The detection of DNA-binding proteins by protein blotting

Cell-surface collagen-binding protein in the procaryote Achromobacter iophagus

Collagen and its high-molecular-weight fragments specifically induce an extracellular collagenase (EC 3.4.24.8) in the Gram-negative Achromobacter iophagus. During the induction process the inducer is concentrated on the bacterial outer membrane. Two-dimensional electrophoresis of 125I-labelled outer membrane proteins has shown that, in particular, the amount of one protein which is already present on the surface of non-induced bacteria increases quantitatively when the inducer is added. After 125I-labelling of the cell membrane and its solubilization, the same protein is retained selectively on a gelatin-Sepharose column. It has isoelectric point of 4.9-5.1 and molecular weight of 40000. This molecular weight is close to that of the 35000 of the collagenase subunit. However, their non-identity was proved in three independent ways: upon two-dimensional electrophoresis, only those proteins in the range corresponding to the collagenase dimer (Mr 70000-80000) react with fluorescent anticollagenase antibody system, whereas the spot of the collagen-binding protein (mr 40000) is negative; the solubilized collagen-binding protein is not retained by anticollagenase-Sepharose affinity chromatography; in vivo, it is not protected by anti-collagenase antibodies against lactoperoxidase iodination. A hypothesis for the possible role of the collagen-binding protein in the induction of collagenase is proposed.

Antibodies to tumour eluates react preferentially with non-lymphoid tumours

Rabbit antisera raised against eluates from a murine fibrosarcoma were characterised using a 125I-protein A assay and a wide variety of target cells. The sera bound preferentially to rodent tumours of non-lymphoid origin, whereas monkey and human cells did not react. Murine lymphoid cells and macrophages (normal or transformed) and normal liver and kidney cells all bound low amounts of the antibody, while embryonic cells were intermediate in reactivity. Target cell treatments indicated that the surface antigens being detected were sensitive to proteolysis and calcium depletion. In addition actively growing cells bound more antibody than resting cells. Double binding assays with sera specific for plasma membrane components suggested the eluate antigens may play a structural role. Immunofluorescent studies demonstrated that surface antigens detected by the antisera capped and were lost and this was followed by synthesis and surface re-expression. Sera such as these, which can distinguish between normal and malignant cells in the rodent, have obvious applications in many aspects of tumour-related investigations.

Is the major Drosophila heat shock protein present in cells that have not been heat shocked?

When eukaryotic cells are exposed to elevated temperatures they respond by vigorously synthesizing a small group of proteins called the heat shock proteins. An essential element in defining the role of these proteins is determining whether they are unique to a stressed state or are also found in healthy, rapidly growing cells at normal temperatures. To date, there have been conflicting reports concerning the major heat-induced protein of Drosophila cells, HSP 70. We report the development of monoclonal antibodies specific for this protein. These antibodies were used to assay HSP 70 in cells incubated under different culture conditions. The protein was detectable in cells maintained at normal temperatures, but only when immunological techniques were pushed to the limits of their sensitivity. To test for the possibility that these cells contain a reservoir of protein in a cryptic antigenic state (i.e., waiting posttranslational modification for use at high temperature), we treated cells with cycloheximide or actinomycin D immediately before heat shock. HSP 70 was not detected in these cells. Finally, we tested for the presence of a reservoir of inactive messages by using a high stringency hybridization of 32P-labeled cloned gene sequences to electrophoretically separated RNAs. Although HSP 70 mRNA was detectable in rapidly growing cells, it was present at less than 1/1,000th the level achieved after induction.

Cloning and expression in Escherichia coli of the gene coding for the protein linked to the ends of Bacillus subtilis phage phi 29 DNA

A phi 29 DNA fragment containing gene 3, coding for the 5'-terminal protein, and several other early genes has been cloned in a pBR322 derivative plasmid (pKC30) under the control of the pL promoter of bacteriophage lambda. Four polypeptides of Mr 27000, 18500, 17500 and 12500 were labelled with [35S]methionine after heat induction, accounting for about 15% of the de novo synthesized protein. The Mr 27000 and 12500 proteins were characterized as p3, the 5'-terminal protein, and p4, involved in the control of late transcription, respectively. Protein p3 synthesized in Escherichia coli was active in the in vitro formation of the initiation complex p3-dAMP when supplemented with extracts from Bacillus subtilis infected with a sus3 mutant.

Analysis of spiroplasma proteins: contribution to the taxonomy of group IV spiroplasmas and the characterization of spiroplasma protein antigens

Spiroplasma strains of group IV were compared by two-dimensional protein analyses on polyacrylamide gels. Although considerable diversity was evident, the assemblages studied were less heterogeneous than the known strains of group I. Two electrophoretic techniques were used to identify spiroplasma proteins that had been used to immunize rabbits. These included monoclonal antibodies prepared against Spiroplasma citri. In the first technique, protein antigens were purified by immunoaffinity chromatography, then identified with SDS-PAGE. In the second technique, spiroplasma proteins were first separated by SDS-PAGE, then antigens were identified by antibody binding to blot-transferred proteins. Finally, two-dimensional protein electrophoresis has been used as a source of immunogens to characterize monospecific antibodies against individual S. citri proteins.

Ribosomal proteins of Streptomyces aureofaciens producing tetracycline

Three different two-dimensional polyacrylamide gel electrophoretic systems were employed for identification of individual ribosomal proteins of Streptomyces aureofaciens. Proteins of small subunits were resolved into 21 spots. Larger ribosomal subunits contained 35 proteins. The separated ribosomal proteins from 50 S subunits were transferred on nitrocellulose membranes for immunochemical estimations. Antibodies developed against 50 S proteins of S. aureofaciens and Escherichia coli were used for identification of structural homologies between 50 S proteins of the two species. Results of the experiments indicate that about one half of the 50 S proteins of S. aureofaciens share common immunochemical determinants with corresponding proteins of 50 S subunits of E. coli. Evidence is presented that acidic ribosomal protein SL5 of large ribosomal subunits of S. aureofaciens can be assembled to E. coli P0 cores lacking proteins L7/L12. Reconstitution of the P0 cores with proteins SL5 or L7/L12 led to restoration of 78% activity in polyphenylalanine synthesis.

Tissue-specific distribution of DNA-binding nuclear proteins

The DNA-binding capacity of nuclear proteins of mouse cells was examined by the protein-blotting method. Under conditions in which the lac repressor specifically binds to the lac operator, the DNA-binding nuclear proteins from different tissues showed a tissue-specific distribution, suggesting that the species and amounts of nuclear proteins with DNA binding activity differ in different tissues. When cloned eukaryotic genes were used for binding, eukaryotic DNA showed stronger binding than prokaryotic DNA. Competition experiments suggested that many nuclear proteins have different DNA binding properties from that of the prokaryotic repressor.

Purified Epstein-Barr virus Mr 340,000 glycoprotein induces potent virus-neutralizing antibodies when incorporated in liposomes

The purified Mr 340,000 glycoprotein component of Epstein-Barr (EB) virus-induced membrane antigen complex incorporated into liposomes was shown to be a potent immunogen in mice. High-titer antisera were induced that (i) are specific for membrane antigen components without absorption, (ii) bind the antigens induced by three different EB virus isolates, and (iii) neutralize the ability of the virus to transform fetal cord blood lymphocytes in vitro. The development of this immunogenic form of purified antigen provides an important step towards a potential subunit vaccine against Epstein-Barr virus infection.

Protein D1 preferentially binds A + T-rich DNA in vitro and is a component of Drosophila melanogaster nucleosomes containing A + T-rich satellite DNA

Our previous work [Levinger, L. & Varshavsky, A. (1982) Cell 28, 375-385] has shown that D1, a 50-kilodalton chromosomal protein of Drosophila melanogaster, is specifically associated with isolated nucleosomes that contain a complex A + T-rich satellite DNA with buoyant density of 1.688 g/ml. We show here that D1 is also a component of nucleosomes containing a simple-sequence, pure A + T satellite DNA, buoyant density 1.672 g/ml. Furthermore, using a modification of a protein blotting technique in which proteins are not exposed to dodecyl sulfate denaturation, we have found that D1 preferentially binds to A + T-rich double-stranded DNA in vitro, and it is apparently the only abundant nuclear protein in cultured D. melanogaster cells that possesses this property. Synthetic poly[d(A-T)].poly[d(A-T)] and poly(dA).poly(dT) duplexes effectively compete in vitro with A + T-rich D. melanogaster satellite DNAs for binding to D1, whereas total Escherichia coli DNA is an extremely poor competitor. These findings strongly suggest that D1 is a specific component of A + T-rich, tandemly repeated, heterochromatic regions, which constitute up to 15-20% of the total D. melanogaster genome. Possible functions of D1 protein include compaction of A + T-rich heterochromatin and participation in microtubule-centromere interactions in mitosis. In addition, D1 may prevent nonspecific binding to A + T-rich satellite DNA of other nuclear proteins that have a preference for AT-DNA, such as RNA polymerase or regulatory proteins, and may also participate in the higher-order chromatin organization outside tandemly repetitive regions by binding to nonrandomly positioned stretches of A + T-rich DNA.

Preparation, characterization, and properties of monoclonal antibodies against the lac carrier protein from Escherichia coli

Monoclonal antibodies directed against the lac carrier protein purified from the membrane of Escherichia coli were prepared by somatic cell fusion of mouse myeloma cells with splenocytes from an immunized mouse. Several clones produce antibodies that react with the purified protein as demonstrated by solid-phase radioimmunoassay and by immunoblotting experiments; culture supernatants from the clones inhibit active transport of lactose in isolated membrane vesicles. Five stable clones were selected for expansion, formal cloning, and production of ascites fluid, and the antibodies secreted in vivo by each clone also were found to inhibit lactose transport. Antibody from hybridoma 4B1, an IgG2a immunoglobulin, inhibits active transport of lactose in proteoliposomes reconstituted with purified lac carrier and in right-side-out membrane vesicles. In contrast, the antibody has no effect on the generation of the proton electrochemical gradient by membrane vesicles nor does it alter the ability of vesicles containing the lac carrier to bind p-nitrophenyl-alpha-D-galactopyranoside. In order to achieve 50% inhibition of transport activity, a 2- to 3-fold molar excess of antibody to lac carrier is required, regardless of the amount of lac carrier in the membrane. Thus, the concentration of antibody required for a given degree of inhibition is proportional to the amount of lac carrier in the membrane. Finally, antibody-induced inhibition occurs within seconds, an observation suggesting that the epitope is accessible on the surface of the membrane.

Cell attachment on replicas of SDS polyacrylamide gels reveals two adhesive plasma proteins

A novel procedure that detects adhesive proteins in complex mixtures was used to characterize such proteins in plasma. The proteins are separated by SDS PAGE and transferred to nitrocellulose filters. Cells incubated on these filters attach to those proteins that have adhesive properties. When applied to human plasma proteins this procedure reveals, in addition to fibronectin, a cell-attachment protein with a polypeptide molecular weight of 70,000. Using a monoclonal antibody that inhibits attachment of cells to fibronectin, we show that this polypeptide is not a fragment of fibronectin and we present evidence that it is a component of the serum spreading factor. Therefore, as defined by our assay, this protein and fibronectin are the major attachment proteins for fibroblastic cells in plasma or serum.

Expression of cloned calf prochymosin gene sequence in Escherichia coli

An expression plasmid for calf prochymosin (prorennin) cDNA was constructed. The plasmid (pCR301) contains the lacUV5 promoter in front of the fused gene in which the codons for the N-terminal four amino acids of prochymosin cDNA were replaced with those for the N-terminal ten amino acids of beta-galactosidase. Synthesis of the fused protein with the expected Mr was detected immunologically in Escherichia coli harboring pCR301. The product seemed to be localized in the cell membrane of the bacterial host.

Tissue-specific and species-specific monoclonal antibodies to avian red cell nuclear proteins

In order to identify potential red cell-specific regulatory proteins and to define additional red cell-specific markers, we have isolated a series of hybridomas that produce monoclonal antibodies that react with nuclear preparations from avian red blood cells. Several antibodies have been well characterized for their tissue- and species-specific reactions by using solid-phase and protein-transfer radioimmunoassays as well as immunofluorescence. These antibodies should allow isolation and characterization of individual nuclear proteins that are tissue and species specific and may prove useful for the study of gene expression in the erythropoietic system. The majority of the well-characterized antibodies appear to have tissue-specific properties. However, three antibodies react with all tested chicken tissues; one of these reacts with multiple peptides in a pattern that varies qualitatively and quantitatively between the tissues. This may reflect a common protein domain or modification that is used in several different tissues for similar functions but is nevertheless present in an overall protein framework that is tissue specific. Because the major fraction of the hybridomas initially produced is tissue specific, we presume that the immune system selects for tissue-specific determinants. This property of the immune system may prove to be a useful general feature of this type of analysis.

Phosphorylation and dephosphorylation alter the structure of D2 hybrid T antigen

D2 hybrid T antigen is a protein closely related to simian virus 40 large T antigen and is synthesized in large quantities in cells infected with Ad2+D2, an adenovirus-simian virus 40 hybrid. We have analyzed the effects of phosphorylation on the structure and DNA binding of this protein. On nondenaturing pore-gradient gels, the purified protein migrated with an apparent molecular weight of 135,000, with a minor band at 330,000 molecular weight. In vitro phosphorylation catalyzed by the protein kinase activity associated with the protein resulted in a structural change so that most of it migrated with an apparent molecular weight of 740,000. Treatment of the phosphorylated form of the protein with alkaline phosphatase (which removed 95% of the phosphate) caused the disappearance of the 740,000-molecular-weight form and reappearance of the smaller forms. Partial tryptic digestion showed that D2 T antigen has two major regions of phosphorylation, only one of which was phosphorylated in vitro. The region phosphorylated in vitro was responsible for the aggregation of D2 T antigen and was tentatively assigned to the N-terminal part of the protein. As shown by protein blotting onto nitrocellulose filters, it was mainly the form of 740,000 molecular weight that bound to simian virus 40 DNA. However, sucrose gradient analyses showed that only a fraction of the in vitro-phosphorylated protein bound to DNA, suggesting that aggregation alone is not sufficient for binding.

Identification of protein bands in polyacrylamide gel by protein printing

Previously described methods for identification of proteins separated in cylindrical polyacrylamide gels have been found to be costly in time and antiserum and difficult to apply to small amounts of protein as are found in cerebrospinal fluid. We describe a method which involves printing of the proteins on the cut surface of the gel onto nitrocellulose paper. The protein bands of the imprint can then be identified using labelled antibodies. We have found this to be economical and quick, and it has permitted sensitive and reliable identification of proteins in unconcentrated cerebrospinal fluid and aqueous humour.

Quantification of an Epstein-Barr virus-associated membrane antigen component

A method is described for the preparation of a 125I-labelled membrane antigen (MA) component (gp340) from B95-8 cell membranes using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). Good yields of antigenic material were obtained when renaturation of the [125I]gp340 was carried out by removal of SDS in the presence of urea and subsequent removal of the urea. The availability of purified, radiolabelled gp340 has provided the essential basis for the development of a radioimmunoassay which, for the first time, permits quantification of this antigen. The assay has been used to demonstrate that cell membrane MA is a better source of gp340 for large-scale work than is the Epstein-Barr virus envelope and to measure the increase in expression of gp340 following treatment of cells with 12-O-tetradecanoyl-phorbol-13-acetate (TPA).

Isolation and characterization of a Pseudomonas aeruginosa PAO mutant defective in the structural gene for the LIVAT-binding protein

A mutant of Pseudomonas aeruginosa PAO which has a defect in the structural gene for a binding protein for leucine, isoleucine, valine, alanine, and threonine (LIVAT-binding protein) was isolated and characterized. DL-4-azaleucine was taken up via the high-affinity branched-chain amino acid transport system (LIV-I), but not via the low affinity system (LIV-II), and then inhibited the growth of P. aeruginosa cells. This finding enabled us to select mutants defective in the LIV-I transport system alone. Among such mutants, strain PAO3530 was found to produce an altered LIVAT-binding protein. The shock fluid of this strain contained a normal level of the protein which corresponded to the wild-type LIVAT-binding protein as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by an immunological test. However, the shock fluid showed almost no binding activity for branched-chain amino acids, suggesting that strain PAO3530 has a defect in the structural gene for the LIVAT-binding protein. The mutation locus (bra-310) was mapped in a region between cnu-9001 and oru-325 on the chromosome of P. aeruginosa PAO by conjugation mediated by plasmid FP5 or R68.45.

Mitochondrial DNA sequences of primates: tempo and mode of evolution

We cloned and sequenced a segment of mitochondrial DNA from human, chimpanzee, gorilla, orangutan, and gibbon. This segment is 896 bp in length, contains the genes for three transfer RNAs and parts of two proteins, and is homologous in all 5 primates. The 5 sequences differ from one another by base substitutions at 283 positions and by a deletion of one base pair. The sequence differences range from 9 to 19% among species, in agreement with estimates from cleavage map comparisons, thus confirming that the rate of mtDNA evolution in primates is 5 to 10 times higher than in nuclear DNA. The most striking new finding to emerge from these comparisons is that transitions greatly outnumber transversions. Ninety-two percent of the differences among the most closely related species (human, chimpanzee, and gorilla) are transitions. For pairs of species with longer divergence times, the observed percentage of transitions falls until, in the case of comparisons between primates and non-primates, it reaches a value of 45. The time dependence is probably due to obliteration of the record of transitions by multiple substitutions at the same nucleotide site. This finding illustrates the importance of choosing closely related species for analysis of evolutionary process. The remarkable bias toward transitions in mtDNA evolution necessitates the revision of equations that correct for multiple substitutions at the same site. With revised equations, we calculated the incidence of silent and replacement substitutions in the two protein-coding genes. The silent substitution rate is 4 to 6 times higher than the replacement rate, indicating strong functional constraints at replacement sites. Moreover, the silent rate for these two genes is about 10% per million years, a value 10 times higher than the silent rate for the nuclear genes studied so far. In addition, the mean substitution rate in the three mitochondrial tRNA genes is at least 100 times higher than in nuclear tRNA genes. Finally, genealogical analysis of the sequence differences supports the view that the human lineage branched off only slightly before the gorilla and chimpanzee lineages diverged and strengthens the hypothesis that humans are more related to gorillas and chimpanzees than is the orangutan.

Methionine sulfoximine in vivo modifies the tRNALys pool of developing rat brain

tRNA was extracted from brains of 3-, 8-, and 18-day-old rats that were injected intracerebrally, 45 min before death, with [3H]methyl methionine or [8-3H]guanosine, and intraperitoneally, 3 h before death, with L-methionine-dl-sulfoximine (MSO), a methylation-activating convulsant agent. Although there was no effect of age or of MSO on the per gram yield of tRNA, its specific radioactivity (dpm/A260) was highest at 3 days in both the control and the MSO groups. Age- and MSO-related changes in tRNALys content of the brain tRNA pool were investigated by means of benzoylated DEAE-cellulose (BDC) and reverse-phase chromatography (RPC). BDC chromatography revealed tRNALys species in the brains of the MSO-treated animals that were absent in control brains. Of particular interest was the finding that differences in RPC-5 chromatographic mobility between control and MSO-tRNALys species were abolished by conversion of lysyl-tRNA, suggesting that the MSO-elicited change(s) in tRNALys structure involved the binding site(s) for lysine. Two additional findings were made: (a) lysine acceptance by the [3H]methyl-labeled tRNALys purified from brains of the MSO-treated animals was higher than that of controls at 18 days; and (b) omission of the BDC chromatographic step accentuated the differences in mobility on RPC-5 columns between tRNALys species of control and MSO-treated animals. Lastly, we found that some tRNALys species of control and MSO-treated brains contained significantly different proportions of N2-methyl guanine and 1-methyl adenine, relative to controls. These MSO-elicited changes in the methyl base content of tRNALys of immature rat brain are the first evidence of an alteration of brain tRNA structure by a centrally acting excitatory agent.

Macromolecular complex of aminoacyl-tRNA synthetases from sheep liver. Identification of the methionyl-tRNA synthetase component by affinity labeling

Both the tRNA aminoacylation and amino-acid-dependent ATP-PPi exchange activities of monomeric trypsin-modified methionyl-tRNA synthetase from sheep liver are lost upon incubation with oxidized initiator tRNAMet. The inactivation, which reflects the formation of a Schiff's base between the 5'-terminal adenosine of tRNA and a lysine within the catalytic site of the enzyme, is accompanied by the covalent attachment of one tRNA molecule per enzyme molecule. The affinity labeling method is applied to the sheep liver complex of Mr 10(6) carrying seven aminoacyl-tRNA synthetase activities, from which the monomeric trypsin-modified methionyl-tRNA synthetase (Mr 68 000) was derived. Upon incubation with oxidized initiator tRNAMet, the methionyl-tRNA synthetase activity of the complex is lost. Of the eleven polypeptide chains composing the high-molecular-weight complex, only one polypeptide chain with Mr 103 000 reacts with the modified tRNAMet. The blocking by periodate-treated tRNA of the methionyl-tRNA synthetase activity in the complex has no effect on the other aminoacyl-tRNA synthetase activities. This strongly argues in favor of the independent parallel functioning of the seven aminoacyl-tRNA synthetases associated in a high-molecular-weight complex.

Hyperthermia and the heat-shock proteins of HeLa cells

When HeLa cells are subject to hyperthermia, the synthesis of specific heat-shock proteins (HSP) is induced under a variety of thermal conditions. HSP synthesis does not occur at temperatures above 43 degrees C but requires return to a culture temperature of 37 degrees C. Maximal induction appears to be achieved if a brief hyperthermia treatment (10 min, 45 degrees -46 degrees C) is followed by 2 h "development" at 37 degrees C. The induction process requires transcription but not DNA replication, and general cell metabolism is probably also required, as induction does not occur if the heat-treated cells are returned to 4 degrees (rather than 37 degrees C) for development. A small proportion of the HSPs of 72-74 Kd are found in nuclei, but do not appear to bind to DNA. The bulk of these proteins, as well as those at 100 Kd, are cytoplasmic, but none are preferentially associated with mitochondria. Increased synthesis of the 100Kd and 72-74 Kd HSPs was also triggered by pretreatment of the cells with 5 x 10(-5) M sodium arsenite.

Detection of a cellular polypeptide associated with adenovirus-coded VA RNA using in vitro labeling of proteins cross-linked to RNA

Ultraviolet light induced RNA-protein cross-linking for identification of polypeptides interacting with RNA in intact cells (Wagenmakers et al. 1980), is limited by the intensity of the label in the proteins or in residual nucleotides remaining attached to the proteins after RNase treatment of the RNA-protein complexes. Here we report a method, where th cross-linked RNA-protein complexes are treated with RNase T1 and the T1-oligonucleotides covalently linked to the proteins are labeled in the 5' terminus using gamma-32P-ATP and T4 polynucleotide kinase. The cross-linked proteins can then readily be identified owing to the incorporated 32P label. As examples, proteins associated with polyadenylated mRNA, hnRNA and adenoviral VA RNA were identified. A protein with a molecular weight of approximately 50,000 is found associated with adenovirus-coded VA RNA. This was confirmed by binding assays, in which labeled VAI RNA is incubated with proteins from uninfected and adenovirus infected HeLa cells immobilized on nitrocellulose sheets.

Immunocytochemical localization of tubulin, actin, and myosin in axonemes of ciliated cells from quail oviduct

Tubulin, actin, and myosin have been localized in isolated demembranated ciliated cells from quail oviduct by immunocytochemistry in both light and electron microscopy by using purified antibodies. The peripheral doublets and the central tubules are stained by the antitubulin whereas the kinetosomes are poorly stained. Actin antibodies clearly stain the axonemes, but only on the proximal-half portion, whereas myosin antibodies stain a small area of the axonemes just above the ciliary neck region.

Higher order DNA structure in macronuclear chromatin of the hypotrichous ciliate Oxytricha nova

On lysis of macronuclei from the ciliated protozoan Oxytricha at 0.5-2 M NaCl, the DNA, which is normally found as discrete molecules ranging from 0.5 to 20 kilobases, appears in high molecular weight aggregates. Various treatments of the macronuclear lysate (i.e., nucleases, proteases, variation of salt, pH, and temperature) indicate that preservation of the aggregate structure depends on both nucleic acid-nucleic acid and nucleic acid-protein interactions. Purification of the DNA-protein complex after lysing the nuclei in 2 M NaCl shows that one major nuclear protein copurifies with the DNA. As shown by DNA-protein binding experiments, this protein has a high affinity for DNA; however, no evidence for sequence specificity of the protein binding was obtained. Chromatin reconstitution experiments suggest that the protein in itself is not sufficient for DNA aggregation in nuclei, but other factors, possibly the native chromatin structure, are required. Electron microscopy of the purified DNA-protein complex showed structures similar to those observed previously with in vitro-aggregated purified macronuclear DNA (14). A model is presented in which the terminal inverted repeat sequences found on all macronuclear DNA molecules interact with each other forming multistranded DNA complexes. The formation of these structures may be accelerated and stabilized by a protein in vivo.

Antibodies Against the Genome-Linked Protein VPg of Cowpea Mosaic Virus Recognize a 60,000-Dalton Precursor Polypeptide

We have prepared a rabbit antiserum specifically directed against the genome-linked protein (VPg) of cowpea mosaic virus by injecting an hydrolysate of purified virion RNA. Using this antiserum as a probe in combination with “Western” (protein) blots of subcellular fractions of cowpea mosaic virus-infected cowpea ( Vigna unguiculata ) cells, we have detected a bottom component RNA-encoded, 60,000-dalton polypeptide which is membrane bound and presumably represents the immediate precursor of VPg.