21. Germline mutations give rise to a familial platelet disorder that results in a predisposition to acute myelogenous leukemia (FPD/AML). Most of the RUNX1 mutants were defective in DNA binding but still active in beta binding, a characteristic that is consistent with the 3-dimensional structural findings and may explain the dominant inhibitory effects. Although genuine haploinsufficiency of RUNX1 was observed in some cases, a greater majority of mutant RUNX1 proteins may also act in a dominant-negative manner, possibly creating a higher propensity for leukemia development. The stronger dominant-negative effect was also deduced to be the major mechanism of the chimeric genes created by chromosomal translocations. The decrement of RUNXI activity may be a common underlying cause for RUNX1-related leukemias. However, because these RUNX1 abnormalities per se are insufficient for leukemogenesis, cooperating genetic alteration(s) should be intensively sought for further mechanistic insights and future clinical applications.

Mechanisms of transcriptional regulation by Runt domain proteins

Runt domain proteins have vital roles in regulating transcription in developmental pathways extending from sex determination and segmentation in fruit fly embryos to the development of blood and bone in mammals. Many of the insights into the mechanisms by which these proteins act to regulate transcription originate either from studies on the Drosophila runt gene, the founding member of this family, or from work on the mammalian PEBP2/CBF transcription factor. Genetic experiments in the Drosophila system reveal that runt functions both to activate and to repress transcription of different downstream target genes and indicate that different mechanisms are used in the regulation of different specific downstream target genes. These studies have also identified other nuclear factors that work with Runt in some of these pathways. Studies in mammalian systems have provided additional evidence for the complexity of transcriptional regulation by Runt domain proteins and have identified other transcription factors that cooperate with Runt domain proteins to regulate the activity of different specific cis-regulatory enhancers. The emerging view from studies in both systems is that these proteins act as context-dependent regulators of transcription, activating or repressing gene expression dependent upon the constititution of a particular promoter/enhancer in a particular cell type. These results have yielded new insights into the molecular mechanisms that control animal development and provide a framework for investigating fundamental issues in eukaryotic transcriptional regulation.

Three-dimensional reconstruction of transcription termination factor rho: orientation of the N-terminal domain and visualization of an RNA-binding site

The Escherichia coli rho transcription termination protein is a hexameric helicase, and is believed to function by separating an RNA-DNA hybrid. Unlike hexameric DNA helicases, where a single strand of DNA passes through the central channel, it has been proposed that the RNA wraps around the outside of the ring. We have generated a three-dimensional reconstruction of rho, and localized a tRNA molecule bound to the primary RNA-binding site to the outside of the ring. An atomic structure of the N-terminal domain of rho fits into our reconstruction uniquely, with the residues involved in RNA-binding on the outside of the ring. Although rho shares a common structural core with the F1-ATPase and other hexameric helicases, there has been a divergence in function due to rho's N-terminal domain, which has no homology to other helicases.

Immunoglobulin motif DNA recognition and heterodimerization of the PEBP2/CBF Runt domain

The polyomavirus enhancer binding protein 2 (PEBP2) or core binding factor (CBF) is a heterodimeric enhancer binding protein that is associated with genetic regulation of hematopoiesis and osteogenesis. Aberrant forms of PEBP2/CBF are implicated in the cause of the acute human leukemias and in a disorder of bone development known as cleidocranial dysplasia. The common denominator in the natural and mutant forms of this protein is a highly conserved domain of PEBP2/CBF alpha, termed the Runt domain (RD), which is responsible for both DNA binding and heterodimerization with the beta subunit of PEBP2/CBF. The three-dimensional structure of the RD bound to DNA has been determined to be an S-type immunoglobulin fold, establishing a structural relationship between the RD and the core DNA binding domains of NF-kappaB, NFAT1, p53 and the STAT proteins. NMR spectroscopy of a 43.6 kD RD-beta-DNA ternary complex identified the surface of the RD in contact with the beta subunit, suggesting a mechanism for the enhancement of RD DNA binding by beta. Analysis of leukemogenic mutants within the RD provides molecular insights into the role of this factor in leukemogenesis and cleidocranial dysplasia.

Molecular insights into PEBP2/CBF beta-SMMHC associated acute leukemia revealed from the structure of PEBP2/CBF beta

PEBP2/CBF is a heterodimeric transcription factor essential for genetic regulation of hematopoiesis and osteogenesis. DNA binding by PEBP2/CBF alpha is accomplished by a highly conserved DNA binding domain, the Runt domain (RD), whose structure adopts an S-type immunoglobulin fold when bound to DNA. The supplementary subunit beta enhances DNA binding by the RD in vitro, but its role in the control of gene expression has remained largely unknown in vivo. Chromosome 16 inversion creates a chimeric gene product fusing PEBP2/CBF beta to a portion of the smooth muscle myosin heavy chain (PEBP2/CBF beta-SMMHC) that is causally associated with the onset of acute myeloid leukemia in humans. The three-dimensional structure of PEBP2/CBF beta has been determined in solution and is shown to adopt a fold related to the beta-barrel oligomer binding motif. Direct analysis of a 43.6 kD ternary RD-beta-DNA complex identifies the likely surface of beta in contact with the RD. The structure of PEBP2/CBF beta enables a molecular understanding of the capacity of PEBP2/CBF beta-SMMHC to sequester PEBP2/CBF alpha in the cytoplasm and therefore provides a molecular basis for understanding leukemogenic transformation.

A WW domain-containing yes-associated protein (YAP) is a novel transcriptional co-activator

A protein module called the WW domain recognizes and binds to a short oligopeptide called the PY motif, PPxY, to mediate protein-protein interactions. The PY motif is present in the transcription activation domains of a wide range of transcription factors including c-Jun, AP-2, NF-E2, C/EBPalpha and PEBP2/CBF, suggesting that it plays an important role in transcriptional activation. We show here that mutation of the PY motif in the subregion of the activation domain of the DNA-binding subunit of PEBP2, PEBP2alpha, abolishes its transactivation function. Using yeast two-hybrid screening, we demonstrate that Yes-associated protein (YAP) binds to the PY motif of PEBP2alpha through its WW domain. The C-terminal region of YAP fused to the DNA-binding domain of GAL4 showed transactivation as strong as that of GAL4-VP16. Exogenously expressed YAP conferred transcription-stimulating activity on the PY motif fused to the GAL4 DNA-binding domain as well as to native PEBP2alpha. The osteocalcin promoter was stimulated by exogenous PEBP2alphaA and a dominant negative form of YAP strongly inhibited this activity, suggesting YAP involvement in this promoter activity in vivo. These results indicate that the PY motif is a novel transcription activation domain that functions by recruiting YAP as a strong transcription activator to target genes.

Transcription termination factor Rho contains three noncatalytic nucleotide binding sites

The active form of transcription termination factor rho from Escherichia coli is a homohexamer, but several studies suggest that the six subunits of the hexamer are not functionally identical. Rho has three tight and three weak ATP binding sites. Based on our findings, we propose that the tight nucleotide binding sites are noncatalytic and the weak sites are catalytic. In the presence of RNA, the rho-catalyzed ATPase rate is fast, close to 30 s-1. However, under these conditions the three tightly bound nucleotides dissociate from the rho hexamer at a slow rate of 0.02 s-1, indicating that the three tight nucleotide binding sites of rho do not participate in the fast ATPase turnover. These slowly exchanging nucleotide binding sites of rho are capable of hydrolyzing ATP, but the resulting products (ADP and Pi) bind tightly and dissociate from rho about 1500 times slower than the fast ATPase turnover. Both RNA and excess ATP in solution are necessary for stabilizing nucleotide binding at these sites. In the absence of RNA, or when solution ATP is hydrolyzed to ADP, a faster dissociation of nucleotides was observed. Based on these results, we propose that the rho hexamer is similar to the F1-ATPase and T7 DNA helicase-containing noncatalytic sites that do not participate in the fast ATPase turnover. We propose that the three tight sites on rho are the noncatalytic sites and the three weak sites are the catalytic sites.

Biallelic and heterozygous point mutations in the runt domain of the AML1/PEBP2alphaB gene associated with myeloblastic leukemias

The AML1 gene encoding the DNA-binding alpha-subunit in the Runt domain family of heterodimeric transcription factors has been noted for its frequent involvement in chromosomal translocations associated with leukemia. Using reverse transcriptase-polymerase chain reaction (RT-PCR) combined with nonisotopic RNase cleavage assay (NIRCA), we found point mutations of the AML1 gene in 8 of 160 leukemia patients: silent mutations, heterozygous missense mutations, and biallelic nonsense or frameshift mutations in 2, 4, and 2 cases, respectively. The mutations were all clustered within the Runt domain. Missense mutations identified in 3 patients showed neither DNA binding nor transactivation, although being active in heterodimerization. These defective missense mutants may be relevant to the predisposition or progression of leukemia. On the other hand, the biallelic nonsense mutants encoding truncated AML1 proteins lost almost all functions examined and may play a role in leukemogenesis leading to acute myeloblastic leukemia.

The quaternary geometry of transcription termination factor rho: assignment by chemical cross-linking

Transcription termination factor rho from Escherichia coli is a ring-shaped homohexamer of 419 amino acid subunits and catalyzes an ATP-dependent release of nascent RNA transcripts. Previous chemical cross-linking studies suggested that the rho hexamer might have D3 symmetry with three isologous dimers as protomers. However, our recent mutational analysis of rho alongside its putative structural homology to F1-ATPase rather argued for C6 symmetry. To resolve this discrepancy, we have re-investigated the pattern of cross-linking of rho using various cross-linkers with different functional groups and spacer lengths. Upon reaction with dimethyl suberimidate followed by SDS-polyacrylamide gel electrophoresis, rho protein generated a series of cross-linked oligomers up to hexamers, of which dimers migrated as distinct doublet bands of approximately equal intensities. However, the lower band became much stronger than the upper one with dimethyl adipimidate and difluorodinitrobenzene, and vice versa with disuccinimidyl glutarate, disuccinimidyl suberate and disulfosuccinimidyl tartarate. Furthermore, the trimeric products also produced doublet bands, whose relative intensities were again variable with cross-linkers, but in an inverse correlation with those of the dimer bands. These results combined with theoretical considerations support a C6 symmetry model in which cross-linking is assumed to occur stochastically at one of two alternative sites within each subunit interface with variable relative frequencies depending on cross-linkers. The D3 symmetry is excluded, for the putative trimeric subspecies should always retain mutually equal intensities in that case. Detailed inspections of the cross-linking kinetics further revealed a moderate characteristic of C3 symmetry for the rho hexamer such that the collective as well as relative rates of cross-linking at the two available sites could fluctuate between alternating interfaces. The final model designated as C3/6 is also compatible with other functional and structural properties known for rho.

Redox regulation of the DNA binding activity in transcription factor PEBP2. The roles of two conserved cysteine residues

Transcription factor PEBP2/CBF consists of a DNA binding subunit, alpha, and a regulatory subunit, beta. The alpha subunit has an evolutionarily conserved 128-amino acid region termed "Runt domain" that is responsible for both DNA binding and heterodimerization with the beta subunit. The Runt domain in all mammalian submembers of the alpha subunit contains two conserved cysteine residues, and its DNA binding activity undergoes redox regulation. To investigate the mechanism of this redox regulation, we performed site-directed mutagenesis of the two conserved cysteines in the Runt domain of the mouse PEBP2alphaA homolog. Substitution of Cys-115 to serine resulted in a partially impaired DNA binding, which remained highly sensitive to a thiol-oxidizing reagent, diamide. Conversely, the corresponding substitution of Cys-124 caused an increased DNA binding concomitant with an increased resistance to diamide. In contrast, substitution of either cysteine to aspartate was destructive to DNA binding to marked extents. These results have revealed that both Cys-115 and Cys-124 are responsible for the redox regulation in their own ways with low and high oxidizabilities, respectively. We have also found that two cellular thiol-reactive proteins, thioredoxin and Ref-1, work effectively and synergistically for activation of the Runt domain. Interestingly, the beta subunit further enhanced the activation by these proteins and reciprocally prevented the oxidative inactivation by diamide. These findings collectively suggest the possibility that the Runt domain's function in vivo could be dynamically regulated by the redox mechanism with Trx, Ref-1, and the beta subunit as key modulators.

A simple screening for mutant DNA binding proteins: application to murine transcription factor PEBP2alpha subunit, a founding member of the Runt domain protein family

Mouse transcription factor PEBP2 (polyomavirus enhancer-binding protein (2) is composed of two distinct subunits alpha and beta. The alpha subunit has an ability to bind the specific DNA sequences, which is enhanced by formation of a heterodimer with the beta subunit. The DNA binding and heterodimerization activities of the alpha subunit are both localized within a 128-amino-acid (aa) region termed as the Runt domain for its homology to the Drosophila segmentation gene runt. To characterize the molecular determinants for these activities, the Runt domain was randomly mutagenized and produced in E. coli as a secreted form. Using E. coli culture supernatant, the DNA binding and heterodimerization of mutant Runt domains were analyzed by gel retardation assay. Nine randomly picked single-aa substitution mutants showed various functional alterations in DNA binding and heterodimerization either separately or simultaneously. This observation suggests that the structure of Runt domain is highly ordered and is quite sensitive to modulations in its primary structure. The method presented here provides a simple and quick method to characterize a large number of mutant DNA binding proteins.

Functional dissection of the alpha and beta subunits of transcription factor PEBP2 and the redox susceptibility of its DNA binding activity

The mouse transcription factor PEBP2 is a heterodimer of two subunits: a DNA binding subunit alpha and its partner subunit beta. The alpha subunit shares a region of high homology, termed the Runt domain, with the products of the Drosophila melanogaster segmentation gene runt and the human acute myeloid leukemia-related gene AML1. To study the molecular basis for the DNA binding and heterodimerization functions of this factor, we constructed series of deletions of the alpha and beta subunits and examined their activities by electrophoretic mobility shift and affinity column assays. The minimal functional region of the alpha subunit for DNA binding and dimerization was shown to coincide with the Runt domain. On the other hand, the region of the beta subunit required for heterodimerization was localized to the N-terminal 135 amino acids. Furthermore, it was found that the DNA binding activity of the Runt domain is regulated by a reduction/oxidization (redox) mechanism and that its reductively activated state, which is extremely labile, is stabilized by the beta subunit. These findings add a new layer to the mechanism and significance of the regulatory interplay between the two subunits of PEBP2.

Structural and functional dissections of transcription termination factor rho by random mutagenesis

Transcription termination factor rho from Escherichia coli is a homohexamer of 419 amino acid subunits and catalyzes an ATP-dependent release of nascent RNA transcripts. A rho monomer has three distinct domains functioning independently at the first approximation: the amino-terminal one quarter containing a primary RNA-binding site, the central 270-amino acids region constituting an ATP-binding domain with homologies to F1-ATPase, and the carboxy-terminal remainder with unknown function(s). To further delineate the structural and functional organizations of rho protein, we undertook its random mutagenesis using error-prone polymerase chain reactions with the carboxy-terminal 100-amino acid region chosen as the initial target. From 14 mutants identified, rho protein was purified and characterized in vitro. Of these, 11 mutants are defective in termination in vivo and show decreased activities in various partial functions examined: ATP binding; RNA binding; and ATPase activities dependent on three cofactors with decreasing efficacies, poly(C), lambda cro RNA and poly(U). A few of them are also affected in the putative secondary RNA-binding site that is functionally coupled to ATP hydrolysis. By contrast, the three other mutants are hyperactive in termination, poly(U)-dependent ATPase activity, and RNA interaction at the primary site. In these properties, the hyper-terminating mutants strikingly resemble the "super rho" mutant formerly found in the amino-terminal domain. Taken together, these findings indicate that the carboxy-terminal region plays a pivotal role in functionally coupling the RNA and ATP-binding domains, plausibly by acting as an interface for their interaction within or across individual subunits. In light of the reported X-ray crystallographic structure of F1-ATPase, we propose a model for the tertiary and quaternary structure of rho that is consistent with the observed mutational effects as well as a number of structural and functional properties characteristic of rho.

Cloning, mapping and expression of PEBP2 alpha C, a third gene encoding the mammalian Runt domain

PEBP2/CBF is a heterodimeric transcription factor composed of alpha and beta subunits. Previously, we reported two distinct mouse genes, PEBP2 alpha A and PEBP2 alpha B, which encode the alpha subunit. PEBP2 alpha B is the homologue of human AML1, encoding the acute myeloid leukemia 1 protein. AML1 and human PEBP2/CBF beta were detected independently at the breakpoints of two characteristic chromosome translocations observed frequently in two subtypes of acute myeloid leukemia. The PEBP2 alpha proteins contain a 128-amino-acid (aa) region highly homologous to the Drosophila melanogaster segmentation gene runt. The evolutionarily conserved region, named the Runt domain, harbors DNA-binding and heterodimerizing activities. In this study, we identified the third Runt-domain-encoding gene, PEBP2 alpha C, which maps to 1p36.11-p36.13 in the human chromosome and encodes a 415-aa protein. PEBP2 alpha C forms a heterodimer with PEBP2 beta, binds to the PEBP2 site and transactivates transcription, similar to PEBP2 alpha A and PEBP2 alpha B.

Subcellular localization of the alpha and beta subunits of the acute myeloid leukemia-linked transcription factor PEBP2/CBF

Each of the two human genes encoding the alpha and beta subunits of a heterodimeric transcription factor, PEBP2, has been found at the breakpoints of two characteristic chromosome translocations associated with acute myeloid leukemia, suggesting that they are candidate proto-oncogenes. Polyclonal antibodies against the alpha and beta subunits of PEBP2 were raised in rabbits and hamsters. Immunofluorescence labeling of NIH 3T3 cells transfected with PEBP2 alpha and -beta cDNAs revealed that the full-size alpha A1 and alpha B1 proteins, the products of two related but distinct genes, are located in the nucleus, while the beta subunit is localized to the cytoplasm. Deletion analysis demonstrated that there are two regions in alpha A1 responsible for nuclear accumulation of the protein: one mapped in the region between amino acids 221 and 513, and the other mapped in the Runt domain (amino acids 94 to 221) harboring the DNA-binding and the heterodimerizing activities. When the full-size alpha A1 and beta proteins are coexpressed in a single cell, the former is present in the nucleus and the latter still remains in the cytoplasm. However, the N- or C-terminally truncated alpha A1 proteins devoid of the region upstream or downstream of the Runt domain colocalized with the beta protein in the nucleus. In these cases, the beta protein appeared to be translocated into the nucleus passively by binding to alpha A1. The chimeric protein containing the beta protein at the N-terminal region generated as a result of the inversion of chromosome 16 colocalized with alpha A1 to the nucleus more readily than the normal beta protein. The implications of these results in relation to leukemogenesis are discussed.

Human cytoplasmic isoleucyl-tRNA synthetase: selective divergence of the anticodon-binding domain and acquisition of a new structural unit

We show here that the class I human cytoplasmic isoleucyl-tRNA synthetase is an exceptionally large polypeptide (1266 aa) which, unlike its homologues in lower eukaryotes and prokaryotes, has a third domain of two repeats of an approximately 90-aa sequence appended to its C-terminal end. While extracts of Escherichia coli do not aminoacrylate mammalian tRNA with isoleucine, expression of the cloned human gene in E. coli results in charging of the mammalian tRNA substrate. The appended third domain is dispensable for detection of this aminoacylation activity and may be needed for assembly of a multisynthetase complex in mammalian cells. Alignment of the sequences of the remaining two domains shared by isoleucyl-tRNA synthetases from E. coli to human reveals a much greater selective pressure on the domain needed for tRNA acceptor helix interactions and catalysis than on the domain needed for interactions with the anticodon. This result may have implications for the historical development of an operational RNA code for amino acids.

PEBP2 alpha B/mouse AML1 consists of multiple isoforms that possess differential transactivation potentials

A murine transcription factor, PEBP2, is composed of two subunits, alpha and beta. There are two genes in the mouse genome, PEBP2 alpha A and PEBP2 alpha B, which encode the alpha subunit. Two types of the alpha B cDNA clones, alpha B1 and alpha B2, were isolated from mouse fibroblasts and characterized. They were found to represent 3.8- and 7.9-kb transcripts, respectively. The 3.8-kb RNA encodes the previously described alpha B protein referred to as alpha B1, while the 7.9-kb RNA encodes a 387-amino-acid protein, termed alpha B2, which is identical to alpha B1 except that it has an internal deletion of 64 amino acid residues. Both alpha B1 and alpha B2 associate with PEBP2 beta and form a heterodimer. The alpha B2/beta complex binds to the PEBP2 binding site two- to threefold more strongly than the alpha B1/beta complex does. alpha B1 stimulates transcription through the PEBP2 site about 40-fold, while alpha B2 is only about 25 to 45% as active as alpha B1. Transactivation domain is located downstream of the 128-amino-acid runt homology region, referred to as the Runt domain. Mouse chromosome mapping studies revealed that alpha A, alpha B, and beta genes are mapped to chromosomes 17, 16, and 8, respectively. The last two genes are syntenic with the human AML1 on chromosome 21q22 and PEBP2 beta/CBF beta on 16q22 detected at the breakpoints of characteristic chromosome translocations of the two different subtypes of acute myeloid leukemia. These results suggest that previously described chimeric gene products, AML1/MTG8(ETO) and AML1-EAP generated by t(8;21) and t(3;21), respectively, lack the transactivation domain of AML1.

PEBP2/PEA2 represents a family of transcription factors homologous to the products of the Drosophila runt gene and the human AML1 gene

cDNAs representing the alpha subunit of polyomavirus enhancer binding protein 2 (PEBP2; also called PEA2) were isolated. The products of the cDNAs are highly homologous to that of Drosophila segmentation gene runt (run) for an N-proximal 128-amino acid region showing 66% identity. The run homology region encompasses the domain capable of binding to a specific nucleotide sequence motif and of dimerizing with the companion beta subunit. The human AML1 gene related to t(8;21) acute myeloid leukemia also had a run homology region. Together with the beta subunit, which increases the affinity of the alpha subunit to DNA without binding to DNA by itself, PEBP2 represents a newly discovered family of transcription factor. The major species of PEBP2 alpha mRNA was expressed in T-cell lines but not in B-cell lines tested. Evidence indicated that PEBP2 functions as a transcriptional activator and is involved in regulation of T-cell-specific gene expression.

Molecular cloning and characterization of PEBP2 beta, the heterodimeric partner of a novel Drosophila runt-related DNA binding protein PEBP2 alpha

Polyomavirus enhancer binding protein, PEBP2 (PEA2), is a heterodimer of two distinct subunits, alpha and beta, of which the former directly binds to DNA and the latter acts auxiliary to enhance the DNA binding. Recent cloning studies has revealed that the alpha subunit is homologous to the products of the Drosophila segmentation gene runt and the human AML1 gene, and that it functions as a major regulator for the T cell-specific gene expression. We have currently cloned cDNAs for the beta subunit. The isolated cDNAs contain three isoforms that are presumed to arise from alternative RNA splicing and encode polypeptides consisting of 187, 182, and 155 amino acids, respectively. These polypeptides neither show any significant homology with known other proteins including the alpha subunit nor have any known DNA-binding and dimerization domains. Thus, PEBP2, as the complex of these subunits, is thought to constitute an entirely novel category of heteromeric transcriptional regulator together with the Runt and AML1 proteins. Gel retardation assays of the cDNA-encoded proteins produced in an in vitro translation system or in Escherichia coli demonstrated that the larger two beta isoforms, but not the smallest one, can dimerize with the alpha subunit. Furthermore, this heterodimerization was shown to cause a marked increase in the intrinsic DNA binding affinity of the alpha subunit.

Isolation of PEBP2 alpha B cDNA representing the mouse homolog of human acute myeloid leukemia gene, AML1

Breakpoints of the t(8;21) chromosome translocation in acute myeloid leukemia are clustered within the human gene, AML1, located on chromosome 21 [Miyoshi, H., Shimizu, K., Maseki, N., Kaneko, Y. & Ohki, M. (1991). Proc. Natl. Acad. Sci. USA, 88, 10431-10434]. The product of AML1 has a region about 130 amino acids long that is highly homologous to the Drosophila segmentation gene runt (runt homology region). The cDNA isolated from mouse fibroblasts encoding the alpha-subunit of polyomavirus enhancer binding protein 2 (PEBP2/PEA2) revealed that it also has a runt homology region (E. Ogawa et al., submitted). In this study, a different cDNA clone presumed to represent the mouse homolog of human AML1 (PEBP2 alpha B) was isolated from a cDNA library derived from B cells. The deduced amino acid sequence of PEBP2 alpha B is 99% identical to that of AML1 for the first 241 residues, including the runt homology region, though their sequences diverge thereafter. On the other hand, PEBP2 alpha B and PEBP2 alpha share only 92% and 82% homologies at the amino acid and nucleotide levels respectively, even for the runt homology region, indicating that these proteins are encoded by distinct genes. While PEBP2 alpha is highly expressed in T-cell lines but not in most of the B-cell lines and functions as an activator of T-cell-specific genes, PEBP2 alpha B is expressed in both types of cells. A possible functional relationship between PEBP2 alpha and PEBP2 alpha B is discussed in relation to leukemogenic potential of AML1.

Human T cell L-plastin bundles actin filaments in a calcium-dependent manner

The amino acid sequences deduced from cDNA analyses revealed that human leucocyte L-plastin phosphorylated in response to interleukin 1, 2 closely resembles a chicken intestinal microvilli protein, fimbrin, that bundles actin filaments [de Arruda et al. (1990) J. Cell Biol. 111, 1069-1079]. In the present work, it was observed that unphosphorylated L-plastin isolated from human T cells bundled F-actin just as fimbrin does. L-Plastin acted on T cell beta-actin, but hardly acted on muscle alpha-actin or chicken gizzard gamma-actin, whereas fimbrin bundled muscle alpha-actin. Unlike fimbrin, L-plastin's actin-bundling action was strictly calcium-dependent: the bundles were formed at pCa 7, but not at pCa 6. Under suitable conditions, approximately one molecule of L-plastin bound to 8 molecules of actin monomer in the actin filament.

Polyclonal and monoclonal antibodies monospecific to MMTV LTR orf protein produced in E. coli

Monoclonal and polyclonal antibodies specific to an open reading frame of the mouse mammary tumor virus long terminal repeat were generated using an open reading frame-beta-galactosidase fusion protein produced in E. coli. Both antibodies reacted with the open reading frame-beta-galactosidase fusion protein but not with beta-galactosidase alone using an immunoblotting technique. It is concluded that these antibodies were specific for the protein encoded by the open reading frame of the mouse mammary tumor virus long terminal repeat.

Differential expression of subspecies of polyomavirus and murine leukemia virus enhancer core binding protein, PEBP2, in various hematopoietic cells

The core sequence of the enhancer of murine leukemia virus (MuLV) long terminal repeat is highly conserved in a large number of MuLV strains and appears to play an essential role when SL3-3 or Moloney strains induce T cell lymphoma in mice. We found by using the electrophoretic mobility shift assay that a polyomavirus enhancer core-binding protein, PEBP2, bound to this core motif of MuLV. We also noted that PEBP2 in several hematopoietic cell lines derived from B lymphocyte, macrophage and myelocyte lineages migrated significantly faster than the authentic PEBP2 detected in NIH3T3 fibroblasts. Interestingly, PEBP2 detected in the cell lines of T lymphocyte lineage appeared to contain both types, which were indistinguishable in electrophoretic mobility from those of NIH3T3 and of B lymphocyte, macrophage and myelocyte lineages. The treatment of the nuclear extract containing PEBP2 with phosphatase generated PEBP3, which is a subcomponent of PEBP2 and retained the same DNA-binding specificity as PEBP2. The altered mobility of hematopoietic cell-derived or T lymphocyte-derived PEBP2 was found to be due to the alteration of the mobility of PEBP3. Based on the distinct mobility of PEBP2/3 of T lymphocytes from those of other hematopoietic cells, we discuss the implication of PEBP2 in MuLV-induced T cell leukemia and T cell-specific gene expression.

Molecular evolution of phosphoenolpyruvate carboxylase for C4 photosynthesis in maize: comparison of its cDNA sequence with a newly isolated cDNA encoding an isozyme involved in the anaplerotic function

Molecular events associated with the evolution of an enzyme for C4 photosynthesis were investigated. In maize, at least three isozymes of phosphoenolpyruvate carboxylase [EC 4.1.1.31] are known: the C4-form, the C2-form and the root-form, being named according to their physiological roles and pattern of tissue distribution [Ting, I.P. & Osmond, C.B. (1973) Plant Physiol. 51, 448-453]. A cDNA clone which presumably encodes the C3-form isozyme was newly isolated and analyzed. Comparison of the sequences of the C3-form and C4-form isozymes revealed that (i) the homologies in the nucleotide and deduced amino acid sequences were 71 and 77%, respectively, and (ii) the gene for the C4-form isozyme evolved under strong G/C pressure. The genes for these isozymes were found to be located apart on different chromosomes. A phylogenetic tree was constructed using 8 amino acid sequences of phosphoenolpyruvate carboxylases from various sources. The topology of the tree indicated that, at least in monocots such as maize and sorghum, the genes for the C4-form and C3-form isozymes diverged from their common ancestral gene earlier than the monocot-dicot divergence (about 2 x 10(8) yr ago), though the divergence of maize (C4 plant) from wheat (C3 plant) is supposed to have occurred much later (6 x 10(7) yr ago).

Purification of a mouse nuclear factor that binds to both the A and B cores of the polyomavirus enhancer

We have previously identified a protein factor, PEBP2 (polyomavirus enhancer-binding protein), in the nuclear extract from mouse NIH 3T3 cells which binds to the sequence motif, PEA2, located within the polyomavirus enhancer A element. Upon cellular transformation with activated oncogene c-Ha-ras, this factor frequently undergoes drastic molecular modifications into an altered form having a considerably reduced molecular size. In this study, the altered form, PEBP3, was purified to near homogeneity. The purified PEBP3 comprised two sets of families of polypeptides, alpha-1 to alpha-4 and beta-1 to beta-2, which were 30 to 35 kilodaltons and 20 to 25 kilodaltons in size, respectively. Both kinds of polypeptides possessed DNA-binding activities with exactly the same sequence specificity. Individual alpha or beta polypeptides complexed with DNA showed faster gel mobilities than did PEBP3. However, the original gel retardation pattern was restored when alpha and beta polypeptides were mixed together in any arbitrary pair. These observation along with the results of UV- and chemical-cross-linking studies led us to conclude that PEBP3 is a heterodimer of alpha and beta subunits, potentially having a divalent DNA-binding activity. Furthermore, PEBP3 was found to bind a second, hitherto-unnoticed site of the polyomavirus enhancer that is located within the B element and coincides with the sequence previously known as the simian virus 40 enhancer core homology. From comparison of this and the original binding sites, the consensus sequence for PEBP3 was defined to be PuACCPuCA. These findings provided new insights into the biological significance of PEBP3 and PEBP2.

65-kilodalton protein phosphorylated by interleukin 2 stimulation bears two putative actin-binding sites and two calcium-binding sites

We have previously characterized a 65-kilodalton protein (p65) as an interleukin 2 stimulated phosphoprotein in human T cells and showed that three endopeptide sequences of p65 are present in the sequence of l-plastin [Zu et al. (1990) Biochemistry 29, 1055-1062]. In this paper, we present the complete primary structure of p65 based on the cDNA isolated from a human T lymphocyte (KUT-2) cDNA library. Analysis of p65 sequences and the amino acid composition of cleaved p65 N-terminal peptide indicated that the deduced p65 amino acid sequence exactly coincides with that of l-plastin over the C-terminal 580 residues [Lin et al. (1988) Mol. Cell. Biol. 8, 4659-4668] and has a 57-residue extension at the N-terminus to l-plastin. Computer-assisted structural analysis revealed that p65 is a multidomain molecule involving at least three intriguing functional domains: two putative calcium-binding sites along the N-terminal 80 amino acid residues; a putative calmodulin-binding site following the calcium-binding region; and two tandem repeats of putative actin-binding domains in its middle and C-terminal parts, each containing approximately 240 amino acid residues. These results suggest that p65 belongs to actin-binding proteins.

A ubiquitous repressor interacting with an F9 cell-specific silencer and its functional suppression by differentiated cell-specific positive factors

A mutant of polyomavirus, F9-5000, capable of growing in F9 cell [M. Vasseur et al., J. Virol., 43: 800-808, 1982 (1)], has a deletion in the enhancer from nucleotide 5119 to nucleotide 5142. The oligonucleotide corresponding to the deleted region (delta F9-5000 element) showed silencer activity on gene expression in F9 cells. Mobility shift assay revealed a nuclear factor, PEBP4, in F9 nuclear extract which bound to the delta F9-5000 element. Mutations introduced into the PEBP4 binding site specifically abolished its binding as well as the inhibitory effect on gene expression. After F9 cells were induced to differentiate, two more factors, PEBP2 and PEBP1, a member of AP1 family, became detectable in addition to PEBP4, and at the same time the delta F9-5000 element lost silencer activity and acquired an enhancer activity. The recognition sequence of PEBP2 as well as that of PEBP1 overlapped with that of a repressor, PEBP4. PEBP4 and PEBP3, a factor related to PEBP2, were shown to compete for binding to delta F9-5000. Interplay of a ubiquitous negative factor and differentiation-induced positive factors may represent one aspect of the gene regulation during embryonic development.

Phosphoenolpyruvate carboxylase prevalent in maize roots: isolation of a cDNA clone and its use for analyses of the gene and gene expression

In search of the gene family for phosphoenolpyruvate carboxylase (PEPC) [EC 4.1.1.31] in C4 plants, we isolated from a maize root cDNA library a new cDNA clone that cross-hybridized with a cDNA for PEPC involved in the C4 photosynthesis (Yanagisawa et al. (1988) FEBS Lett. 229, 107-110). Alignment of the nucleotide sequence with that of the probe cDNA revealed the absence of sequence homology in the 3' non-coding region between the two cDNAs. Southern blot hybridization probed with this specific sequence indicated that the corresponding gene is unique in the maize genome. Northern blot hybridization using the same probe showed that this gene-family member was expressed most strongly in roots and also in green leaves to a lesser extent, but not significantly in etiolated leaves.

Mutant rho factors with increased transcription termination activities. II. Identification and functional dissection of amino acid changes

We have determined the nucleotide sequences of three mutant rho genes encoding hyperfunctional rho proteins (rho S) together with their parent allele, rho-ts702. These mutant rho factors contain the following amino acid changes as deduced from their sequences: (1) the thermo-labile mutant, rho-ts702, has Thr304 substituting for Ala; (2) rho S-77 and rho S-81, which are selectively altered in the primary polynucleotide binding site, share an identical mutation, Leu3----Phe; (3) rho S-82, which is altered in both the primary and secondary polynucleotide binding sites, carries three amino acid substitutions together, Leu3----Phe, Asp156----Asn and Thr323----Ile. Dissection and functional characterization of each mutation in rho S-82 have revealed that Ile323 alone is responsible for alterations in both the secondary RNA interaction and the terminator selectivity observed with the original mutant, rho S-82. Taken together, these results not only confirm our proposal in the accompanying paper that the primary and secondary RNA binding sites differently contribute in determining the overall efficiency and site-specificity of termination, respectively, but also support the possibility that these binding sites exist as structurally distinct domains in rho protein. In contrast, Asn156 was shown to cause decreased termination efficiency, though it had no influence on RNA interactions. Thus, this amino acid residue appears to be associated with still another rate-determining step of termination, for instance, interactions between rho and RNA polymerase. On the basis of Chou-Fasman secondary structure predictions as well as amino acid sequence comparison with F1-ATPase, we discuss how the proposed domains are structurally and functionally related to the putative ATPase reactive center of rho protein.

Mutant rho factors with increased transcription termination activities. I. Functional correlations of the primary and secondary polynucleotide binding sites with the efficiency and site-selectivity of rho-dependent termination

We have characterized rho proteins from mutants of Escherichia coli, rho s-81 and rho s-82, which are hyperactive in termination. The two mutant rho proteins are differentially altered both in termination activities and in RNA interactions. rho s-81 generally elicits enhanced termination on various templates such as phage T7 DNA and a DNA restriction fragment containing the trpE intracistronic rho-dependent terminators, either measured as a whole or examined for individual sites. On the other hand, rho s-82 has strikingly different preferences toward individual termination sites, exhibiting overall termination activities higher or lower than normal, depending on templates. From measurements of the rho ATPase activity with T7 RNA and various homoribopolymers as cofactors, both mutant rho proteins are shown to have broadened RNA base specificities in contrast to the stringent requirement for cytosine observed with the wild-type rho. Functional tests on the two kinds of polynucleotide binding sites known for rho have indicated that rho s-81 is mainly altered in the primary site, whereas rho s-82 is simultaneously affected in the secondary binding site as well as the primary site. Thus, we conclude that the primary and secondary sites contribute distinctly in determining the overall efficiency and site-specificity of termination, respectively. Further analysis of detailed termination points at the trpE and lambda tR1 terminators has revealed that major RNA transcripts generated by the wild-type rho and rho s-81 are notably rich in adenine and poor in cytosine for the 3'-terminal five to ten nucleotides, whereas those preferentially terminated by rho s-82 are conversely richer in cytosine than adenine. This finding suggests that rho may recognize the RNA-DNA hybrid region at the 3' end of a nascent transcript in its secondary binding reaction.

Regulatory effect of a synthetic CRP recognition sequence placed downstream of a promoter

A series of plasmids were constructed in which a promoter was introduced into a lac-based operon fusion vector. A perfectly symmetrical oligonucleotide of 22-bp corresponding to an idealized binding site for cAMP receptor protein (CRP) of E. coli was chemically synthesized. The synthetic CRP site was placed between the promoter and the lacZ structural gene with varying distances from the promoter. Specific binding of cAMP-CRP complex to the synthetic CRP site was shown by a gel retardation and a DNase I footprinting assays. Plasmid constructs were transformed into crp+ and crp- cells carrying a chromosomal deletion of the lac genes. The regulatory effect of the inserted CRP site was examined by comparing the beta-galactosidase activity and the levels of RNA transcript in two cells harboring the plasmids. We found a strong inhibitory effect of the CRP site in the presence of cAMP and CRP when it was placed close to the promoter. When the CRP site was placed far downstream of the promoter, a moderate repression of transcription was observed.

Further analysis of cDNA clones for maize phosphoenolpyruvate carboxylase involved in C4 photosynthesis. Nucleotide sequence of entire open reading frame and evidence for polyadenylation of mRNA at multiple sites in vivo

Four clones of cDNA for phosphoenolpyruvate carboxylase [EC 4.1.1.31] were obtained from a maize green leaf cDNA library by colony hybridization. The largest cDNA was of full-length (3335 nucleotides), being 243 nucleotides longer than the cDNA cloned previously [(1986) Nucleic Acids Res. 14, 1615-1628]. Alignment of the sequence for the N-terminal coding region found in two of the four clones with the sequence reported previously, established the sequence of the entire coding region for the enzyme. The sequencing of 3'-untranslated region of the clones revealed that the poly(A) tract is attached at multiple sites in vivo.

Construction and characterization of plasmid and lambda phage vector systems for study of transcriptional control in Escherichia coli

We constructed a family of lambda phage and plasmid vectors which facilitate cloning and quantitative analysis of transcriptional regulator in both single and multiple copies. Their expression system was modified from the ara-trp-lac fusion operon of plasmid pMC81 [Casadaban and Cohen, J. Mol. Biol. 138 (1980) 179-207], which is designed to assay both promoters and terminators with a single vehicle. To eliminate transcriptional and translational polar effects liable to occur in the original fusion operon upon insertion of a foreign nucleotide sequence, intracistronic Rho-dependent terminators, that are present within the trpB gene and distal to the cloning site were deleted, and DNA spacers containing stop codons were introduced immediately before and after the cloning site. In analysis of the cloned trp regulatory region, the lambda phage system faithfully reproduced the tight regulation by tryptophan characteristic to the natural trp operon on the E. coli chromosome, whereas the plasmid counterpart exhibited a substantially relaxed response. Comparative studies on the relative strengths of various promoters and terminators have further demonstrated that the lambda phage vector system permits accurate assays of exceptionally strong promoters like Ptrp and lambda pL without disturbing the bacterial growth, while being sensitive enough for detecting low-level transcription under the control of weak promoters or potent terminators. Cloning with the lambda phage vector can be greatly facilitated by transferring the target regulatory site precloned with the plasmid onto the phage genome through in vivo recombination.

Nucleotide sequence and molecular evolution of mouse retrovirus-like IAP elements

We determined the nucleotide (nt) sequences of cDNA and genomic clones for murine intracisternal type A particle (IAP) elements, which are retrovirus-like repetitive sequences in rodent genomes. The nucleotide sequence of the cDNA resembled that of retrovirus RNA genomes in its lack of the U5 sequence within the 3' long terminal repeat. By sequence comparison of our clones with reported rodent IAP elements, we located the probable gag, pol and env gene regions. The sequences for the pol, env and the 3' two-thirds of the gag region were conserved among the IAP elements. In the regions, synonymous substitutions occurred more frequently than non-synonymous ones, which suggested that the regions in question were functionally constrained until fairly recently. The rate of nucleotide substitutions in the regions was estimated to be 6-10 X 10(-9) nt per site per year, and significantly higher than that of the cellular genes. These rates may exemplify a characteristic of the nucleotide substitutions for an endogenous retrovirus. The sequence homology between the IAP element and IgE-binding factor gene is discussed.

Construction of a new plasmid vector that can express cloned cDNA in all translational reading frames

Construction of a bacterial expression vector, pSI4001, is described. The vector contains the lac promoter-operator and three sets of ribosome-binding sites (RBSs) tandemly arranged in all possible reading frames. cDNA can be directly cloned downstream from these translational start points in the fixed and proper orientation by using the method of Okayama and Berg [Mol. Cell. Biol. 3 (1982) 280-289]. The open reading frame of any cDNA inserted may be automatically aligned in phase with either of the three ATG start codons, thus enabling its expression with a maximum theoretical probability of unity. Fusion with the lacZ gene (coding for beta-galactosidase) has shown that at least two of the three translation initiation sites exhibit high expression capacities and the remaining one can also function at a lower but significant rate. We used the vector to construct a bovine pituitary cDNA library, from which clones coding for prolactin were detected by immunological screening with an efficiency as high as two in three clones. The construction with triple RBSs should also provide a unique experimental model to study the regulation of overlapping translations.

Cloning and sequence analysis of cDNA encoding active phosphoenolpyruvate carboxylase of the C4-pathway from maize

A recombinant clone, pM52, containing cDNA for maize phosphoenolpyruvate carboxylase (PEPCase, EC 4.1.1.31) was isolated from a maize leaf cDNA library constructed using an expression vector in Escherichia coli. The screening of the clone was conveniently performed through its ability to complement the phenotype (glutamate requirement) of PEPCase-negative mutant of E. coli. The enzyme encoded by this clone was identical with the major PEPCase in maize, a key enzyme in the C4-pathway, as judged from its allosteric properties and immunological reactivity. The cloned cDNA (3093 nucleotides in length) contained an open reading frame of 2805 nucleotides, the 3'-untranslated region of 222 nucleotides and the poly(dA) tract of 64 nucleotides. The deduced amino acid sequence (935 residues) of the enzyme showed higher homology with that of an enterobacterium, E. coli (43%) than that of a cyanobacterium (blue-green alga), Anacystis nidulans (33%).

Molecular cloning of cDNA for rat mitochondrial 3-oxoacyl-CoA thiolase

Messenger RNA of rat 3-oxoacyl-CoA thiolase (acetyl-CoA acyltransferase), a mitochondrial matrix enzyme involved in fatty acid beta-oxidation, was enriched by immunoprecipitation of rat liver free polysomes and recombinant plasmids were prepared from the enriched mRNA by a modification of the vector-primer method of Okayama and Berg. The transformants were initially screened for 3-oxoacyl-CoA thiolase cDNA sequences by differential colony hybridization with [32P]cDNAs, synthesized from the immunopurified and unpurified mRNAs. The cDNA clones for 3-oxoacyl-CoA thiolase were identified by hybrid-arrested translation and hybrid-selected translation. One of the clones, designated pT1-1, contained a 700-base insert and hybridized to a mRNA species of 1.6 X 10(3) bases in rat liver. The transformants were rescreened using the cDNA insert of pT1-1 as a hybridization probe and a clone (pT1-19) with a 1.5 X 10(3)-base insert was obtained. Activity and concentration of 3-oxoacyl-CoA thiolase mRNA were quantified by in vitro translation and dot-blot analysis using the cDNA insert as a hybridization probe. The level of translatable and hybridizable mRNA in rat liver was increased about 5.1-fold and 4.6-fold, respectively, after administration of di-(2-ethylhexyl)phthalate, a potent inducer of the enzyme. The 3-oxoacyl-CoA thiolase mRNA levels thus determined correlated closely with levels of the activity and amount of this enzyme.

Construction of a cDNA to the hamster CAD gene and its application toward defining the domain for aspartate transcarbamylase

cDNA complementary to hamster mRNA encoding the CAD protein, a multifunctional protein which carries the first three enzymes of pyrimidine biosynthesis, was constructed. The longest of these recombinants (pCAD142) covers 82% of the 7.9-kilobase mRNA. Portions of the cDNA were excised and replaced by a lac promoter-operator-initiation codon segment. The resultant plasmids were transfected into an Escherichia coli mutant defective in aspartate transcarbamylase, the second enzyme of the pathway. Complementation of the bacterial defect was observed with as little as 2.2 kilobases of cDNA sequence, corresponding to the 3' region of the mRNA. DNA sequencing in this region of the hamster cDNA reveals stretches which are highly homologous to the E. coli gene for the catalytic subunit of aspartate transcarbamylase; other stretches show no homology. The highly conserved regions probably reflect areas of protein structure critical to catalysis, while the nonconserved regions may reflect differences between the quaternary structures of E. coli and mammalian aspartate transcarbamylases, one such difference being that the bacterial enzyme in its native form is allosterically regulated and the mammalian enzyme is not.

Expression of the gag gene of human T-cell leukemia virus type I in Escherichia coli and its diagnostic use

An expression plasmid, pHY202, was constructed which directs the synthesis of a fusion protein encoded by the gag sequence of human T-cell leukemia virus type I (HTLV-I) inserted into the lacZ' gene. Escherichia coli cells harboring pHY202 produced the 43-kDal LacZ'-Gag fusion protein with a yield of approx. 0.3% of total soluble proteins. The fusion protein is specifically recognized by monoclonal antibodies against the Gag proteins p19 and p24, and could be applicable for the diagnosis of HTLV-I infection, because almost all sera from HTLV-I carriers gave a positive response in the enzyme-linked immunosorbent assay (ELISA) employing the LacZ'-Gag hybrid protein purified by immunoaffinity column chromatography.

Isolation and characterization of rho mutants of Escherichia coli with increased transcription termination activities

A novel type of rho mutants, rhos, with increased transcription termination activities have been isolated. A termination defective rho mutation rho-ts702 (formerly designated nitA702), which causes temperature-sensitive cell growth, was found to be dominant over the wild-type allele in relieving mutational polarity. The rhos mutations were derived as temperature-resistant revertants of rho-ts702 carried by lambda transducing phage. They exhibited dominance over rho-ts702 leading to restoration of polarity. When the rhos mutations were introduced into the Escherichia coli chromosome, they caused increased polarity in the trp and lac operons. The rhos mutants were classified into two groups in terms of their terminator specificity: The first group demonstrated increased termination efficiencies against all terminators tested, whereas the second exhibited various efficiencies, either more than or less than the normal level depending on the terminator. The cellular content of p protein in each rhos strain was significantly lower than that in the rho+ strain. Moreover, in an in vitro transcription system, purified ps proteins showed increased termination activities against the trpE pseudoterminators. These results indicate that the rhos phenotype is due to qualitative alterations, rather than quantitative increases, of the p protein. The reduced content of ps enforces the current notion that the rho gene is autogenously regulated by rho-dependent transcriptional attenuation.

Overproduction of transcription termination factor Rho in Escherichia coli

A plasmid system has been constructed which allows high-level expression of the rho gene of Escherichia coli under the control of the pL promoter and the N-antitermination regulatory system of bacteriophage lambda. The pL-directed synthesis of Rho crucially depends on the lambda N gene product and is promoted most effectively when this product is supplied from the N gene cloned on a separate compatible plasmid with a moderate copy number. The requirement for N can be circumvented partly, but not completely, by deletion of the region preceding the rho structural gene. Attempts were also made to optimize the construction of rho-expression plasmids by adjusting the orientation and location of pL and rho inserts on the pBR322 vector. With optimal conditions, Rho protein is overexpressed 100-fold and can become as much as 10% of the total cellular protein. Using this plasmid system, Rho can be purified with a yield of more than 20 mg from 10 g of induced cells.

Genomic structure of HTLV (human T-cell leukemia virus): detection of defective genome and its amplification in MT-2 cells

We studied the genomic structure of human T-cell leukemia virus (HTLV) in the HTLV producer cell line MT-2. Southern blotting revealed that at least eight HTLV proviruses were integrated in the chromosomes of MT-2 cells. The genomic structure of these proviruses was analyzed using fragments of cloned HTLV that were specific to gag, pol, env, pXs and U3R genes as probes. We have identified a complete genome of HTLV in MT-2 (non-defective type). However, seven of the eight proviruses had defective genomes. Provirus T2-a contains only the U3R (LTR) of HTLV and T2-b corresponds to the non-defective genome. T2-c possesses only a portion of env, and pXs and U3R. T2-d consists of gag, pol, part of env and U3R. On the other hand, T2-e, f, g and h consist of gag, pXs and U3R. Northern blotting experiments with mRNA from MT-2 cells supported the evidence of amplification of the gag-pXs gene of HTLV. 26S mRNA is considered to be a subgenomic species of 35S RNA. 32S mRNA may represent the T2-d provirus which lacks a portion of env and pXs, while 20S mRNA was a subgenomic species. The gag-pXs gene may correspond to 24S mRNA, the amount which was amplified in MT-2 cells.

Function of transcription termination factor rho in a model transcription system using synthetic deoxyribonucleic acid as template

The function of a transcription termination factor, rho, has been studied by using several synthetic DNAs with simple repetitive base sequences as templates for transcription. rho actually exhibits various effects on transcription depending on the base sequence of the template: (1) rho terminates poly(A) synthesis with poly(dA) x poly(dT), poly(dT), or oligo(dT), leading to release of RNA from RNA polymerase. rho also inhibits the synthesis of other homoribopolymers such as poly(U) directed by poly(dA) x poly(dT) and poly(C) and poly(I) directed by poly(dG) x poly(dC), presumably by a similar mechanism. (2) rho inhibits the synthesis of another homoribopolymer, poly(G), directed by poly(dG) x poly(dC) at the step of initiation rather than propagation of transcription. (3) rho stimulates rather than inhibits the synthesis of poly(A-C) and poly(G-U) directed by poly[d(A-C)] x poly[d(G-T)], presumably by enhancing the dissociation of transcription complexes. (4) rho has no influence on the synthesis of poly(A-U) and poly(G-C) directed by poly[d(A-T)] and poly[d(G-C)], respectively. In the first case, but not otherwise, the effect of rho is coupled with its RNA-dependent nucleosidetriphosphate phosphohydrolase activity, as is rho-mediated transcription termination on natural templates. The implication of these results is discussed in reference to the current view that rho acts on transcription complexes that have ceased elongation and causes release of RNA in an energy-requiring reaction.

Studies of RNA release reaction catalyzed by E. coli transcription termination factor rho using isolated ternary transcription complexes

Protein factor rho catalyzes site-specific termination of transcription in a reaction requiring hydrolysis of nucleoside triphosphate with eventual release of RNA from RNA polymerase and DNA template. We have characterized the rho-catalyzed RNA release reaction using isolated transcription complexes. Transcription complexes containing T7 D111 DNA, RNA polymerase, and 3H-labeled nascent RNA were formed and isolated by gel filtration on an Agarose 5M column. When the ternary complexes were incubated with rho factor in the presence of ATP, or dATP, significant amounts of nascet RNA were released from the complexes as determined in a membrane filtration assay. Gel electrophoretic analysis of RNA has revealed that rho releases selected species of discrete-sized RNA from among those originally present in the ternary complexes. These results show that rho essentially acts to release RNA from those ternary complexes which have come to pause, and that this reaction proceeds in a discrete step separately from the pausing of RNA synthesis. Under the conditions used, the extent of RNA release widely varied at individual pausing sites and thus the action of rho exhibited certain site-selectivity.