The EF-G-like GTPase Snu114p regulates spliceosome dynamics mediated by Brr2p, a DExD/H box ATPase

Binding of a pre-mRNA substrate triggers spliceosome activation, whereas the release of the mRNA product triggers spliceosome disassembly. The mechanisms that underlie the regulation of these rearrangements remain unclear. We find evidence that the GTPase Snu114p mediates the regulation of spliceosome activation and disassembly. Specifically, both unwinding of U4/U6, required for spliceosome activation, and disassembly of the postsplicing U2/U6.U5.intron complex are repressed by Snu114p bound to GDP and derepressed by Snu114p bound to GTP or nonhydrolyzable GTP analogs. Further, similar to U4/U6 unwinding, spliceosome disassembly requires the DExD/H box ATPase Brr2p. Together, our data define a common mechanism for regulating and executing spliceosome activation and disassembly. Although sequence similarity with EF-G suggests Snu114p functions as a molecular motor, our findings indicate that Snu114p functions as a classic regulatory G protein. We propose that Snu114p serves as a signal-dependent switch that transduces signals to Brr2p to control spliceosome dynamics.

Spb4p, an essential putative RNA helicase, is required for a late step in the assembly of 60S ribosomal subunits in Saccharomyces cerevisiae

Spb4p is a putative ATP-dependent RNA helicase that is required for synthesis of 60S ribosomal subunits. Polysome analyses of strains genetically depleted of Spb4p or carrying the cold-sensitive spb4-1 mutation revealed an underaccumulation of 60S ribosomal subunits. Analysis of pre-rRNA processing by pulse-chase labeling, northern hybridization, and primer extension indicated that these strains exhibited a reduced synthesis of the 25S/5.8S rRNAs, due to inhibition of processing of the 27SB pre-rRNAs. At later times of depletion of Spb4p or following transfer of the spb4-1 strain to more restrictive temperatures, the early pre-rRNA processing steps at sites A0, Al, and A2 were also inhibited. Sucrose gradient fractionation showed that the accumulated 27SB pre-rRNAs are associated with a high-molecular-weight complex, most likely the 66S pre-ribosomal particle. An HA epitope-tagged Spb4p is localized to the nucleolus and the adjacent nucleoplasmic area. On sucrose gradients, HA-Spb4p was found almost exclusively in rapidly sedimenting complexes and showed a peak in the fractions containing the 66S pre-ribosomes. We propose that Spb4p is involved directly in a late and essential step during assembly of 60S ribosomal subunits, presumably by acting as an rRNA helicase.

Oocyte polarity depends on regulation of gurken by Vasa

Vasa, a DEAD box mRNA helicase similar to eIF4A, is involved in pole plasm assembly in the Drosophila oocyte and appears to regulate translation of oskar and nanos mRNAs. However, several vasa alleles exhibit a wide range of early oogenesis phenotypes. Here we report a detailed analysis of Vasa function during early oogenesis using novel as well as previously identified hypomorphic vasa alleles. We find that vasa is required for the establishment of both anterior-posterior and dorsal-ventral polarity of the oocyte. The polarity defects of vasa mutants appear to be caused by a reduction in the amount of Gurken protein at stages of oogenesis critical for the establishment of polarity. Vasa is required for translation of gurken mRNA during early oogenesis and for achieving wild-type levels of gurken mRNA expression later in oogenesis. A variety of early oogenesis phenotypes observed in vasa ovaries, which cannot be attributed to the defect in gurken expression, suggest that vasa also affects expression of other mRNAs.

Dbp7p, a putative ATP-dependent RNA helicase from Saccharomyces cerevisiae, is required for 60S ribosomal subunit assembly

Putative ATP-dependent RNA helicases are ubiquitous, highly conserved proteins that are found in most organisms and they are implicated in all aspects of cellular RNA metabolism. Here we present the functional characterization of the Dbp7 protein, a putative ATP-dependent RNA helicase of the DEAD-box protein family from Saccharomyces cerevisiae. The complete deletion of the DBP7 ORF causes a severe slow-growth phenotype. In addition, the absence of Dbp7p results in a reduced amount of 60S ribosomal subunits and an accumulation of halfmer polysomes. Subsequent analysis of pre-rRNA processing indicates that this 60S ribosomal subunit deficit is due to a strong decrease in the production of 27S and 7S precursor rRNAs, which leads to reduced levels of the mature 25S and 5.8S rRNAs. Noticeably, the overall decrease of the 27S pre-rRNA species is neither associated with the accumulation of preceding precursors nor with the emergence of abnormal processing intermediates, suggesting that these 27S pre-rRNA species are degraded rapidly in the absence of Dbp7p. Finally, an HA epitope-tagged Dbp7 protein is localized in the nucleolus. We propose that Dbp7p is involved in the assembly of the pre-ribosomal particle during the biogenesis of the 60S ribosomal subunit.

Zebrafish vasa homologue RNA is localized to the cleavage planes of 2- and 4-cell-stage embryos and is expressed in the primordial germ cells

Identification and manipulation of the germ line are important to the study of model organisms. Although zebrafish has recently emerged as a model for vertebrate development, the primordial germ cells (PGCs) in this organism have not been previously described. To identify a molecular marker for the zebrafish PGCs, we cloned the zebrafish homologue of the Drosophila vasa gene, which, in the fly, encodes a germ-cell-specific protein. Northern blotting revealed that zebrafish vasa homologue (vas) transcript is present in embryos just after fertilization, and hence it is probably maternally supplied. Using whole-mount in situ hybridization, we investigated the expression pattern of vas RNA in zebrafish embryos from the 1-cell stage to 10 days of development. Here we present evidence that vas RNA is a germ-cell-specific marker, allowing a description of the zebrafish PGCs for the first time. Furthermore, vas transcript was detected in a novel pattern, localized to the cleavage planes in 2- and 4-cell-stage embryos. During subsequent cleavages, the RNA is segregated as subcellular clumps to a small number of cells that may be the future germ cells. These results suggest new ways in which one might develop techniques for the genetic manipulation of zebrafish. Furthermore, they provide the basis for further studies on this novel RNA localization pattern and on germ-line development in general.

Xp54, the Xenopus homologue of human RNA helicase p54, is an integral component of stored mRNP particles in oocytes

In investigating the composition of stored (maternal) mRNP particles in Xenopus oocytes, attention has focussed primarily on the phosphoproteins pp60/56, which are Y-box proteins involved in a general packaging of mRNA. We now identify a third, abundant, integral component of stored mRNP particles, Xp54, which belongs to the family of DEAD-box RNA helicases. Xp54 was first detected by its ability to photocrosslink ATP. Subsequent sequence analysis identifies Xp54 as a member of a helicase subfamily which includes: human p54, encoded at a chromosomal breakpoint in the B-cell lymphoma cell line, RC-K8; Drosophila ME31B, encoded by a maternally-expressed gene, and Saccharomyces pombe Ste13, cloned by complementation of the sterility mutant ste13. Expression studies reveal that the gene encoding Xp54 is transcribed maximally at early oogenesis: no transcripts are detected in adult tissues, other than ovary. Using a monospecific antibody raised against native Xp54, its presence in mRNP particles is confirmed by immunoblotting fractions bound to oligo(dT)-cellulose and separated by rate sedimentation and buoyant density. On isolating Xp54 from mRNP particles, it is shown to possess an ATP-dependent RNA helicase activity. Possible functions of Xp54 are discussed in relation to the assembly and utilization of mRNP particles.

Dbp3p, a putative RNA helicase in Saccharomyces cerevisiae, is required for efficient pre-rRNA processing predominantly at site A3

In Saccharomyces cerevisiae, ribosomal biogenesis takes place primarily in the nucleolus, in which a single 35S precursor rRNA (pre-rRNA) is first transcribed and sequentially processed into 25S, 5.8S, and 18S mature rRNAs, leading to the formation of the 40S and 60S ribosomal subunits. Although many components involved in this process have been identified, our understanding of this important cellular process remains limited. Here we report that one of the evolutionarily conserved DEAD-box protein genes in yeast, DBP3, is required for optimal ribosomal biogenesis. DBP3 encodes a putative RNA helicase, Dbp3p, of 523 amino acids in length, which bears a highly charged amino terminus consisting of 10 tandem lysine-lysine-X repeats ([KKX] repeats). Disruption of DBP3 is not lethal but yields a slow-growth phenotype. This genetic depletion of Dbp3p results in a deficiency of 60S ribosomal subunits and a delayed synthesis of the mature 25S rRNA, which is caused by a prominent kinetic delay in pre-rRNA processing at site A3 and to a lesser extent at sites A2 and A0. These data suggest that Dbp3p may directly or indirectly facilitate RNase MRP cleavage at site A3. The direct involvement of Dbp3p in ribosomal biogenesis is supported by the finding that Dbp3p is localized predominantly in the nucleolus. In addition, we show that the [KKX] repeats are dispensable for Dbp3p's function in ribosomal biogenesis but are required for its proper localization. The [KKX] repeats thus represent a novel signaling motif for nuclear localization and/or retention.

Native gel analysis of ribonucleoprotein complexes from a Leishmania tarentolae mitochondrial extract

Two polypeptides of 50 and 45 kDa were adenylated by incubation of a mitochondrial extract from Leishmania tarentolae with [alpha-32P]ATP. These proteins were components of a complex that sedimented at 20S in glycerol gradients and migrated as a single band of approximately 1800 kDa in a native gel. The facts that RNA ligase activity cosedimented at 20S and that the ATP-labeled p45 and p50 polypeptides were deadenylated upon incubation with a ligatable RNA substrate suggested that these proteins may represent charged intermediates of a mitochondrial RNA ligase. Hybridization of native gel blots with guide RNA (gRNA) probes showed the presence of gRNA in the previously identified T-IV complexes that sedimented in glycerol at 10S and contained terminal uridylyl transferase (TUTase) activity, and also in a previously unidentified class of heterodisperse complexes that sedimented throughout the gradient. gRNAs were not detected in the p45 + p50-containing 1800 kDa complex. The heterodisperse gRNA-containing complexes were sensitive to incubation at 27 degrees C and appear to represent complexes of T-IV subunits with mRNA. Polyclonal antiserum to a 70 kDa protein that purified with terminal uridylyl transferase activity was generated, and the antiserum was used to show that this p70 polypeptide was a component of both the T-IV and the heterodisperse gRNA-containing complexes. We propose that the p45 + p50-containing 1800 kDa complex and the p70 + gRNA-containing heterodisperse complexes interact in the editing process. Further characterization of these various complexes should increase our knowledge of the biochemical mechanisms involved in RNA editing.

Changes of enzymes and factors involved in DNA synthesis during wheat embryo germination

We have previously purified and characterized wheat germ DNA polymerases A and B. To determine the role played by DNA polymerases A and B in DNA replication, we have measured the level of their activities during wheat embryo germination. The level of cellular proteins known to be associated with DNA synthesis such as PCNA and DNA primase were also investigated. The activity of DNA polymerase A gradually increased reaching a maximal level at 12 h after germination. Three days later, only a residual activity was detected. DNA polymerase B showed the same pattern during germination with very similar changes in activity. Our results indicate a striking correlation between maximal activities of DNA polymerase A, DNA polymerase B and optimal levels of DNA synthesis. These results support a replicative role of these enzymes. The activity of wheat DNA primase that copurifies with DNA polymerase A also increases during wheat germination. Taking together all its properties, and in spite of its behaviour with some inhibitors. DNA polymerase A may be considered as the plant counterpart of animal DNA polymerase alpha. Concerning DNA polymerase B we have previously shown that PCNA stimulates its processivity. Besides studying the changes of DNA polymerases A and B and DNA primase we have also studied changes in PCNA during germination. We show that PCNA is present in wheat embryos at a constant relatively high level during the first 24 h of germination. After 48 h, the absence of PCNA is concomitant with an important decrease in DNA polymerase B activity. In this report we confirm the behaviour of DNA polymerase B as a delta-like activity.

Leukophysin: an RNA helicase A-related molecule identified in cytotoxic T cell granules and vesicles

Leukophysin (LKP) is a 28-kDa protein of CTL and U937 monocytic cells that is located in the membrane of high density granules as well as lighter cytoplasmic granules or vesicles. mAbs to KLP were used to clone a full length cDNA clone with an open reading frame coding for a 235-amino acid polypeptide with a molecular mass of 24.3 kDa and two potential transmembrane regions. The nucleotide sequence was highly homologous to the 3' end of human RNA helicase A. Expression of the LKP was confirmed as a reverse transcriptase-PCR product that may be an alternately spliced product of RNA helicase A. The cDNA contained a repetitive motif that was similar to synaptophysin 1, a protein that is important for synaptic vesicle exocytosis. A polyclonal Ab directed against the 17 carboxyl-terminal amino acids of LKP detected the same 28-kDa granule membrane protein as the D545, one of the mAbs used to clone the cDNA. In addition, the D545 mAb reacted strongly with the GST fusion protein of the bacterially expressed LKP cDNA. In confocal immunofluorescence studies, the anti-LKP peptide Ab reacted with granzyme A-negative granules and vesicles in CD8+ CTL lymphocytes from normal and Chediak-Higashi patients. Thus, based on the expression of the C-terminal LKP epitope, vesicular structures an granules have been detected in CTL that are distinct from classical granzyme-containing cytolytic granules.

Mechanisms leading to and the consequences of altering the normal distribution of ATP(CTP):tRNA nucleotidyltransferase in yeast

CCA1 codes for mitochondrial, cytosolic, and nuclear ATP(CTP):tRNA nucleotidyltransferase. Studies reported here examine the mechanisms leading to and the consequences of altering the distribution of this important tRNA processing enzyme. We show that the majority of Cca1p-I, translated from the first in-frame ATG, is in mitochondria but surprisingly, there is a small contribution to nuclear and cytosolic tRNA processing by this form as well. The majority of Cca1p-II and Cca1p-III, translated from ATG2 and ATG3, respectively, is in the cytosol but both are also located in the nucleus for processing precursors. Altering the cytosolic/nuclear distribution of Cca1p by fusing the SV40 nuclear localization signal to the 5' end of CCA1 causes a growth defect and results in the accumulation of end-shortened tRNAs in the cytosol. These results suggest an important role for Cca1p in the cytosol of eukaryotes, presumably in the repair of 3' CCA termini. These experiments also demonstrate that individual tRNAs are affected differently by reduced cytosolic nucleotidyltransferase and that cells resuming exponential growth are more severely affected than those continuing exponential growth.

The human breast cell DNA synthesome: its purification from tumor tissue and cell culture

In this report, we describe for the first time the isolation and purification of a multiprotein complex for DNA replication from MDA MB-468 human breast cancer cells. This complex, which we designate the DNA synthesome, fully supports the in vitro replication of simian virus 40 (SV40) origin-containing DNA in the presence of the viral large T-antigen. Since the SV40 virus utilizes the host's cellular proteins for its own DNA replication, our results indicate that the DNA synthesome may play a role not only in viral DNA synthesis but in human breast cell DNA replication as well. Our studies demonstrate that the following DNA replication proteins constitute the DNA synthesome: DNA polymerase alpha, DNA primase, DNA polymerase delta, proliferating cell nuclear antigen, replication protein A, replication factor C, DNA topoisomerases I, II, and DNA polymerase epsilon. In addition, we successfully isolated the DNA synthesome from human breast tumor tissue as well as from xenografts from nude mice injected with the human breast cancer cell line MCF-7. The DNA synthesome purified from the breast cancer tissues fully supports SV40 DNA replication in vitro. Furthermore, our results obtained from a novel forward mutagenesis assay suggest that the DNA synthesome isolated from a nonmalignant breast cell line mediates SV40 DNA replication by an error-resistant mechanism. In contrast, the DNA synthesome derived from malignant breast cells and tissue exhibited a lower fidelity for DNA synthesis in vitro. Overall, our data support the role of the DNA synthesome as mediating breast cell DNA replication in vitro and in vivo.

Nucleic acid compartmentalization within the cell nucleus by in situ transferase-immunogold techniques

In the present review, we report on recent results obtained by in situ transferase-immunogold techniques as to the ultrastructural distribution of DNA and RNA within the cell nucleus. Special emphasis is placed on the various nucleolar components and the various enigmatic structures of the extranucleolar region: interchromatin granules, coiled bodies, and simple nuclear bodies. These data are discussed in the light of our current understanding of the functional organization of the cell nucleus.

A 130 kDa polypeptide immunologically related to the 180 kDa catalytic subunit of DNA polymerase alpha-primase complex is detected in early embryos of Drosophila

An immunocytochemical method using specific antibodies was employed to detect DNA polymerase alpha-primase complex in Drosophila melanogaster embryos during the first 13 nuclear division cycles. A monoclonal antibody specific to the 72 kDa polypeptide stained interphase nuclei, but not metaphase chromosome, while at late anaphase and thereafter staining of the chromosome was regained. On the other hand, a polyclonal antibody specific to the 180 kDa polypeptide stained not only the interphase nuclei but also the cytoplasmic regions surrounding interphase nuclei. These results suggest that the distributions of the 180 kDa and the 72 kDa polypeptides of DNA polymerase alpha-primase complex are different. We detected the 180 kDa and the 72 kDa polypeptides in the extract prepared from a single Drosophila embryo by Western blotting, and a 130 kDa polypeptide immunologically related to the 180 kDa polypeptide was also detected in the extract. These polypeptides (180, 130, and 72 kDa) in the embryos were detected at similar levels at interphase and at the mitotic phase. These three polypeptides were also detected in unfertilized eggs, showing that they were maternally stored. The 130 kDa polypeptide was detected till cycle 10, then began to decrease, and finally disappeared at cycle 14, whereas the 180 kDa and the 72 kDa polypeptides were present without marked fluctuation in quantity throughout the developmental stages. Even in unfertilized eggs, the level of the 130 kDa polypeptide decreased gradually with a similar time course to that in fertilized ones, but the levels of the 180 kDa and the 72 kDa polypeptides remained unchanged. This is the first report suggesting the existence of the 130 kDa polypeptide in vivo in the early embryos of Drosophila. The significance of the 130 kDa polypeptide is discussed.

Characterization of DNA polymerase-alpha/primase complex from developing embryonic chicken brains

DNA polymerase-alpha and primase activities present in a complex, have been isolated, partially purified, and characterized from embryonic chicken brain. DNA polymerase-alpha activity, characterized by its sensitivity to N-ethyl-maleimide, high sedimentation coefficient (11.3 S), and acidic isoelectric point (5-5.5) was found in all embryonic ages. Primase activity, the enzyme responsible for the initiation of DNA synthesis, co-sedimented with DNA polymerase-alpha activity on a continuous glycerol velocity gradient. A complex containing both DNA polymerase-alpha and primase activities was isolated by DE-23 cellulose column chromatography of cell-free extracts of different embryonic ages of chicken brain. In addition to the primase complexed with DNA polymerase-alpha, a free primase activity was isolated by DE-23 cellulose column chromatography of an ammonium sulfate (0-45%; w/v) precipitated fraction of embryonic chicken brain cell-free extract. DNA polymerase-alpha activity from developing chicken brains in the embryonic stage was purified by immuno-affinity column chromatography. Of all the single-stranded DNA templates tested, primase activity was found to be maximally active with poly dC. Primase activity was not inhibited by a high concentration of alpha-amanitin. The results obtained may provide insight into further understanding of regulation of chromosomal DNA replication in developing tissues.

Characterization of two classes of ribonucleoprotein complexes possibly involved in RNA editing from Leishmania tarentolae mitochondria

The molecular mechanism of RNA editing in trypanosomatid mitochondria is an unsolved problem. We show that two classes of ribonucleoprotein complexes exist in a mitochondrial extract from Leishmania tarentolae and appear to be involved in RNA editing. The 'G' class of RNP complexes consists of 170-300 A particles which contain guide RNAs and proteins, show little terminal uridylyl transferase (TUTase) activity and exhibit an in vitro RNA editing-like activity. The 'T' class consists of approximately six RNP complexes, the endogenous RNA of which can be self-labeled with [alpha-32P]UTP. The most abundant T complex, T-IV, is visualized by electron microscopy as 80-140 A particles. This complex exhibits TUTase activity in the native gel and contains guide RNAs. Both G and T complexes are possibly involved with RNA editing in vivo. These results are a starting point for the analysis of the biochemistry of RNA editing.

The B subunit of the DNA polymerase alpha-primase complex in Saccharomyces cerevisiae executes an essential function at the initial stage of DNA replication

The four-subunit DNA polymerase alpha-primase complex is unique in its ability to synthesize DNA chains de novo, and some in vitro data suggest its involvement in initiation and elongation of chromosomal DNA replication, although direct in vivo evidence for a role in the initiation reaction is still lacking. The function of the B subunit of the complex is unknown, but the Saccharomyces cerevisiae POL12 gene, which encodes this protein, is essential for cell viability. We have produced different pol12 alleles by in vitro mutagenesis of the cloned gene. The in vivo analysis of our 18 pol12 alleles indicates that the conserved carboxy-terminal two-thirds of the protein contains regions that are essential for cell viability, while the more divergent NH2-terminal portion is partially dispensable. The characterization of the temperature-sensitive pol12-T9 mutant allele demonstrates that the B subunit is required for in vivo DNA synthesis and correct progression through S phase. Moreover, reciprocal shift experiments indicate that the POL12 gene product plays an essential role at the early stage of chromosomal DNA replication, before the hydroxyurea-sensitive step. A model for the role of the B subunit in initiation of DNA replication at an origin is presented.

On the association of DNA primase activity with the nuclear matrix in HeLa S3 cells

We have reinvestigated the association of DNA primase activity with the nuclear matrix prepared from exponentially growing HeLa S3 cells. We have found that 25-30 per cent of the nuclear primase activity resists extraction with 2 M NaCl and digestion with Dnase I. Unlike previous investigations, done with the same cell line, the results showed that nuclear matrix-bound DNA primase activity represented less than 10 per cent of the total cell activity. Association of high levels of primase activity with the nuclear matrix was strictly dependent on a 37 degrees C incubation of isolated nuclei prior to subfractionation. Evidence was obtained that the method used for preparing nuclei can have a dramatic effect on the amount of primase activity which is recovered both in the postnuclear supernatant and in isolated nuclei, thus seriously affecting the interpretation of the results about the quantity of DNA primase activity bound to the nuclear matrix.

Characterization of rotavirus replication intermediates: a model for the assembly of single-shelled particles

The segmented double-stranded (ds)RNA genome of the rotaviruses is replicated asymmetrically with viral mRNA serving as the template for minus-strand RNA synthesis. To identify intermediate structures in rotavirus replication, subviral particles (SVPs) purified from the cytoplasm of simian rotavirus SA11-infected cells were assayed for RNA polymerase activity in a cell-free system that supports viral RNA replication. Intact SVPs containing newly made RNA were resolved by electrophoresis under nondenaturing conditions on 0.6% agarose gels (50 mM Tris-glycine, pH 8.8). This gel system was found to separate without disrupting SA11 single- and double-shelled virions and virion-derived core particles. SVPs from the cell-free system that contained newly made dsRNA migrated in the agarose gels at positions between virion-derived cores and intermediate of single- and double-shelled virions. SVPs containing newly made dsRNA were eluted from the gel and analyzed for protein content by electrophoresis on polyacrylamide gels. The results showed that three distinct types of replication intermediates (RIs) were present in SA11-infected cells. The smallest intermediate (precore RI, 45 nm, 220 S) contained the structural proteins VP1, VP3, and VP9 and the nonstructural proteins NS53, NS35, and NS34. A second intermediate (core RI, 60 nm, 310 S) contained the core proteins VP1, VP2, and VP3 and the proteins VP9, NS35 and NS34. The largest RI (single-shelled RI, 75 nm, 420 S) contained the inner shell proteins VP1, VP2, VP3, and VP6 and the proteins VP9, NS35 and NS34. Analysis of the formation and turnover of RIs in infected cells pulse-labeled with 35S-amino acids supports a hypothesis that rotavirus single-shelled particles are assembled in vivo by the sequential addition of VP2 and VP6 to precore RIs consisting of VP1, VP3, VP9, NS35, and NS34.

Two domains distantly related to protein-tyrosine kinases in the vesicular stomatitis virus polymerase

We have carried out an exhaustive search for amino acid sequence similarities between vesicular stomatitis virus (VSV) proteins and database entries. Unexpectedly, we found that the L polymerase protein contains two blocks of sequence (residues 725-1102 and 1291-1671) with distant but statistically significant similarity to the catalytic domain of tyrosine-specific protein kinases. The first kinase-like region is most similar to members of the Abl subfamily, Fes and Fps (26.6% and 27.3% identity, respectively), whereas the second region is closest to members of the platelet-derived growth factor receptor (PDGFR) subfamily, PDGFR and Kit (30.4% and 25.9% identity, respectively). Multiple alignment of the catalytic domain of these kinases to all three rhabdovirus L protein sequences available (VSV Indiana, VSV New Jersey, and rabies) revealed that the polymerases contain many but not all residues well conserved in the protein kinase family. Similarity was highest for VSV Indiana and lowest for rabies. We conclude that the kinase-like regions in the rhabdoviral L proteins are probably very distantly related to the protein kinase family. The similarities could either reflect contemporary protein kinase activity or represent some other function(s) associated with these large multifunctional polymerase proteins. Our findings also shed new light on questions of the origins and evolution of RNA viruses.

A novel primase-free form of murine DNA polymerase alpha induced by infection with minute virus of mice

Two species of DNA polymerase alpha free of primase activity were identified in extracts of Ehrlich mouse cells that had been infected with minute virus of mice. Primase-free forms of DNA polymerase alpha eluted with 150 and 180 mM NaCl during ion-exchange chromatography on DEAE-cellulose columns, exhibited sedimentation coefficients of 11 S and 8.2 S, respectively, and were inhibited by aphidicolin, N2-(p-n-butylphenyl)-9-(2-deoxy-beta-D-ribofuranosyl)guanine 5'-triphosphate, and 2-(p-n-butylanilino)-9-(2-deoxy-beta-D-ribofuranosyl)adenine 5'-triphosphate. The ratio of primase-free DNA polymerase alpha to the DNA polymerase alpha-primase complex increased from 1.5 to greater than 100 during the course of infection, and free primase was produced during the MVM replicative cycle.

RNA-dependent RNA polymerase activity in two morphologically different white clover cryptic viruses

RNA-dependent RNA polymerase activities were detected in purified particles of white clover cryptic viruses 1 and 2. The polymerases of the two viruses had different requirements for optimum activity. Enzyme activity was dependent upon the presence of virus particles, Mg2+, and the four ribonucleoside triphosphates, and was insensitive to actinomycin D, alpha-amanitin, and rifampicin. The labeled reaction products were dsRNAs as indicated by CF 11 column chromatography and by their ionic-strength-dependent sensitivity to hydrolysis by RNase A and resistance to S1 nuclease. The dsRNAs synthesized in vitro had the same electrophoretic mobilities as the corresponding viral templates.

DNA repair synthesis in mouse spermatogenesis involves DNA polymerase beta activity

The role of DNA polymerase alpha-DNA primase complex and DNA polymerase beta in DNA replication and ultraviolet-induced DNA repair synthesis has been analyzed in mouse spermatogenesis. Autoradiographic experiments with germ cells in culture, indicating an involvement of DNA polymerase alpha and/or delta in DNA replication, and of DNA polymerase beta in DNA repair synthesis, have been confirmed by studying partially purified enzymes. These findings support the idea that, different from other biological systems, in meiotic and post meiotic male mouse germ cells DNA polymerase beta is the main DNA polymerase form needed for DNA repair.

Identification and subcellular localization of the polypeptide for chick DNA primase with a specific monoclonal antibody

Polypeptides responsible for activities of chick embryo DNA primase and DNA polymerase alpha were identified using monoclonal antibodies specific to these two enzymes. The 4-8H antibody neutralized DNA polymerase alpha activity measured on activated DNA template and also ribonucleoside triphosphate-dependent DNA synthesis on single-stranded DNA template (DNA primase-DNA polymerase alpha combined activity) to a partial extent (about 30%), but did not affect DNA primase activity. The 4-2D antibody, although it did not affect DNA polymerase alpha activity, did neutralize both DNA primase activity and DNA primase-DNA polymerase alpha combined activity extensively (up to 70%). Immunoblotting analysis of the DNA primase-DNA polymerase alpha complex showed that 4-2D and 4-8H antibodies recognize 60-kDa and 160-180-kDa polypeptides, respectively. An immunoaffinity column made of either of these antibodies retained DNA primase-DNA polymerase alpha complex. When the enzyme was eluted from the 4-8H column with alkaline solution, DNA primase was eluted prior to DNA polymerase alpha. In the case of 4-2D antibody column chromatography, the elution order of two enzymes was reversed. Results indicate that two enzymes in the complex which was retained in the antibody column were dissociated by lower alkaline pH than that dissociated the antigenic enzymes from the corresponding antibodies. In both cases, the fractions with DNA primase activity contained exclusively 60-kDa polypeptide, while those with DNA polymerase alpha contained 160-180-kDa polypeptides. Thus, DNA primase resided in 60-kDa polypeptide and was recognized by 4-2D antibody while DNA polymerase alpha resided in 160-180-kDa polypeptides and was recognized by 4-8H antibody. Immunofluorescence made with the DNA primase-specific 4-2D antibody as well as with 4-8H antibody appeared in granular structures which were tightly bound to the nuclear matrix. These nuclear fluorescences were much reduced in quiescent cells. Furthermore, since the fluorescence made by these antibodies was induced by adding serum to the quiescent cells in serum-deprived cultures, the expression of DNA primase and its organization in the structures on the nuclear matrix are regulated in correlation to the proliferating stage of cells, as observed with DNA polymerase alpha.

Immunochemical detection of a primase activity related subunit of DNA polymerase alpha from human and mouse cells using the monoclonal antibody

A hybrid cell line (HDR-854-E4) secreting monoclonal antibody (E4 antibody) against a subunit of human DNA polymerase alpha was established by immunizing mice with DNA replicase complex (DNA polymerase alpha-primase complex) prepared from HeLa cells. The E4 antibody immunoprecipitates DNA replicase complex from both human and mouse cells. The E4 antibody neutralizes the primase activity as assessed either by the direct primase assay (incorporation of [alpha-32P]AMP) or by assay of DNA polymerase activity coupled with the primase activity using unprimed poly(dT) as a template. The E4 antibody does not neutralize DNA polymerase alpha activity with the activated calf thymus DNA as a template. Western immunoblotting analysis shows that the E4 antibody binds to a polypeptide of 77 kilodaltons (kDa) which is tightly associated with DNA polymerase alpha. The 77-kDa polypeptide was distinguished from the catalytic subunit (160 and 180 kDa) for DNA synthesis which was detected by another monoclonal antibody, HDR-863-A5. Furthermore, it is unlikely that the 77-kDa peptide is the primase, since we found that the E4 antibody also immunoprecipitates the mouse 7.3S DNA polymerase alpha which has no primase activity, and Western immunoblotting analysis shows that the 77-kDa polypeptide is a subunit of the 7.3S DNA polymerase alpha. Furthermore, after dissociation of the primase from mouse DNA replicase by chromatography on a hydroxyapatite column in the presence of dimethyl sulfoxide and ethylene glycol, the 77-kDa polypeptide is associated with DNA polymerase alpha, and not with the primase.(ABSTRACT TRUNCATED AT 250 WORDS)

[Possible functional role of the DD-domain of RNA-dependent polymerases]

An attempt to study the functional role of one of the most conservative domains found in all RNA-dependent RNA and DNA polymerases of plant and animal viruses (the so called "DD-domain") was made. A structure similar to the "DD-domain" was found in a minor T7 phage tail protein--gpII. Antibodies against this phage protein have been raised and used to probe "DD-domain" in molecules of avian myeloblastose virus reverse transcriptase and E. coli RNA-dependent RNA polymerase. The antibodies are shown to inhibit the activity of these enzymes under certain conditions. At the same time inhibition of the reverse transcriptase reaction causes the decrease in length of the most high molecular cDNA-products as well. The experimental data obtained are discussed in view of the suggested hypothesis on the probable functional role of the "DD-domain" of RNA-dependent polymerases.

Immunological analysis of the polypeptide structure of calf thymus DNA polymerase-primase complex

Five major polypeptides are found in immunoaffinity-purified calf thymus DNA polymerase-DNA primase complex: 185, 160, 68, 55, and 48 kDa. Individual polypeptides purified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were used to produce antibodies in rabbits to aid in identifying the relationships between these polypeptides by immunoblotting and enzyme neutralization procedures. Immunoblot analyses showed that the 160-kDa peptide is derived from the 185-kDa peptide and the 48-kDa peptide is derived from the 68-kDa peptide while antibodies to the 55-kDa peptide do not cross-react with other peptides found in the complex. Direct enzyme neutralization studies demonstrated that antibodies to 185- and 160-kDa peptides inhibit DNA polymerase activity in the complex, confirming earlier suggestions that these peptides are the catalytic peptides for DNA polymerase. DNA primase activity in the complex is inhibited by antibodies to 68-, 55-, and 48-kDa peptides and to a lesser extent by antibodies to the 160-kDa peptide. Free DNA primase isolated from the complex was estimated to have a native molecular weight of about 110,000. The 55- and 48-kDa peptides are found to be associated with the free primase activity. Rabbit antibodies to both 55- and 48-kDa peptides are inhibitory to this primase activity. From these results we suggest that the native calf thymus DNA polymerase-DNA primase complex contains only three unique peptides with the 185-kDa peptide as the catalytic peptide of DNA polymerase and the 55- and 68-kDa peptides constituting the primase peptides. A model illustrating the roles of these peptides in initiation and replication of DNA is presented.

DNA primase of human mitochondria is associated with structural RNA that is essential for enzymatic activity

DNA primase isolated from human mitochondria sediments in glycerol density gradients at 30S and 70S. These unusually high sedimentation coefficients are a result of association of the primase activity with RNA. Treatment of primase with nuclease not only affects its sedimentation behavior, but also inactivates the primase activity. The major RNA species that cofractionates with primase activity is shown by direct sequence analysis to be cytosolic 5.8S ribosomal RNA (rRNA). Specific degradation of endogenous 5.8S rRNA using ribonuclease H and oligonucleotides complementary to 5.8S rRNA results in reduction of primase activity. Other small RNAs may play a structural role in the formation of an active DNA primase complex.

Preferential binding of DNA primase to the nuclear matrix in HeLa cells

Studies of the spatial organization of DNA replication have provided increasing evidence of the importance of the nuclear matrix. We have previously reported a relationship between rates of DNA synthesis and the differential binding of DNA polymerase alpha to the nuclear matrix over the S-phase. We now report the detection of DNA primase bound to the HeLa nuclear matrix. Matrix-bound primase was measured both indirectly, by the incorporation of [32P]dAMP into an unprimed single-stranded template, poly(dT), and directly, by the incorporation of [3H]AMP into matrix DNA. Characteristics of this system include a requirement for ATP, inhibition by adenosine 5'-O-(thiotriphosphate), a primase inhibitor, and insensitivity to aphidicolin and alpha-amanitine, inhibitors of polymerase alpha and RNA polymerase, respectively. Subcellular quantification of primase and polymerase alpha activity revealed that while most (approximately 72%) primase activity is bound to the matrix, only a minority (approximately 32%) of polymerase alpha activity is matrix-bound. Treatment of the nuclear matrix with beta-D-octylglucoside allowed the solubilization of approximately 54% of primase activity and approximately 39% of the polymerase alpha activity. This data provides further evidence of a structural and functional role for the nuclear matrix in DNA replication. The ability to solubilize matrix-bound replicative enzymes may prove to be an important tool in the elucidation of the spatial organization of DNA replication.

Chronic granulocytic leukemia: correlation of blastic transformation type with karyotypic evolution

Over a 3-year period, 26 patients with Philadelphia chromosome-positive chronic granulocytic leukemia were studied cytogenetically in both the chronic and blastic transformation phases of the disease; a further three patients were studied only after blastic transformation. Sixteen were considered to have adequate evidence of the type of transformation and form the basis of the report, where chromosome changes have been correlated with the morphological type of blastic transformation. Seven patients developed a myeloblastic transformation, seven a lymphoblastic transformation, and two an erythroblastic transformation. All patients in the myeloid group acquired one or more of the nonrandom changes associated with CGL blastic transformation, viz. +8,i(17q), +19, +22q-. Patients in the lymphoblastic group acquired structural abnormalities, apparently random in nature and usually in a small percentage of cells. The two patients with erythroblastic transformation developed markedly hyperdiploid cells (greater than 50 chromosomes) with both numerical and structural abnormalities. Patients in the lymphoblastic group appeared to have a slightly better prognosis than the myeloid group, whilst the patients with erythroblastic transformation had a very poor prognosis.

Polypeptide structure of DNA primase from a yeast DNA polymerase-primase complex

An immunoaffinity chromatographic procedure was developed to purify DNA polymerase-DNA primase complex from crude soluble extracts of yeast cells. The immunoabsorbent column is made of mouse monoclonal antibody to yeast DNA polymerase I covalently linked to Protein A-Sepharose. Purification of the complex involves binding of the complex to the immunoabsorbent column and elution with concentrated MgCl2 solutions. After rebinding to the monoclonal antibody column free primase activity is selectively eluted with a lower concentration of MgCl2. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate showed the presence of five major peptides, p180, p140, p74, p58, and p48 in the immunoaffinity-purified DNA polymerase-DNA primase complex. Free primase and free polymerase fractions obtained by fractionation on the immunoabsorbent column were analyzed on activity gels and immunoblots. These analyses showed that p180 and p140 are DNA polymerase peptides. Two polypeptides of 58 and 48 kDa co-fractionated with the free yeast DNA primase. From sucrose gradient analysis we estimate a molecular weight of 110 kDa for the native DNA primase.

Mouse DNA polymerase alpha-primase terminates and reinitiates DNA synthesis 2-14 nucleotides upstream of C2A1-2(C2-3/T2) sequences on a minute virus of mice DNA template

The distribution of termination and initiation sites in a 5081-nucleotide minute virus of mice DNA template being copied by a highly purified mouse DNA polymerase alpha-DNA primase complex in the presence of GTP has been examined. The 3'-hydroxyl termini (17 in all) were clustered at six sites that were located 2-14 nucleotides upstream of C2A2C2, C2AC3, or C2A2T2 sequences. When either [alpha-32P]- or [gamma-32P]GTP was included in the DNA polymerase reaction mixtures, nascent DNA became radiolabeled. Analysis of the 32P-labeled material following treatment of the DNA with tobacco acid pyrophosphatase, bacterial alkaline phosphatase, or ribonuclease T1 revealed the presence of oligoribonucleotide chains averaging 5-7 nucleotides long and beginning with 5' GTP residues. Eight presumptive DNA primase initiation sites were located opposite C4 or C5 sequences 3-9 nucleotides upstream of one of the three closely related hexanucleotides C2A2C2, C2AC3, and C2A2T2. RNA-DNA junctions were found 3-10 nucleotides downstream of DNA primase initiation sites. The results indicate that hexanucleotides having the general formula C2A1-2(C2-3/T2), herein referred to as psi, are involved in promoting termination of DNA synthesis and/or de novo initiation of RNA-primed DNA chains by DNA polymerase alpha-primase.

Initiation, elongation and pausing of in vitro DNA synthesis catalyzed by immunopurified yeast DNA primase: DNA polymerase complex

Yeast DNA primase and DNA polymerase I can be purified by immunoaffinity chromatography as a multipeptide complex which can then be resolved into its functional components and further reassembled in vitro. Isolated DNA primase synthesizes oligonucleotides of a preferred length of 9-10 nucleotides and multiples thereof on a poly(dT) template. In vitro reconstitution of the DNA primase:DNA polymerase complex allows the synthesis of long DNA chains covalently linked to RNA initiators shorter than those synthesized by DNA primase alone. The SS (single-stranded) circular DNA of phage M13mp9 can also be replicated by the DNA primase:DNA polymerase complex. Priming by DNA primase occurs at multiple sites and the initiators are utilized by the DNA polymerase moiety of the complex, so that almost all the SS template is converted into duplex form. The rate of DNA synthesis catalyzed by isolated yeast DNA polymerase I on the M13mp9 template is not constant and is characterized by distinct pausing sites, which partly correlate with secondary structures on the template DNA. Thus, replication of M13mp9 SS DNA with the native primase:polymerase complex gives rise to a series of DNA chains with significantly uniform termini specified by the primase start sites and the polymerase stop sites.

Comparison of the virus-associated polymerases of cytoplasmic polyhedrosis virus types 1 and 2

A detailed comparison was made of the virus-associated polymerase activities of cytoplasmic polyhedrosis virus (CPV) types 1 and 2 which had previously been shown to differ in their response to the methyl donor S-adenosyl-L-methionine (AdoMet). While the type 1 CPV polymerase was approximately twice as active as the type 2 CPV enzyme in the presence of AdoMet, temperature, pH and divalent cation optima of the two enzymes were similar. Both viruses synthesized in vitro single-stranded RNA copies of only one strand of the double-stranded RNA genome. In addition, each RNA segment of both viruses was transcribed in approximately equal amounts by weight. The results suggest that most features of CPV polymerase activity are highly conserved, even among CPV types which show substantial antigenic and biochemical differences.

RNA-dependent RNA polymerase activity in coronavirus- infected cells

An enzymatic activity which incorporates [3H]UMP into acid-precipitable material in the presence of endogenous template was found in the cytoplasm of porcine cells infected with the transmissible gastroenteritis virus of swine. This activity was not found in uninfected control cells, nor was it found in purified virus. The activity was associated with the mitochondrial fraction of infected cells, suggesting that the enzyme is membrane bound. The activity required the presence of all three ribonucleoside triphosphates in addition to [3H]UTP, and it was not inhibited by actinomycin D. The heated product was digested by RNase but not by DNase. Mg2+ was required for enzymatic activity, and its optimal concentration was approximately 5 mM. The size of the in vitro products was compared by electrophoresis with that of in vivo-synthesized virus-specified RNA to confirm the viral specificity of the polymerase activity. Virus-specified RNA from infected cells consisted of 10 species of single-stranded, polyadenylated RNA with molecular weights of 6.8 X 10(6), 6.2 X 10(6), 3.15 X 10(6), 1.40 X 10(6), 1.05 X 10(6), 0.94 X 10(6), 0.66 X 10(6), 0.39 X 10(6), 0.34 X 10(6), and 0.24 X 10(6). In vitro synthesized RNA consisted of a high-molecular-weight species, of apparently higher molecular weight than genomic RNA, and two single-stranded species that electrophoretically comigrated with the species of 1.40 X 10(6) and 0.66 X 10(6) molecular weight made in vivo.

Identification of poliovirus polypeptide P63 as a soluble RNA-dependent RNA polymerase

A poliovirus-specific RNA-dependent RNA polymerase was isolated from a cytoplasmic extract of infected HeLa cells and was shown to copurify with a single virus-specific protein. The polymerase was isolated from cells labeled with [35S]-methionine and was fractionated from other soluble cytoplasmic proteins by ammonium sulfate precipitation, phosphocellulose chromatography, gel filtration on Sephacryl S-200, and chromatography on hydroxylapatite. The activity of the enzyme was measured by using either polyadenylic acid or poliovirion RNA as a template in the presence of an oligouridylic acid primer. A single virus-specific protein that had an apparent molecular weight of 63,000 (p63) was found to copurify with this activity. Host-coded proteins were present in reduced molar amounts relative to p63. Noncapsid viral protein 2 (NCVP2) and other viral proteins were clearly separated from p63 by gel filtration on Sephacryl S-200. Polymerase activity coeluted from the column precisely with p63. NCVP2 was totally inactive as an RNA polymerase and did not stimulate the polymerase activity of p63. The purified enzyme sedimented at about 4S on a glycerol gradient and thus appeared to be a monomer of p63. Two-dimensional gel electrophoresis of the polymerase protein indicated that it had an isoelectric point of about 7.5. Thus, the viral polypeptide, p63, as defined by the above physical parameters, is an RNA-dependent RNA polymerase that can copy poliovirion RNA when oligouridylic acid is used as a primer.

Picornaviral VPg sequences are contained in the replicase precursor

It has previously been shown that the RNA replicase of encephalomyocarditis virus contains two virus-coded proteins, D and E, which are produced in two successive proteolytic steps: (i) C leads to D + ?; and (ii) D leads to p22 + E. It is here shown (i) that virus protein H (molecular weight, 12,000) is the previously unidentified product of the first step and (ii) that VPg, a protein linked covalently to the virion RNA, yields two tryptic peptides found in protein C but not in protein D. The results suggest that VPg is derived by cleavage of protein C and that protein H may be intermediate. Preliminary experiments with VPg sequences in polioviral noncapsid protein 1b, the counterpart of encephalomyocarditis viral protein C, were inconclusive.

Inhibition of mengovirus RNA-dependent RNA polymerase by an isatinisothiosemicarbazone and a piperidine-thiocarbonyl-hydrazone derivative in a cell-free system

The inhibitory action of two antiviral compounds used at the maximum tolerated dosis in a cellular system led to a complete suppression of the infectious mengovirus yield and to a 100% plaque reduction. The products of the RNA polymerase reaction catalyzed by the microsomal-mitochondrial fraction of mengovirus-infected FL cells were analyzed by linear sucrose gradient centrifugation and polyacrylamid gel electrophoresis. The results showed that the inhibitors cause a general reduction of the synthesis of single-stranded viral RNA. The influence on double-stranded viral RNA was more obvious in the case of the isatinisothiosemicarbazone derivative.

Levels of aminoacyl-tRNA synthetases, tRNA nucleotidyltransferase and ATP in germinating lupin seeds

Transfer RNAs in dry lupin seeds are aminoacylated to a low extent (Kedzierski, W. and Pawełkiewicz, J. (1977) Phytochemistry 16, 503-504) and are partly degraded at the acceptor terminus (Dziegielewski, T. and Pawełkiewicz, J. (1977) Bull. Acad. Polon. Sci. Ser. Biol. 7, 4oo-435). Increase in the levels of tRNA aminoacylation and disappearance of defective tRNA molecules during seed germination are not accompanied by significant changes in the levels of phenylalanyl-, arginyl-, valyl-tRNA synthetases and tRNA nucleotidyltransferase. Additionally, no inhibitor of aminoacylation of valine tRNA has been detected in dry seeds. However, dry seeds contain very low ATP amounts, which increase dramatically during germination. The above results suggest that a very low ATP level is a factor limiting the aminoacylation and reparation of tRNA molecules at early stages of seed germination.

Poliovirus polyuridylic acid polymerase and RNA replicase have the same viral polypeptide

A poliovirus-specific polyuridylic acid [poly(U)] polymerase that copies a polyadenylic acid template complexed to an oligouridylic acid primer was isolated from the membrane fraction of infected HeLa cells and was found to sediment at 4 to 5S on a linear 5 to 20% glycerol gradient. When the poly(U) polymerase was isolated from cells labeled with [(35)S]methionine and was analyzed by glycerol gradient centrifugation and polyacrylamide gel electrophoresis, the position of only one viral protein was found to correlate with the location of enzyme activity. This protein had an apparent molecular weight of 62,500 based on its electrophoretic mobility relative to that of several molecular weight standards and was designated p63. When the poly(U) polymerase was isolated from the soluble fraction of a cytoplasmic extract, the activity was found to sediment at about 7S. In this case, however, both p63 and NCVP2 (77,000-dalton precursor of p63) cosedimented with the 7S activity peak. When the 7S polymerase activity was purified by phosphocellulose chromatography, both p63 and NCVP2 were found to co-chromatograph with poly(U) polymerase activity. The poliovirus replicase complexed with its endogenous RNA template was isolated from infected cells labeled with [(35)S]methionine and was centrifuged through a linear 15 to 30% glycerol gradient. The major viral polypeptide component in a 26S peak of replicase activity was p63, but small amounts of other poliovirus proteins were also present. When the replicase-template complex was treated with RNase T1 before centrifugation, a single peak of activity was found that sedimented at 20S and contained only labeled p63. Thus, p63 was found to be the only viral polypeptide in the replicase bound to its endogenous RNA template, and appears to be active as a poly(U) polymerase either as a monomer protein or as a 7S complex.

Ribonucleic acid polymerase activity associated with purified calf rotavirus

The presence of an RNA-dependent RNA polymerase is demonstrated in purified rotavirus particles. Optimum polymerase activity was found between 45 to 50 degrees C, at pH 8, and in the presence of 10 mM-magnesium ions. The polymerase product was highly sensitive to pancreatic RNase (97%) in low or high salt concentration. The enzyme was activated by EDTA treatment of intact particles or heat shock. The similarities between reovirus, blue-tongue virus and rotavirus polymerases are discussed.

Purification and polypeptide composition of Semliki Forest virus RNA polymerase

A purification method for Semliki Forest virus-specified RNA-dependent RNA polymerase from BHK cells is described. The procedure entails (i) the preparation of a crude cell lysate by Dounce homogenization of cells 3-5 h post-infection, (ii) differential centrifugation to give a 15 000 g 'mitochondrial' pellet, (iii) equilibrium centrifugation on discontinuous sucrose gradients (Friedman et al. 1972) to give a membranous band of density 1-16 g/ml, (iv) solubilization with Triton N-101 and velocity centrifugation to give a 25S solubilized polymerase complex and (v) affinity chromatography through an oligo (dT)-cellulose matrix bearing immobilized 42S virus particle RNA. The overall purification was approx. 360-fold with a 5% recovery of activity. Of the various intermediate fractions in the purfication procedure, only the relatively crude post-nuclear supernatant fraction was competent to synthesize the major single-stranded RNAs found in infected cells. Other fractions incorporated precursor only into replicative intermediate (RI) or replicative from (RF). Analysis of the product RF showed that it was of the same size and could bind to the same extent to oligo (dT)-cellulose as the RF isolated directly from lysates of infected cells. Displacement hybridization and ribonuclease digestion suggested that the purified polymerase could only complete previously initiated progeny positive strands using negative strands as template and, even in its most highly purified form, was still tightly bound to its template. Analysis on polyacrylamide slab gels revealed the presence of three 35S-labelled polypeptides in the purified polymerase preparation, but a polypeptide which had identical electrophoretic mobility to the lowest mol. wt. polypeptide of the purified polymerase was also present in material from mock-fected cells which had been taken through the purification procedure. From these results we conclude that only two virus-specified polypeptides are present in the polymerase. A scheme for the synthesis of these polypeptides is presented in the accompanying paper.