Electron-microscopic demonstration of terminal and internal initiation sites for cDNA synthesis on vitellogenin mRNA

cDNA synthesized on purified vitellogenin mRNA from Xenopus liver was hybridized to the template in formamide/urea at 22 degrees C to avoid degradation of the RNA. The hybrids formed were visualized by spreading for electron microscopy. Contour length measurements proved that most of the RNA molecules in the hybrids were still intact showing the expected molecular weight of 2.3 x 10(6). The hybridized cDNA corresponded on the average to 12% of the RNA length. In about 80% of the molecules the cDNA was located at one end. Since cDNA synthesis was primed by oligo(dT), the terminal duplex region marks the 3' end of the vitellogenin mRNA molecule. Internal duplex regions were mainly located at a specific position starting about 2800 nucleotides from the 3' end. Since the cDNA hybridizing at the internal position could specifically be synthesized on a vitellogenin RNA fragment isolated on poly(U)-Sepharose as an oligo(A)-containing RNA, we conclude that cDNA synthesis is not only initiated by the poly(A) of the 3' end, but also by a specific internal sequence.     

Complexity and characterization of polyadenylated RNA in the mouse brain

The complexity of nuclear RNA, poly(A)hnRNA, poly(A)mRNA, and total poly(A)RNA from mouse brain has been measured by saturation hybridization with nonrepeated DNA. These DNA populations were complementary, respectively, to 21, 13.5, 3.8, and 13.3% of the DNA. From the RNA Cot required to achieve half-sturation, it was estimated that about 2.5–3% of the mass of total nuclear RNA constituted most of the complexity. Similarly, complexity driver molecules constituted 6–7% of the mass of the poly(A)hnRNA. 75–80% of the poly(A)mRNA diversity is contained in an estimated 4–5% of the mass of this mRNA. Poly(A)hnRNA constituted about 20% of the mass of nuclear RNA and was comprised of molecules which sedimented in DMSO-sucrose gradients largely between 16S and 60S. The number average size of poly(A)hnRNA determined by sedimentation, electron microscopy, or poly(A) content was 4200–4800 nucleotides. Poly(A)mRNA constituted about 2% of the total polysomal RNA, and the number average size was 1100–1400 nucleotides. The complexity of whole cell poly(A)RNA, which contains both poly(A)hnRNA and poly(A)mRNA populations, was the same as poly(A)hnRNA. This implies that cytoplasmic polyadenylation does not occur to any apparent qualitative extent and that poly(A)mRNA is a subset of the poly(A)hnRNA population. The complexity of poly(A)hnRNA and poly(A)mRNA in kilobases was 5 × 10⁵ and 1.4 × 10⁵, respectively. DNA which hybridized with poly(A)mRNA renatures in the presence of excess total DNA at the same rate as nonrepetitive tracer DNA. Hence saturation values are due to hybridization with nonrepeated DNA and are therefore a direct measure of the sequence complexity of poly(A)mRNA. These results indicate that the nonrepeated sequence complexity of the poly(A)mRNA population is equal to about one fourth that observed for poly(A)hnRNA.     

The preparation and characterization of locust vitellogenin messenger RNA and the synthesis of its complementary DNA.

Poly(A)+ (polyadenylated) RNA was isolated from vitellogenic female-locus fat-body by LiCl/urea extraction and poly(U)-Sepharose 4B affinity chromatography. Agarose-gel electrophoresis of this poly(A)+ RNA under denaturing conditions shows the presence of a high-molecular-weight species (greater than 31 S, 7100 nucleotides) as the major species, which is absent from the RNA prepared from male-locust fat-body. Inclusion of this poly(A)+ RNA in a mRNA-dependent reticulocyte-lysate system directs the synthesis of polypeptides that could be immunoprecipitated with monospecific antibodies against locust egg vitellin. DNA complementary (cDNA) to the poly(A)+ RNA was synthesized, and back-hybridization of the cDNA to its template reveals a major abundant species comprising about 45% of the total poly(A)+ RNA hybridizing with R0t 1/2 of 2 x 10(-2) mol . litre-1 . s. Abundant cDNA isolated from the total cDNA hybridizes to poly(A)+ RNA with a R0t 1/2 of 9 x 10(-3) mol . litre-1 . s. There are 9.1 x 10(3) copies of vitellogenin mRNA per cell of vitellogenic female-locust fat-body, comprising 55% of the poly(A)+ RNA and equivalent to 0.7% of total cellular RNA.     

Poly(adenylic acid)-containing and -deficient messenger RNA of mouse liver

RNA was isolated and fractionated into poly(A)-containing and -deficient classes by oligo(dT) chromatography. Approximately 99% of the poly(A) material bound to the oligo(dT); that which did not bind contained substantially shorter poly(A) chains. All RNA fractions retained an ability to initiate cell-free translation, with the poly(A)-deficient fraction containing half the total translational activity, i.e., mRNA. Two-dimensional polyacrylamide gel analysis of the cell-free translation products revealed three classes of mRNA: 1, mRNA preferentially containing poly(A), including the abundant liver mRNA species; 2, poly(A)-deficient mRNA, including many mid- and low-abundant mRNAs exhibiting less than 10% contamination in the poly(A)-containing fraction fraction; and 3, bimorphic species of mRNA proportioned between both the poly(A)-containing and -deficient fractions. Poly(A)-containing and bimorphic mRNA classes were further characterized by cDNA hybridizations. The capacity of various RNA fractions to prime cDNA synthesis was determined. Compared to total RNA, the poly(A)-containing RNA retained 70% of the priming capacity, while 20% was found in the poly(A)-deficient fraction. Poly(A)-containing, poly(A)-deficient, and total RNA fractions were hybridized to cDNAs synthesized from (+)poly(A)RNA. Poly(A)-containing RNA hybridized with an average R0t 1/2 approximately 20 times faster than total RNA. Poly(A)-deficient RNA hybridized with an average R0t 1/2 approximately 3-4 times slower than total RNA. These R0t 1/2 shifts indicated that in excess of three-quarters of the total hybridizable RNA was recovered in the poly(A)-containing fraction and that less than one-quarter was recovered in the poly(A)-deficient RNA fraction. Abundancy classes were less distinct in heterologous hybridizations. In all cases the extent of hybridization was similar, indicating that while the amount of various mRNA species varied among the RNA fractions, most hybridizing species of RNA were present in each RNA fraction. cDNA to the abundant class of mRNAs was purified and hybridized to both (+)- and (-)poly(A)RNA. Messenger RNA corresponding to the more abundant species was enriched in the poly(A)-containing fraction at least 2-fold over the less abundant species of mRNA, with less than 10% of the abundant mRNAs appearing inthe poly(A)-deficient fraction.     

The synthesis of polyadenylic acid-containing ribonucleic acid by isolated mitochondria from Ehrlich ascites cells.

The synthesis of poly(A)-containing RNA by isolated mitochondria from Ehrlich ascites cells was studied. Isolated mitochondria incorporate [3H]AMP or [3H]UTP into an RNA species that adsorbs on oligo (dT)-cellulose columns or Millipore filters. Hydrolysis of the poly(A)-containing RNA with pancreatic and T1 ribonucleases released a poly(A) sequence that had an electrophoretic mobility slightly faster than 4SE. In comparison, ascites-cell cytosolic poly(A)-containing RNA had a poly(A) tail that had an electrophoretic mobility of about 7SE. Sensitivity of the incorporation of [3H]AMP into poly(A)-containing RNA to ethidium bromide and to atractyloside and lack of sensitivity to immobilized ribonuclease added to the mitochondria after incubation indicated that the site of incorporation was mitochondrial. The poly(A)-containing RNA sedimented with a peak of about 18S, with much material of higher s value. After denaturation at 70 degrees C for 5 min the poly(A)-containing RNA separated into two components of 12S and 16S on a 5-20% (w/v) sucrose density gradient at 4 degrees C, or at 4 degrees and 25 degrees C in the presence of formaldehyde. Poly(A)-containing RNA synthesized in the presence of ethidium bromide sedimented at 5-10S in a 15-33% (w/v) sucrose density gradient at 24 degrees C. The poly(A) tail of this RNA was smaller than that synthesized in the absence of ethidium bromide. The size of the poly(A)-containing RNA (approx. 1300 nucleotides) is about the length necessary for that of mRNA species for the products of mitochondrial protein synthesis observed by ourselves and others.     

Purification and properties of biologically active rainbow trout testis protamine mRNA.

At least two classes of protamine mRNA are present in both trout testis polysomal RNA and RNA from the postribosomal supernatant fraction of trout testis hormogenate both of which direct the synthesis of protamine in a Krebs II ascites S-30. One contains poly(A) tracts and the other is devoid of poly(A). Sucrose gradient analyses showed that the poly(A) containing protamine mRNA (poly(A) (+)) sedimented IN THE 6 S region with a shoulder in the 4 S region while the protamine mRNA devoid of poly(A) (poly(A) (-)) appeared to sediment at about 4 S and could not be resolved from tRNA. Analysis of the poly(A) (+) protamine mRNA by boundary sedimentation in an analytical ultracentrifuge showed a sedimentation coefficient of 5.7 S, a value which gives rise to an estimate of 165 to 170 nucleotides per molecule. The poly(A) (+) protamine mRNA migrated as a single species in formamide-containing polyacrylamide gels and its mobility in relation to markers of tRNA (4 S) and 5 S RNA was consistent with its sedimentation velocity of 6 S. The RNA present in the major band on an aqueous polyacrylamide gel was extracted and shown to code for protamine in a wheat germ cell-free system.     

Methylated nucleotides block 5' terminus of HeLa cell messenger RNA.

Polyadenylylated [poly(A)+] mRNA from HeLa cells that were labeled with [3H-methyl]-methionine and 14C-uridine was isolated by poly(U)-Sepharose chromatography. The presence of approximately two methyl groups per 1000 nucleotides of poly(A)+ RNA was calculated from the 3H/14C ratios and known degrees of methylation of 18S and 28S ribosomal RNAs. All four 2'-O-methylribonucleosides, but only two base-methylated derivatives, 7-methylguanosine (7MeG) and 6-methyladenosine (6MeA), were identified. 6MeA was the major component accounting for approximately 50% of the total methyl-labeled ribonucleosides. 7MeG, comprising about 10% of the total, was present exclusively at the 5' terminus of the poly(A)+ RNA and could be removed by periodate oxidation and beta elimination. Evidence for a 5' to 5' linkage of 7MeG to adjacent 2'-O-methylribonucleosides through at least two and probably three phosphates to give structures of the type 7MeG5'ppp5pNMep- and 7MeG5'ppp5'NMepNmep- was presented. The previous finding of similar sequences of methylated nucleotides in mRNA synthesized in vitro by enzymes associated with virus cores indicates that blocked 5' termini may be a characteristic feature of mRNAs that function in eucaryotic cells.     

Construction and partial characterization of a recombinant DNA probe for locust vitellogenin messenger RNA.

Double-stranded DNA complementary to poly(A)-containing RNA from the fat body of adult female locusts, Locusta migratoria, was synthesized. Hybrid molecules containing this cDNA was constructed in the PstI site of the plasmid pAT 153 by the technique of dC . dG tailing and amplified in Escherichia coli K-12 strain HB 101. Ten colonies of bacteria were identified as carrying recombinant plasmids containing DNA complementary to locust vitellogenin mRNA by (a) 'Northern' blot hybridization analysis and (b) hybrid selection of vitellogenin mRNA and immunological detection of the products of translation of the mRNA. Of the ten recombinant plasmids, one, termed plasmid 4E, containing a cDNA insert of about 650 nucleotides, was characterized in greater detail and a partial restriction map obtained. Using this hybrid plasmid it was possible to derive a value for the average content of vitellogenin mRNA in the adult female locust fat body as 1.5 x 10(5) molecules/cell, and to establish that the haploid genome of L. migratoria contains only one or two genes coding for vitellogenin.     

An analysis of the rate of metallothionein mRNA poly(A)-shortening using RNA blot hybridization.

A progressive reduction in the size of rat metallothionein-1 mRNA following induction by copper chloride or dexamethasone was demonstrated on RNA blots, and was shown to be due to shortening of the poly(A)-tail. The rate of poly(A) removal was the same in rat liver and kidney following copper chloride induction, in rat liver following dexamethasone induction, and in mouse liver following copper chloride induction. In mouse liver metallothionein-1 and 2 mRNAs were shortened at the same rate. The reduction of the poly(A) tail was more rapid in the first 5 hours (approximately 20 nucleotides/h) but much slower (approximately 3 nucleotides/h) after the poly(A)-tail had been reduced to about 60 residues. Metallothionein mRNA molecules with poly(A) tail sizes less than 30-40 nucleotides were not observed. Exonuclease digestion of the poly(A)-tail is suggested, at least in the initial rapid phase. It is hypothesized that poly(A)-tails longer than 30 are required for mRNA stability and that much longer poly(A) tails may give newly synthesized mRNA molecules a competitive advantage in protein synthesis.     

Polyadenylic acid-containing RNA and its polyadenylic acid sequences newly synthesized in isolated embryonic cells of Xenopus laevis.

Newly synthesized polyriboadenylic acid [poly(A)]-containing RNA and its poly(A) sequences were isolated and characterized in Xenopus embryonic cells. Upon sedimentation analysis, the poly(A)-containing RNA labeled for 30 min showed a very heterogeneous size distribution ranging from 9 to >40 S. After 5 hr of labeling, the profile became much less heterogeneous and the main component was distributed in the 9–28 S region. The average molecular weight of 6.5–7.0 × 10⁵ daltons was calculated for the 5-hr labeled RNA. This poly(A)-containing RNA, comprising about 10% of the total labeled RNA, was metabolically stable and accumulated linearly for 5 hr. Gel electrophoresis of the RNA revealed the presence of little or no free poly(A) sequences. Most of the poly(A) sequences, which were isolated from 30-min labeled poly(A)-containing RNA migrated as a single discrete component approximately 150 nucleotides long. In contrast, they were slightly smaller (130 nucleotides long) and more heterogeneous, when obtained from the poly(A)-containing RNA labeled for 5 hr. From these results, it may be likely that the embryonic poly(A)-containing RNA is similar in size to the steady-state population of the poly(A)-containing RNA reported to occur in vitellogenic oocytes and cultured kidney cells of the same species.     

Multiple oligo(A) tracts associated with inactive sea urchin maternal mRNA sequences

Maternal RNA of sea urchin eggs and embryos was analyzed for short poly(A) sequences by digesting hybrids formed between [³H]poly(U) and poly(A) with RNase at 4°C. When the undigested [³H]poly(U) is precipitated with CTAB, all (A)_n tracts longer than 6 nucleotides are detected. This assay revealed a poly(A) content severalfold higher than is obtained with a similar assay using RNase at higher temperatures. On polyacrylamide gel electrophoresis, most of the previously undetected (A)_n tracts ran as a peak of oligo(A) of less than 20 nucleotides which accumulated at the dye front. The oligo(A) sequences were resolved into a single peak of (A)₁₀ when sized on Sephadex G100. These (A)₁₀ sequences were associated with large mRNA-sized molecules of about 3000 nucloetides average length which comprised 0.5 to 2% of the total maternal RNA. However, the (A)₁₀ sequences were not in mRNA molecules containing 3′-terminal poly(A) of 50–120 nucleotides nor did they remain in RNA that entered polysomes upon fertilization. However, hybridization studies showed that all sequences represented in the maternal poly(A)-containing RNA appeared to be present in the RNA molecules containing only (A)₁₀ sequences. The results suggest that the (A)₁₀-containing RNA might be incompletely processed mRNA precursor-like molecules.     

Relative amounts of newly synthesized poly(A)+ and poly(A)- messenger RNA during development of Xenopus laevis

The relative amounts of newly synthesized poly(A)⁺ and poly(A)^? mRNA have been determined in developing embryos of the frog Xenopus laevis. Polysomal RNA was isolated and fractionated into poly(A)⁺ and poly(A)^? RNA fractions with oligo(dT)-cellulose. In normal embryos the newly synthesized polysomal poly(A)⁺ RNA has a heterodisperse size distribution as expected of mRNA. The labeled poly(A)^? RNA of polysomes is composed mainly of rRNA and 4S RNA. The amount of poly(A)^? mRNA in this fraction cannot be quantitated because it represents a very small proportion of the labeled poly(A)^? RNA. By using the anucleolate mutants of Xenopus which do not synthesize rRNA, it is possible to estimate the percentage of mRNA which contains poly(A) and lacks poly(A). All labeled polysomal RNA larger than 4S RNA which does not bind to oligo(dT)-cellulose in the anucleolate mutants is considered presumptive poly(A)^? mRNA. The results indicate that about 80% of the mRNA lacks a poly(A) segment long enough to bind to oligo(dT). The poly(A)⁺ and poly(A)^? mRNA populations have a similar size distribution with a modal molecular weight of about 7 × 10⁵. The poly(A) segment of poly(A)⁺ mRNA is about 125 nucleotides long. Analysis of the poly(A)^? mRNA fraction has shown that it lacks poly(A)₁₂₅.     

Polyadenylic acid on poliovirus RNA. II. poly(A) on intracellular RNAs.

The content, size, and mechanism of synthesis of 3'-terminal poly(A) on the various intracellular species of poliovirus RNA have been examined. All viral RNA species bound to poly(U) filters and contained RNase-resistant stretches of poly(A) which could be analyzed by electrophoresis in polyacrylamide gels. At 3 h after infection, the poly(A) on virion RNA, relicative intermediate RNA, polyribosomal RNA, and total cytoplasmic 35S RNA was heterogeneous in size with an average length of 75 nucleotides. By 6 h after infection many of the intracellular RNA's had poly(A) of over 150 nucleotides in length, but the poly(A) in virion RNA did not increase in size suggesting that the amount of poly(A) which can be encapsidated is limited. At all times, the double-stranded poliovirus RNA molecules had poly(A) of 150 to 200 nucleotides. Investigation of the kinetics of poly(A) appearance in the replicative intermediate and in finished 35S molecules indicated that poly(A) is the last portion of the 35S RNA to be synthesized; no nascent poly(A) could be detected in the replicative intermediate. Although this result indicates that poliovirus RNA is synthesized 5' leads to 3' like other RNA's, it also suggests that much of the poly(A) found in the replicative intermediate is an artifact possibly arising from the binding of finished 35S RNA molecules to the replicative intermediate during extraction. The addition of poly(A) to 35S RNA molecules was not sensitive to guanidene.     

Changes in expression of albumin and alpha-fetoprotein genes during rat liver development and neoplasia.

Albumin mRNA was isolated and purified from rat liver polysomes by a combination of immunoprecipitation of specific polysomes, poly(U)-Sepharose 4B chromatography, and fractionation of the resulting poly(A)-containing RNA on a sucrose gradient. alpha-Fetoprotein (AFP) mRNA was isolated from Morris hepatoma 7777 by a similar procedure. The purity of the mRNA preparations was determined by analytical gel electrophoresis under denaturing conditions, analysis of sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the polypeptides synthesized in a wheat germ cell-free system, and the kinetics of hybridization to cDNA transcribed from albumin mRNA and AFP mRNA. The albumin mRNA possessed a chain length of approximately 2265 nucleotides and the AFP mRNA possesed a length of approximately 2235 nucleotides when examined under stringent denaturing conditions on agarose gels containing 10 mM methylmercury hydroxide. Analysis of poly(A) content by a hybridization assay with [3H]poly(U) revealed the presence in albumin mRNA of a poly(A) region containing approximately 100 adenosine residues. The AFP mRNA preparation was found to contain an average poly(A) tract of approximately 190 bases. Thus, albumin mRNA appears to contain approximately 330 untranslated nucleotides, and AFP mRNA appears to contain a similar number (approximately 285) of noncoding, nonpoly(A) bases. The purified albumin and AFP mRNA's were used as templates for synthesis of full-length cDNA hybridization probes. Both of the probes selectively hybridized to their templates with kinetics expected for single RNA species the sizes of albumin and AFP mRNA. ROt analysis was used to quantitate albumin and AFP mRNA sequences during normal liver postnatal development and liver oncogenesis. The number of polysomal AFP mRNA molecules per liver was found to drastically decrease during the first weeks of postnatal life, concomitant with a decline in the AFP synthetic capacity of the livers and in the serum concentrations of AFP. During this period, the concentration of albumin mRNA molecules per cell in the liver remained at high, approximately constant levels. In Morris hepatoma 7777, the concentration of AFP-specifying sequences was at least 10(3)-fold higher than that found in normal adult liver, whereas the content of albumin nRNA was four- to five-fold lower. These changes in concentration of albumin and AFP mRNA sequences closely correlated with a parallel variation in the specific protein synthetic capacity of the tissues.     

Complex population of nonpolyadenylated messenger RNA in mouse brain

The complexity of nonadenylated mRNA [poly(A)-mRNA] has been determined by hybridization with single-copy DNA (scDNA) and cDNA. Our results show that poly(A)- and poly(A)+ mRNA are essentially nonoverlapping (nonhomologous) sequence populations of similar complexity. The sum of the complexities of poly(A)+ mRNA and poly(A)- mRNA is equal to that of total polysomal RNA or total mRNA, or the equivalent of approximately 1.7 x 10(5) different sequences 1.5 kb in length. Poly(A)- mRNA, isolated from polysomal RNA by benzoylated cellulose chromatography, hybridized with 3.6% of the scDNA, corresponding to a complexity of 7.8 x 10(4) different 1.5 kb sequences. The equivalent of only one adenosine tract of approximately 20 nucleotides per 100 poly(A)- mRNA molecules 1.5 kb in size was observed by hybridization with poly(U). cDNA was transcribed from poly(A)- mRNA using random oligonucleotides as primers. Only 1-2% of the single-copy fraction of this cDNA was hybridized using poly(A)+ mRNA as a driver. These results show that poly(A)- mRNA shares few sequences with poly(A)+ mRNA and thus constitutes a separate, complex class of messenger RNA. These measurements preclude the presence of a complex class of bimorphic mRNAs [that is, species present in both poly(A)+ and poly(A)- forms] in brain polysomes.