In vitro synthesis of DNA complementary to rabbit reticulocyte 10S RNA

50th anniversary of the discovery of reverse transcriptase

The simultaneous discovery in 1970 of reverse transcriptase in virions of retroviruses by Howard Temin and David Baltimore was perhaps the most dramatic scientific moment of the second half of the 20th century. Ten years previously, Temin's observation of cells transformed by Rous Sarcoma virus led him to the conclusion that retroviruses replicate through a DNA intermediate he called the provirus. This heretical hypothesis was greeted with derision by fellow scientists; Temin and Baltimore performed a simple experiment, rapidly reproduced, and convincing to all. Its result was a major paradigm shift-reversal of the central dogma of molecular biology. It immediately grabbed the attention of both the scientific and lay press. It also came at a key time for cancer research, at the start of the "War on Cancer." As a theoretical base and fundamental molecular tool, it enabled a decade of (largely fruitless) search for human oncogenic retroviruses but laid the foundation for the discovery of HIV 13 years later, leading to the development of effective therapy. I had the good fortune, as a student in Temin's lab, to witness these events. I am honored to be able to share my recollection on the occasion of their 50th anniversary.

Sixty Years of Discovery

Each of us is a story. Mine is a story of doing science for 60 years, and I am honored to be asked to tell it. Even though this autobiography was written for the Annual Review of Immunology, I have chosen to describe my whole career in science because the segment that was immunology is so intertwined with all else I was doing. This article is an elongation and modification of a talk I gave at my 80th birthday celebration at Caltech on March 23, 2018.

Single-Cell Isolation and Gene Analysis: Pitfalls and Possibilities

During the last two decades single-cell analysis (SCA) has revealed extensive phenotypic differences within homogenous cell populations. These phenotypic differences are reflected in the stochastic nature of gene regulation, which is often masked by qualitatively and quantitatively averaging in whole tissue analyses. The ability to isolate transcripts and investigate how genes are regulated at the single cell level requires highly sensitive and refined methods. This paper reviews different strategies currently used for SCA, including harvesting, reverse transcription, and amplification of the RNA, followed by methods for transcript quantification. The review provides the historical background to SCA, discusses limitations, and current and future possibilities in this exciting field of research.

Functional cDNA expression cloning: pushing it to the limit

The 1970s and the following decade are the era of the birth and early development of recombinant DNA technologies, which have entirely revolutionized the modern life science by providing tools that enable us to know the structures of genes and genomes and to dissect their components and understand their functions at the molecular and submolecular levels. One major objective of the life sciences is to achieve molecular and chemical understandings of the functions of genes and their encoded proteins, which are responsible for the manifestation of all biological phenomena in organisms. In the early 1980s, I developed, together with Paul Berg, a new technique that enables the cloning of full-length complementary DNAs (cDNAs) on the basis of their functional expression in a given cell of interest. I review the development, application and future implications in the life sciences of this gene-cloning technique.

Covalent linkage between ribonucleic Acid primer and deoxyribonucleic Acid product of the avian myeloblastosis virus deoxyribonucleic Acid polymerase

Initiation of deoxyribonucleic acid (DNA) synthesis by the avian myeloblastosis virus DNA polymerase was previously suggested to involve a ribonucleic acid (RNA) primer, the initial product being a DNA molecule joined by a phosphodiester bond to the RNA primer. The existence and nature of such an RNA-DNA joint was investigated by assaying for transfer of a (32)P atom from an alpha-(32)P-deoxyribonucleotide to a 2'(3')-ribonucleotide after alkaline hydrolysis of the polymerase product. Such a transfer was observed, but only from alpha-(32)P-deoxyadenosine triphosphate and only to 2'(3')-adenosine monophosphate. This same transfer was observed in both the endogenous DNA polymerase reaction of purified virions and the reconstructed reaction of purified DNA polymerase plus purified 60 to 70S viral RNA. These results indicate a high level of specificity for the initiation process and support the idea of a low-molecular-weight initiator RNA as part of the 60 to 70S RNA complex.

Reverse transcriptase. The use of cloned Moloney murine leukemia virus reverse transcriptase to synthesize DNA from RNA

Reverse transcriptase (RT) is the key enzyme required for conversion of RNA to DNA. Cloning of Moloney murine leukemia virus (MMLV) RT has enable engineering an RT that lacks endogenous RNase H activity. RT catalyzes cDNA synthesis more efficiently in the absence of RNase H. We describe here a number of properties of MMLV RT and RNase H-minus MMLV RT not summarized in a single location elsewhere, providing a basis for best use of these enzymes in cDNA synthesis. In addition, general guidelines and detailed protocols are provided for use of MMLV RTs in one tube double-stranded cDNA synthesis, in [32P]cDNA synthesis, and in RT-PCR and long RT-PCR.

Inverse polymerase chain reaction. An efficient approach to cloning cDNA ends

The conventional polymerase chain reaction (PCR) requires that DNA sequences at both ends of the region to be amplified be known. Inverse PCR (IPCR) and anchored PCR overcome this limitation and amplify flanking unknown DNA sequences by utilizing inverse amplification and a universal primer, respectively. The major advantage of IPCR is that two gene-specific primers are reserved for specific and efficient amplification of the unknown cDNA ends on the basis of a small stretch of known sequence. The protocol consists of five steps: reverse transcription, synthesis of second strand cDNA, circularization of double strand cDNA, reopen the circle DNA, and amplification of the inverse DNA fragment.

A convenient procedure for the isolation of intact translatable mRNA by potassium iodide gradient centrifugation

High concentrations of KI were found to efficiently protect RNA against degradation by RNases. When a sufficient amount of solid KI was added to cell lysates or subcellular fractions (9 g per 10 ml), the solutions could be stored at room temperature for several days without measurable degradation of mRNA. Ribonucleic acids were selectively sedimented when these KI-containing solutions were centrifuged at 72,000 x g for 24 h. The RNA pellets were found to be readily dissolved in bidistilled water and the redissolved RNA could be immediately submitted to oligo(dT)-cellulose chromatography to isolate the poly(A)-containing RNA. However, extraction with phenol/chloroform was found to be necessary, if total RNA or poly(A)-minus RNA was to be analysed. This procedure was found to be superior to other methods currently in use - especially with respect to the isolation of intact, translatable high-molecular-weight mRNA.

Construction and quality of cDNA libraries prepared from cytoplasmic RNA not enriched in poly(A)+RNA

Poly(A)+RNA and cytoplasmic RNA of Ehrlich ascites tumor cells grown in vivo were used to study the quality and efficiency of cDNA synthesis. It was found that the rates of oligo(dT)-primed and unprimed reverse transcription were very similar in both cases. The size distributions of the cDNA strands prepared from unfractionated RNA reflected the size of cytoplasmic mRNA populations including a significant fraction of long molecules up to 6 kb. The fraction of cDNAs primed on rRNAs by oligo(dT) was found to be as low as 2-3%. Following second-strand synthesis by means of RNase H-induced nick translation by DNA polymerase I the overall yields in double-stranded cDNA were slightly higher when unfractionated cytoplasmic RNA was used as starting template. In repeated experiments we obtained an average yield of 2.2 micrograms of double-stranded cDNA when 70 micrograms of unfractionated cytoplasmic RNA was used as starting material. This amount of cDNA synthesized in one assay was sufficient to construct representative cDNA libraries in different vectors. Southern hybridizations of DNA isolated from cDNA libraries with various radiolabelled probes show that the libraries constructed from cDNA synthesized from cytoplasmic RNA not enriched in poly(A)+RNA contain a high ratio of full-length cDNA clones. The results suggest that representative cDNA libraries of high quality can be constructed without pre-isolation of poly(A)+RNA fractions.

The regulation of protein synthesis in heart muscle under normal conditions and in the adriamycin-cardiomyopathy

The regulation of cardiac protein synthesis, in particular messenger-RNA (mRNA) and polyribosome metabolism, has been investigated in normal rat heart muscle and in the adriamycin-cardiomyopathy by using newly developed methods for the isolation, characterization and in-vitro translation of cardiac polyribosomes and mRNA. The obtained data allow the following conclusions: 1. Normal heart muscle has a high content of polyribosomes (865 micrograms/g) and of mRNA (20-60 micrograms/g), and thus a high rate of protein synthesis. 2. The level of cardiac polyribosomes and mRNA is strictly age-dependent and much higher in young animals (2-3 x). This corresponds to a higher cardiac protein synthesis rate in young animals with a growing heart muscle, and shows that the protein-synthetic reserves of heart muscle decrease sharply with age. 3. Withdrawal of food for 1-3 days results in a pronounced decrease (-50% to -70%) of cardiac polyribosomes and mRNA, demonstrating that the cardiac protein synthesis reacts very sensitively to conditions of starving. 4. The cardiac polyribosomes and mRNA are unevenly distributed in the myocyte. The bulk of these substances is present in the cardiac microsomes, and much less is found in nuclei, myofibrils, mitochondria and in the post-microsomal fraction (=cell-sap) of the cardiac muscle. This shows that the major intracellular site of cardiac protein synthesis is the microsomal fraction of the myocyte. 5. A pool of untranslated mRNA was demonstrated to be present in the cell-sap of the myocyte. This mRNA is to some extent translatable in-vitro and appears to represent mRNA sub-pools with two functions: a) mRNA which is partially broken down or in the process of being broken down, and b) intact mRNA which could have a "reserve-function", e.g., by being utilized to increase cardiac protein synthesis under certain conditions. 6. A method of quantitating small amounts of cardiac mRNA (25-50 ng) has been developed which makes it possible to estimate the mRNA content of cardiac biopsies. 7. These methods were utilized to study the relevance of changes in RNA- and protein synthesis in the development of the adriamycin-cardiomyopathy. It appears that severe decreases in cardiac mRNA and polyribosome levels are a key factor in the pathogenesis of the adriamycin-cardiomyopathy. These decreases are probably caused by the direct binding of adriamycin to cardiac DNA and lead themselves to a persisting decrease in cardiac protein synthesis which in view of the short half-lives of the cardiac contractile proteins (5-12 days) causes a gradual loss of cardiac structure and function.

Molecular mechanisms of nucleotide-sequence rearrangements in cDNA clones of human fibroblast interferon mRNA

The characterization and nucleotide sequencing of cDNA clones of human fibroblast interferon (HFIF) mRNA (Derynck et al., 1980) revealed two types of structural inversions :(i) an inverted repeat of a 3'-proximal segment at the 5'end, or (ii) an inverted 5'-terminal segment. Based on the nucleotide sequence of these inverted clones, we have deduced molecular mechanisms to explain how the rearrangements could have arisen. We propose that the usual small 3'-terminal hairpin does not form after the reverse transcription produces the first cDNA strand. Instead, base pairing occurs between the 3'-terminal segment and the more distal region of the first cDNA strand, forming a much larger hairpin loop. Second-strand synthesis followed by either nicking or unfolding of the loop structure and continuation of the polymerase reaction would lead to the observed types of inversions. The 5'-truncated clone, pHFIF-1, whose new 5'-terminal sequence diverges from that of the corresponding region in the mRNA, can also be explained by such a mechanism.

The isolation of ecdysterone inducible genes by hybridization subtraction chromatography

We have developed a procedure for selectively enriching a mRNA population for inducible sequences. Other than the induced mRNA species, the population of mRNA in control cells is approximately the same as the mRNA population in induced cells. Cytoplasmic mRNA from control cells is bound to oligo (dT)-cellulose and used as a template for reverse transcriptase, the oligo (dT) serving as a primer. After removing the template mRNAs, the cDNA-cellulose column is used to hybridize a population of mRNAs from induced cells. The non-hybridized poly A+ RNAs are greatly enriched in the inducible sequences. We have used this technique of hybridization subtraction chromatography to select a mRNA population enriched for the mRNAs inducible by ecdysterone in Schneider's Line 2 Drosophila cells. This population of RNAs was used to screen a recombinant library. Preliminary results indicate that approximately 10% of the RNA in the probe population represents ecdysterone inducible sequences. Methods are described for optimizing the cDNA synthesis reaction (we obtain greater than or equal to 30% efficiency) and hybridizing RNA to the cDNA-cellulose resin. This method can be used to select induced mRNAs regardless of the way in which the induction is brought about.

Frequency and diversity of RNA sequences in different cell types of the fungus Schizophyllum commune

In the basidiomycete Schizophyllum commune an RNA complexity of 13,500 average-sized (1100 nucleotides) RNA sequences was found by means of RNA excess hybridization to complementary DNA. 600 abundant poly(A)-containing RNA sequences were present in about 100 copies per cell, whereas the rare 13,000 poly(A)-containing RNA sequences were present in about 5 copies per cell on average. Homologous and heterologous cDNA hybridizations to poly(A)-rich RNAs revealed some quantitative, but no significant qualitative differences between the monokaryotic and the dikaryotic cell type. Heterologous hybridization and subsequent hybridizations of remaining single-stranded cDNA to poly(A)-rich RNAs from both cell types showed qualitative differences amounting to 2-3% of the rare sequences in the two cell types. However, from the results of similar hybridizations with total RNAs, i.e. including non-polyadenylated RNA, it could be calculated that the RNA sequences in monokaryons and dikaryons are at least 99% the same. Identity of RNA sequences in monokaryons and dikaryons was also inferred by comparison of the patterns of polypeptides synthesized by translation of total RNA in cell-free systems.

The molecular basis of disorders of human hemoglobin synthesis

The structure and organization of the human globin genes at the nucleotide level has been established by restriction endonuclease digestion of cellular DNA, and by the isolation and purification of these genes in phage vectors. With this approach it has been possible to define alterations at the DNA level resulting in a group of inherited diseases of man known as the thalassemia syndromes, and related disorders. Combined with other known genetic and biochemical data, these studies provide a framework for understanding the pathogenesis of these disorders at the molecular level.

Mammalian epidermal messenger RNA: identification and characterization of the keratin messengers

A messenger RNA fraction which directs the synthesis of epidermal keratins and other skin proteins has been isolated from adult guinea pigs and newborn rats, utilizing techniques designed to minimize degradation by endogenous nucleases. During the initial extraction procedures an inhibitor of translation was identified. This inhibitor could be removed by sedimentation of the RNA through cesium chloride. Electrophoresis of the resulting RNA on denaturing agarose/urea gels indicated that, in addition to 18S and 28S ribosomal RNAs, several minor species ranging in size from 10S to 28S were present. This heterogeneous RNA stimulated the incorporation of radioactive amino acids into protein in a reticulocyte lysate system which had an absolute dependence on added mRNA. A fraction of the RNA was retained on oligo(dT)-cellulose, indicating the presence of poly(adenylic acid) sequences. This poly(A)-rich material was considerably enriched for mRNA activity. Analysis of the products of synthesis indicated that proteins which migrated as keratins in 1- and 2-dimensional electrophoretic systems were major translation products of both the unfractionated material and poly(A)-containing fractions. The minimum sedimentation coefficient of keratin mRNAs was found to be 18S, a value consistent with the molecular weight of the keratin polypeptides.

Subcytoplasmic distribution of thyroglobulin mRNA in normal sheep thyroid

The thyroglobulin 33-S mRNA was isolated from sheep thyroid total polysomes. The 33-S RNA, twice purified on a 1% sodium dodecylsulfate/sucrose gradient, was 30-fold enriched in thyroglobulin messenger activity and was estimated as 50% pure by its messenger activity and 80% pure by the electrophoretic profile. It was used as template for complementary DNA synthesis and hybridized up to 85% of the DNA copy with pseudo-first-order kinetics. Back-hybridization kinetics showed that the purified mRNA corresponds to a major kinetic component with a base sequence complexity of 10000 nucleotides as determined by comparison to globin mRNA. Cross-reactivity of [3H]cDNA with liver RNA is less than 10%. Restriction endonuclease digestion of [3H]cDNA yielded a discrete band pattern. The distribution of thyroglobulin mRNA among free polysomes, membrane-bound polysomes and extrapolysomal pools was analyzed using hybridization to the specific [3H]cDNA probe. Free particles were recovered in the supernatant and membrane-bound particles in the pellet after a brief centrifugation of detergent-free homogenate (5 min at 27000 x g: procedure A; 12 min at 130000 x g: procedure B) with precautions taken to avoid cross-contamination. Using procedure A, 80% of thyroglobulin mRNA sequences were found in the membrane-bound fraction. Using procedure B, where contamination of free particles by membrane-bound particles was avoided by high-speed initial centrifugation and further isolation through a discontinuous sucrose gradient, 95-98% of thyroglobulin mRNA sequences were recovered in membrane-bound polysomes. In total polysomes, 89% of thyroglobulin mRNA sequences were in the polysomal area and shifted to ribosomal subunits after EDTA treatment.

Synthesis of carbonic anhydrase in rabbit and chicken reticulocyte lysates

The synthesis of carbonic anhydrase, the second most abundant soluble protein of red blood cells, is shown to occur in rabbit and chicken reticulocyte lysates. This translation product was identified by chloroform/ethanol extraction, polyacrylamide gel electrophoresis in sodium dodecylsulphate and peptide mapping. In rabbit retic-locyte lysates, predominantly one of the two red cell isozymes, carbonic anhydrase I, is synthesised. The proportion of carbonic anhydrase synthesis (0.2-0.8% of total protein synthesis) in vitro is comparable to that (0.2-1.0%) in vivo for both rabbit and chicken reticulocytes.

Quantification of myosin heavy-chain mRNA during myogenesis

Complementary DNA (cDNA) was synthesized from a 26-S poly(A)-containing RNA fraction consisting predominately of myosin mRNA, and used to quantitate the amounts of myosin mRNA during development. RNA . cDNAmyosin hybridizations were performed with RNA isolated from tertiary cultures of chick fibroblasts, and the proportion of myosin RNA determined. The levels of myosin RNA present in primary chick myoblast cultures undergoing development were also analyzed by RNA . cDNAmyosin hybridizations. Correlation of these values with the rate of myosin heavy chain synthesis indicates that myosin mRNA is subject to post-transcriptional or translational control.

Reverse transcription of thyroglobulin 33-S mRNA

Bovine thyroglobulin 33-S mRNA has been used as a template for the synthesis of a complementary DNA, using RNA-directed DNA polymerase from the avian myeloblastosis virus. The yield of the reaction was relatively poor and the size of the cDNA did not exceed 10 S. Nevertheless, a copy of high specific radioactivity (approximately 10(7) counts. min-1 microgram-1) could be obtained which hybridized specifically back to its template with an rot1/2 value about 5 times higher than that observed in hybridizations between hemoglobin mRNA (alpha + beta chain) and hemoglobin cDNA. This suggests that thyroglobulin mRNA does not contain extensive internal repetitive sequences. Quantification of thyroglobulin mRNA sequences among various RNA preparations from the beef thyroid was performed using cDNA/RNA hybridizations in RNA excess. The results confirmed that thyroglobulin mRNA represents the large majority of mRNA in membrane-bound polysomes and indicated the virtual absence of thyroglobulin sequences on free polyosomes. The cDNA transcribed from mRNA of bovine origin hybridized efficiently with thyroid RNA from goats, dogs and humans. Although the heterologuous hybrids exhibited the expected decrease in thermal stability, the bovine cDNA provides an appropriate probe for studies dealing with the expression of the thyroglobulin gene in various mammals including man.

Globin RNA precursor molecules: biosynthesis and process in erythroid cells

Hybridization of labeled RNA with excess amounts of DNA complementary to globin mRNA, in conjunction with a pulse-chase technique, were used to investigate the biosynthetic pathway of globin mRNA in erythroid cells. Three species of molecules sharing common sequences with globin mRNA were detected in the nuclei of these cells, two of which are larger than the cytoplasmic globin mRNA. One species was approximately 7 times larger than globin mRNA ("27S"), and the other ("15S") was only about twice the size of cytoplasmic globin mRNA. The largest species lacked poly(A) sequences, while the others contained poly(A), After chase, the large RNA species gradually disappeared ( 1/2 = 5 min), while the cytoplasmic 10S species accumulated. From these results a model is proposed describing the biosynthetic pathway of globin RNA transcription: an early transcription product is the large molecule "27S" (approximately 5000 nucleotides long) which is then cleaved into a smaller species "15S" (approximately 1500 nucleotides). This intermediate precursor is then clipped, presumably at the 5' end, and finally converted to the exported "10S" molecule (approximately 750 nucleotides) which accumulates in the cytoplasm.

Study of markers of human erythroid differentiation in hybrid cells

Somatic cell hybrids exhibiting co-expression of the globin genes of two species were generated by fusion of mouse erythroleukemia cells with Chinese hamster or human marrow erythroid cells. In contrast, extinction of the mouse globin genes occurred in hybrids formed between the erythroleukemia cells and human fibroblasts. Direct detection of the human globin genes in human X mouse fibroblast hybrids was achieved by annealing of DNA from these cells to human globin complementary DNA. This method was developed to permit the chromosomal assignment of the human globin genes.

Gene specific priming of complementary DNA synthesis

DNA, complementary to chicken globin mRNA was synthesized using either Avian Myeloblastosis virus reverse transcriptase, or E. coli DNA polymerase I. Transcriptase cDNA sediments at 9 S on sucrose gradients, and is 620 nucleotides in length, representing a complete copy of globin mRNA template. In contrast, Polymerase I cDNA sediments at 4 S, is 100 to 200 nucleotides in length, and is a copy of a small region at the 3'(poly A) end of globin mRNA. Similarly, Transcriptase cDNA and Polymerase I cDNA hybridize to globin mRNA template with characteristic, individual Crot1/2 values. The Crot1/2 value for Transcriptase cDNA hybridization is 7 X 10(-4) mol s 1(-1), and that for Polymerase I cDNA is 5 X 10(-3). This work shows that Avian Myeloblastosis virus reverse transcriptase can use Polymerase I cDNA to prime further cDNA synthesis along the mRNA template. The product of extended cDNA synthesis is identical in length and hybridization properties to oligo (dT) primed transcriptase cDNA.

Deoxyribonucleic acid polymerase from the extreme thermophile Thermus aquaticus

A stable deoxyribonucleic acid (DNA) polymerase (EC 2.7.7.7) with a temperature optimum of 80 degrees C has been purified from the extreme thermophile Thermus aquaticus. The enzyme is free from phosphomonoesterase, phosphodiesterase and single-stranded exonuclease activities. Maximal activity of the enzyme requires all four deoxyribonucleotides and activated calf thymus DNA. An absolute requirement for divalent cation cofactor was satisfied by Mg2+ or to a lesser extent by Mn2+. Monovalent cations at concentrations as high as 0.1 M did not show a significant inhibitory effect. The pH optimum was 8.0 in tris(hydroxymethyl)aminomethane-hydrochloride buffer. The molecular weight of the enzyme was estimated by sucrose gradient centrifugation and gel filtrations on Sephadex G-100 to be approximately 63,000 to 68,000. The elevated temperature requirement, small size, and lack of nuclease activity distinguish this polymerase from the DNA polymerase of Escherichia coli.

Detection of mRNA sequences in nuclear 30S ribonucleoprotein subcomplexes

RNA from nuclear 30S ribonucleoprotein (RNP) complexes of mouse ascites cells has been shows to contain sequences homologous to poly(A) + mRNA by its ability to hybridize with complementary DNA prepared from poly(A) + mRNA template. Analysis of the hybridization kinetics of poly(A) + mRNA with its own complementary DNA revealed several abundancy classes. The total complexity of poly(A) + mRNA from ascites cells was estimated to be approximately 30,000 sequences of average molecular weight (6 X 10(5)). When the hybridization reaction of 30S RNP-RNA with mRNA-specific cDNA was compared to the homologous reaction the majority, and most probably all, of the poly(A) + mRNA sequences were found to be present in the RNA. The kinetics of hybridization suggest that 10-15% of the RNA in this RNP complex is homologous to poly(A) + mRNA. The 30S RNP subcomplexes therefore contain nuclear poly(A) + mRNA sequences as well as the bulk of heterogeneous RNA.

Sequences related to immunoglobulin kappa chain messenger RNA in T cells

We investigated by molecular hybridization whether T cells contain RNA sequences homologous to RNA which codes for immunoglobulin kappa-chain (k-chain). A radioactive probe of complementary DNA (cDNA) was prepared by transcription of purified k-chain mRNA from mouse myeloma MOPC-41 with reverse transcriptase (RNA-dependent-DNA nucleotidyltransferase) from avian myeloblastosis virus. The cDNA probably corresponded only to the constant region and 3'-terminus of k-chain mRNA. Kappa-chain cDNA was found to hybridize efficiently with RNA from both thymus cells and an established culture of thymoma cells. The thymus and thymoma cells contained 99.8% and 100% theta-positive cells, respectively. Quantitatively the average thymus T cell (thymus derived lymphocyte) contained about one half as much k-chain mRNA as the average spleen B cell ("bursa" dependent lymphocyte), whereas the thymoma cells contained only 1/33 as much. Control hybridizations of k-chain cDNA with myeloma and liver RNA support the conclusion that T cells in the thymus and in the thymoma cell line synthesize k-chain mRNA-like molecules. The thermal stability of hybrids of k-chain cDNA with RNA from spleen, thymus, thymoma, and another k-chain producing myeloma tumor was lower than that with MOPC-41 RNA. This finding may be due to the existence of several slightly different ck genes in the mouse as suggested by various control experiments.

Extinction of globin gene expression in human fibroblast x mouse erythroleukemia cell hybrids

We have chosen human fibroblast x mouse erythroleukemia hybrid cells as a model system to examine regulation of unique genes. The globin genes were studied as a marker of erythroid differentiation. Three separate hybrid cell lines were incubated in 2% dimethylsulfoxide, an agent which induces erythroid differentiation of the parental erythroleukemia cells. Neither human nor mouse globin mRNA sequences could be detected by a sensitive molecular hybridization assay which utilized globin complementary D N A. However, td n a from one of the cell lines was shown to contain both the mouse and humand globin genes. Thus, loss of the genes by chromosomal segregation did not account for their failure to be expressed. Cocultivation of the mouse erythroleukemia cells with excess human fibroblasts did not prevent erythroid differentiation of the erythroleukemia cells in the presence of dimethylsulfoxide. Similarly globin gene expression was preserved in tetraploid cells generated by fusion of two erythroleukemia lines. Thus, extinction of globin geneated by fusion of two erythroleukemia lines. Thus, extinction of blobin gene expression in the human fibroblast x erythroleukemia hybrids occurred at the level of mRNA production and appeared to be due to the presence of the fibroblast genome within the hybrial cell.

Immunochemical isolation and characterization of vitellogenin mRNA from liver of estradiol-treated chicks

Vitellogenin mRNA was purified through three steps. A heavy polysome fraction was obtained by discontinuous sucrose density gradient centrifugation, vitellogenin polysomes were immunoprecipitated with affinity-chromatography-purified anti-lipovitellin IgG and goat anti-rabbit IgG, the enriched mRNA was isolated on a poly(U)-Sepharose column. As judged by its specific activity in a reticulocyte lysate system, vitellogenin mRNA has been enriched a 1000-fold with a recovery of 30%. On 99% formamide 3.4% polyacrylamide gels vitellogenin mRNA has an Mr of 2.4-2.5 X 10(6) and codes for a peptide of Mr 240000, which under our incubation conditions is partially degraded to smaller peptides.