Biomedical subjects

H L Moses

Publications and source records attributed to H L Moses.

At least 145 records · Page 8Linked to original sources

PubMed1984

Specific binding to cultured cells of 125I-labeled type beta transforming growth factor from human platelets.

Purified type beta transforming growth factor from human platelets (TGF beta) radioiodinated with 125I-labeled Bolton and Hunter reagent was found to bind to a variety of cultured cells of both epithelial and mesenchymal origin, including normal human fibroblasts and keratinocytes. TGF beta binding sites have also been found on three mouse embryo-derived fibroblast-like cell lines with lower levels of TGF beta binding on the chemically transformed derivatives of these cell lines. A variety of human tumor cell lines was shown to have an inverse correlation between their level of TGF beta binding and their ability to form colonies in soft agar. The mouse embryo-derived AKR-2B (clone 84A) cells reached maximal binding of 125I-labeled TGF beta after 2 hr at 22 degrees C. Scatchard analysis of the equilibrium binding of TGF beta to AKR-2B (clone 84A) cells gives a Kd of 33 pM with approximately equal to 10,500 binding sites per cell. This Kd for TGF beta binding to AKR-2B (clone 84A) cells agreed well with the ED50 of 40 pM for stimulation of colony formation of these cells by TGF beta. The TGF beta binding sites on the AKR-2B cells were shown to be specific for TGF beta with no significant competition with epidermal growth factor, fibroblast growth factor, or insulin and only a small level of competition with high concentrations of platelet-derived growth factor. Partially purified preparations with TGF beta-like activity from mouse embryos and medium conditioned by mouse embryo-derived cells competed effectively for binding to the TGF beta receptor.

Animals↗

PubMed1984

Differential effects of epidermal growth factor, transforming growth factor, and insulin on DNA and protein synthesis and morphology in serum-free cultures of AKR-2B cells.

The effects of epidermal growth factor (EGF), insulin, and a transforming growth factor from fetal bovine serum on protein synthesis, DNA synthesis, and morphology in confluent cultures of AKR-2B mouse cells have been examined. The cells were plated and grown to confluence in serum-containing medium, and then the medium was changed to MCDB 402 containing no undefined supplements for 2 days before the addition of the polypeptides to be tested. Under these conditions, all of the factors listed above stimulated rapid increases in protein synthesis. When added to cultures singly, however, only EGF stimulated significant DNA synthesis. The effect of EGF on DNA synthesis was highly synergistic with insulin. Fetal serum transforming growth factor activity, which stimulated multiple rounds of cell replication in soft agar in serum-containing medium, did not stimulate DNA synthesis in serum-free monolayer cultures of these cells when added alone or in combination with EGF or insulin. Fetal serum transforming growth factor did, however, stimulate a rapid change in the morphology of these cells from flat, nonoverlapping cells to bipolar, multilayered cultures when added to the serum-free medium.

Animals↗

PubMed1983

Comparison of intra- and extracellular transforming growth factors from nontransformed and chemically transformed mouse embryo cells.

Multiple transforming growth factors (TGFs) have been isolated from the serum-free conditioned media and acid-ethanol cell extracts of nontransformed AKR-2B and chemically transformed AKR-MCA mouse cells. The TGF activity was analyzed using Bio-Gel P-60 chromatography in 1 M acetic acid and tested for colony-stimulating activity in soft agar using AKR-2B, AKR-2B (clone 84A), NRK, and AKR-MCA cells as indicators. Both intracellular and extracellular TGF activity from AKR-MCA and AKR-2B cells show a major peak of AKR-2B and epidermal growth factor-potentiated NRK colony-stimulating activity that coelute in the 13,000 +/- 2,000 molecular weight region. In the 24,000 +/- 7,000 molecular weight range, AKR-MCA cells produce intracellular and extracellular NRK colony-stimulating activity that is not potentiated by epidermal growth factor, while the intracellular and extracellular NRK colony-stimulating activity produced by AKR-2B cells requires added epidermal growth factor for colony formation. Also important in determining the growth and morphological characteristics of a cell line, besides the production of a TGF, is the ability of a cell to respond to TGF activity. We have shown that the transformed AKR-MCA cells form more colonies than AKR-2B cells in response to certain TGF activities. This suggests that the increased responsiveness of AKR-MCA cells to TGFs may be important in determining its phenotype.

Animals↗

PubMed1983

Transforming growth factors in solid human malignant neoplasms.

Surgically removed solid human benign and malignant neoplasms and nonneoplastic tissues were examined for the presence of transforming growth factors (TGFs). TGFs are polypeptide growth factor-like substances which cause the appearance of a reversible neoplastic phenotype in nontransformed, anchorage-dependent cells in culture, including the induction of the ability to grow while suspended in semisolid medium. Acid-ethanol extracts from adenocarcinomas of the breast, colon, kidney, and ovary; fibrosarcoma and leiomyosarcoma; Hodgkin's lymphoma; fibroadenoma of the breast; uterine leiomyoma; and nonneoplastic kidney and lung were found to cause growth in soft agar of both nontransformed mouse AKR-2B and rat NRK cells. This colony-stimulating activity, where tested, was heat and acid stable but was destroyed by trypsin and dithiothreitol treatment, indicating that the activity is due to a polypeptide with disulfide bonds. Extracts from several of the tumors provided sufficient material for purification by molecular sieve chromatography. Peaks of colony-stimulating activity from a Bio-Gel P-60 column eluted with 1 M acetic acid were detected in the M, 3,000 to 25,000 range with the apparent molecular weight varying depending on the type of tumor being studied and the indicator cells used. The data suggest that at least three TGFs are present in human tumors. Evidence is presented differentiating these TGFs into TGFa, which has selective activity for stimulating AKR-2B cells, and TGFn, which has selective activity for stimulating NRK cells. The NGFn activity was further subdivided into a TGFns fraction and TGFnl fraction, denoting small (less than 6,000) and large (12,000 to 20,00) apparent molecular weights, respectively. The TGFa and TGFnl activities were present in malignant and nonneoplastic (kidney and lung) tissue, whereas the TGFns activity predominated in benign neoplasms. These TGFs exhibited no competition with epidermal growth factor for binding to the epidermal growth factor receptor, and the TGFnl activity was potentiated by epidermal growth factor.

Animals↗

PubMed1983

Epithelial tissue-derived growth factor-like polypeptides.

SW-13 cells, derived from a human adenocarcinoma of the adrenal cortex, formed only a few small colonies when suspended in soft agar at low cell densities. The number and size of colonies increased dramatically following stimulation with serum-free medium conditioned by SW-13 cells, indicating the possibility of autostimulation in these malignant cells. Evidence is presented suggesting that SW-13 cells form progressively growing soft agar colonies upon stimulation by epithelial tissue-derived growth factor-like polypeptides. Both acid-ethanol extracts and conditioned media from three human carcinoma cell lines (A431, D562, and A549) caused similar increases in colony number and size of SW-13 cells. Extracts from 26 of 32 freshly excised human carcinomas and five freshly excised nonneoplastic human kidneys and one human lung stimulated soft agar growth of SW-13 cells as well. None of the nine extracts from nonepithelial human solid malignant tumors stimulated SW-13 cells. However, a benign nonepithelial tumor (uterine leiomyoma) caused a low level of soft agar growth of SW-13 cells. Cell extract from A204 human sarcoma cells and both conditioned medium and acid-ethanol cell extract from A375 human melanoma cells lacked SW-13 activity, whereas medium conditioned by A204 cells stimulated soft agar growth of SW-13 cells. Chemical and physical treatment data indicated that the epithelial tissue-derived growth factor-like substances are acid- and heat-stable polypeptides with disulfide bonds. The major peak of this activity had an apparent molecular weight of 20,000 to 22,000 and was clearly separable from transforming growth factors reported previously which stimulate colony formation by nontransformed mouse AKR-2B and rat NRK cells. The major peaks of SW-13, NRK, and AKR-2B activity could be separated by high-performance liquid chromatography. This SW-13 activity induced irreversible anchorage-independent growth of SW-13 cells and an increase in DNA synthesis as measured by [3H]thymidine incorporation.

Adenocarcinoma↗

PubMed1982

Mouse embryos contain polypeptide growth factor(s) capable of inducing a reversible neoplastic phenotype in nontransformed cells in culture.

A growth-factor-like substance capable of inducing nontransformed mouse AKR-2B, rat NRK, and EGF-receptorless mouse NR6 cells to form progressively growing colonies in soft agar was identified in acid/ethanol extracts of 17-day mouse embryos. This "mouse embryo factor" (MEF) is similar to previously described transforming growth factors in that it is capable of stimulating DNA synthesis and conferring a reversible transformed morphology on nontransformed cells in vitro. Passage of crude embryo extracts over a Bio-Gel P-60 column gave a major peak of soft agar growth-stimulating activity in the 15,000 molecular weight range with a minor peak at about 22,000. This biological activity was sensitive to treatment with either trypsin or dithiothreitol, but was unaffected by heat (56 degrees C for 30 minutes or 100 degrees C for 3 minutes), indicating that the activity is due to a heat-stable polypeptide(s) with disulfide bonds. Separation of these polypeptide(s) by chromatography on carboxymethyl cellulose revealed two peaks of soft agar growth-stimulating activity which did not cochromatograph with a peak of epidermal growth factor receptor-competing activity. The similarities of this mouse embryo-derived growth factor to previously identified transforming growth factors suggest that both fetal development and neoplastic transformation may be affected by similar mechanisms.

Animals↗

PubMed1982

Serum contains a platelet-derived transforming growth factor.

High concentrations of fetal bovine serum induced colony formation in soft agar by anchorage-dependent, nontransformed mouse AKR-2B and rat NRK cells. The colony-stimulating activity in fetal bovine serum was precipitated by 45% saturated ammonium sulfate and migrated in molecular sieve chromatography as a single peak of activity in the 10,000-15,000 molecular weight range. The colony-stimulating activity was heat and acid stable and was destroyed by trypsin and dithiothreitol, indicating the activity is due to a polypeptide that requires disulfide bonds for biological activity. No competition for binding to the epidermal growth factor receptor was associated with the colony-stimulating activity. Isoelectric focusing revealed activity in the pI 4-5 range. The colony-stimulating activity in serum appeared to be of platelet origin because platelet-poor plasma and platelet-poor plasma-derived serum contained little activity, whereas acid/ethanol extracts of bovine and human platelets had potent colony-stimulating activity. Chromatography of platelet extracts on Bio-Gel P-60 revealed peaks of AKR-2B colony-stimulating activity in the 12,000 and 20,000 molecular weight ranges. The other biological and chemical properties of the platelet colony-stimulating activity were the same as those for the serum activity. The data indicate the presence in serum of a platelet-derived growth factor(s) with properties similar to those of the transforming growth factors.

Animals↗

PubMed1982

Polyadenylylated RNA complementary to a mouse retrovirus-like multigene family is rapidly and specifically induced by epidermal growth factor stimulation of quiescent cells.

Complementary DNA probes prepared from total polysomal poly(A)+RNA populations were used to identify clones of mouse DNA containing sequences whose expression is specifically enhanced after epidermal growth factor (EGF) stimulation of quiescent mouse embryo cells in culture. Three such clones were isolated and used to study changes in the levels of clone-specific poly(A)+RNA in the polysomes of cells after mitogenic stimulation by EGF. RNA complementary to sequences present in these clones increased approximately equal to 10-fold as a fraction of the total poly(A)+RNA by 6 hr after stimulation. All three clones were found by hybridization criteria to contain sequences related to the class of mouse retrovirus or transposon-like elements termed VL30. These VL30-related sequences were further found to be complementary to EGF-inducible poly(A)+RNAs and enhanced expression was detectable as early as 1 hr after EGF stimulation. In contrast, nine additional clones, including an AKR-type murine leukemia provirus DNA clone, contained no detectable VL30 sequence elements and were complementary to poly(A)+RNA species whose relative concentration was essentially constant in quiescent and EGF-stimulated cells. Therefore, VL30 sequence elements appear distinct in that they encompass members whose expression is specifically regulated in response to a defined peptide growth factor.

Animals↗

PubMed1982

Growth factors, growth factor receptors, and cell cycle control mechanisms in chemically transformed cells.

Previous studies have shown that the chemically transformed AKR-MCA and C3H/MCA-58 cell lines spontaneously arrested growth at high saturation density in the G1 phase of the cell cycle because of depletion of low-molecular-weight nutrients (amino acids and glucose) from the medium. The nontransformed parent cell lines, AKR-2B and C3H/10T1/2, spontaneously arrest growth in G1 at low saturation density because of depletion of essential serum growth factors. If prevented from becoming deficient in growth factor by maintenance in medium with mitogens such as epidermal growth factor (EGF), fibroblast growth factor, or the tumor promoter 12-O-tetradecanoylphorbol-13-acetate, the nontransformed AKR-2B cells behave similarly to the transformed cells and arrest growth in G1 at high saturation density because of nutrient deficiency. This suggests that a major difference between the nontransformed and chemically transformed cells is an inability of the latter to achieve growth factor deficiency. In addition, the transformed AKR-MCA and C3H/MCA-58 cells show greatly diminished detectable EGF receptors. These observations could be accounted for by the endogenous production of the response to a growth factor-like substance by the transformed cells. Preliminary data indicate that these chemically transformed cells release a transforming growth factor (TGF) into serum-free medium. Whether TGF is similar or identical to the previously described sarcoma growth factor remains to be established.

Cell Cycle↗

PubMed1982

Cell cycle variation in 125I-labeled epidermal growth factor binding in chemically transformed cells.

Previous studies have shown that the nontransformed AKR-2B cells when arrested in the G1 phase of the cell cycle due to low-molecule-weight nutrient (amino acid) deficiency exhibit a 5- to 10-fold lower level of epidermal growth factor (EGF) receptor activity than do the same cells in the rapidly growing state or arrested in G1 due to growth factor deficiency. The chemically transformed AKR-MCA and C3H/MCA-58 cell lines spontaneously arrest growth in G1 due to nutrient deficiency when grown to saturation density in medium with 10% fetal bovine serum. An examination of 125I-labeled EGF binding in rapidly growing and G1-arrested AKR-MCA and C3H/MCA-58 cells showed that the G1-arrested chemically transformed cells also have a 10- to 20-fold reduction in the amount of 125I-labeled EGF binding relative to the same cells in the rapidly growing state. Stimulation of DNA synthesis in the arrested cells by the addition of serum-free medium caused a 6- to 10-fold increase in 125I-labeled EGF binding. This recovery of receptor activity was inhibited by actinomycin D and cycloheximide, suggesting that new messenger RNA synthesis as well as increased protein synthesis is necessary for the recovery of EGF binding. A comparison of EGF binding in C3H/MCA-58 cells and the nontransformed parent line (C3H/10T 1/2) in the rapidly growing state showed the same approximate level of receptor activity. However, the rapidly growing AKR-MCA cells had approximately one-tenth the amount of EGF binding as did the rapidly growing nontransformed parent line (AKR-2B). Scatchard analysis of binding data showed a 10-fold greater number of receptors in the AKR-2B cells relative to the AKR-MCA cells with a lesser difference in apparent receptor affinity. The chemically transformed BP-3T3, like the other two chemically transformed lines, was also demonstrated to arrest growth spontaneously due to nutrient deficiency with an associated 100-fold decrease in EGF binding. Rapidly growing BP-3T3 cells had only slightly less 125I-labeled EGF binding than did the nontransformed parent line (BALB-3T3) in the rapidly growing state. The data indicate that one mechanism for reduction of EGF binding in chemically transformed cells is the propensity of these cells to arrest growth in G1 at saturation density due to low-molecular-weight nutrient deficiency, a state associated with decreased EGF binding.

Amino Acids↗

PubMed1981

Decreased epidermal growth factor binding in cells growth arrested in G1 by nutrient deficiency.

Previous studies have shown that the nontransformed AKR-2B mouse embryo derived cell line may growth arrest by two separate mechanisms in the G1 phase of the cell cycle--growth factor deficiency arrest (G0) and low molecular weight nutrient deficiency arrest. An examination of epidermal growth factor (EGF) receptors under the different resting or growth conditions has shown that rapidly growing cells or cells arrested due to growth factor deficiency have the expected amount of 125I-EGF binding with approximately 10(5) receptors per cell being present in G0 arrested cells. In contrast, cells arrested due to nutrient deficiency show a reduction in 125I-EGF binding to 10-20% of that observed under the other conditions. This effect appears to be due to decreased receptor number and not to a change in the affinity of the receptor. Stimulation of DNA synthesis by nutrient replenishment causes a tenfold increase in EGF binding 20 hours later, with some increase in binding being detectable as early as six hours. The increase in binding is inhibited by cycloheximide and actinomycin D. This suggests that new mRNA synthesis as well as increased protein synthesis is required for the increase in EGF binding.

Amino Acids↗

PubMed1981

Covalent binding of benzo[a]pyrene metabolites to DNA, RNA and chromatin proteins in the AKR mouse embryo cell-line.

A specific fraction from the nuclei of the AKR mouse embryo cell-line (fraction I) displayed a much greater localization of radioactivity compared to fraction II and III when the chemical carcinogen, [3H]benzo[a]pyrene (B[a]P) was incubated with the cells for 24 h. The radioactivity in fraction I consisted of both covalently and non-covalently bound metabolites. Isolation of the DNA, RNA and protein of fraction I revealed that 94% of the covalently bound radioactivity was to protein, 5% to RNA and 1% to DNA. Analysis of the fraction I proteins by SDS gel electrophoresis revealed that there was more radioactivity covalently bound to the larger proteins than to smaller proteins. Isoelectric focusing (IEF) of the purified proteins displayed two peaks of radioactivity, one at a pH of 5 and the other at 11. The former proteins bound more radioactivity per mass of protein than the latter proteins. Analysis of fraction I histones on acid urea polyacrylamide gels showed that the radioactivity coincided with histones H3 and H2B and low levels of radioactivity associated with histones H1, H2A and H4. Two significant peaks of radioactivity closely migrated near but did not co-migrate with histone H1. The distribution of the bound radioactivity is probably a reflection of the availability of the proteins to the reactive carcinogen metabolites. The possible binding of B[a]P metabolites to phosphorylated histones and to the high mobility of group (HMG) proteins 1 and 2 is discussed.

Animals↗

PubMed1981

Binding of [3H]benzo(a)pyrene metabolites to the AKR mouse embryo cell line nuclear proteins.

Nuclear proteins from AKR-2B mouse embryo cells incubated 24 hr with [3H]benzo(a)pyrene (1 microgram/ml; 10 microCi/ml) were purified to remove nucleic acids and characterized by isoelectric focusing and sodium dodecyl sulfate gel electrophoresis on polyacrylamide gels. Analysis of the nuclear proteins by isoelectric focusing revealed a distribution of protein in the pH gradient with two prominent bands of proteins, one focusing at pH 5 and the other at pH 11, both of which bound significant amounts of the carcinogen. The acid-soluble proteins from the nuclear preparation were analyzed by isoelectric focusing, and the results also revealed two peaks of protein and radioactivity at pH's of 5 and 11. Further analysis of the acid-soluble fraction on acid urea gels revealed that significant radioactivity was associated with proteins that migrated near but slower than histone H1 which may be the high-mobility group proteins 1 and 2. Radioactivity also comigrated with histones H3 and H2B. Low levels of radioactivity were associated with histone H1 and H4.

Animals↗

PubMed1981

Transforming growth factor production by chemically transformed cells.

Evidence is presented indicating that the chemically transformed AKR-MCA and C3H/MCA-58 murine cell lines produce "transforming growth factor(s)" capable of inducing a transformed morphology and the ability to grow in soft agar in nontransformed, anchorage-dependent indicator cells. Serum-free medium conditioned by exposure to the chemically transformed cells was chromatographed on a Bio-Gel P-60 column after dialysis and lyophilization. Using the nontransformed mouse AKR-2B cells as the indicator cells, a peak of soft agar growth-stimulating activity was detected in the molecular weight range of 10,000 to 12,000. The soft agar growth-stimulating activity in pooled fractions from the AKR-MCA cells was shown to be trypsin and dithiothreitol sensitive and relatively heat stable; the activity was not destroyed by heating to 56 degrees for 30 min or to 100 degrees for 3 min. The pooled material also caused stimulation of growth in the soft agar of rat NRK cells and stimulation of DNA synthesis in the AKR-2B cells. The quantity required to give significant competition for binding to the epidermal growth factor receptor was about one order of magnitude greater than that required for stimulation of soft agar growth. Further separation of these polypeptide(s) by carboxymethylcellulose chromatography revealed three apparent peaks of soft agar growth-stimulating activity. Epidermal growth factor receptor-competing activity cochromatographed with the early minor soft agar growth-stimulating peak, whereas the two major peaks of soft agar growth-stimulating activity had no associated detectable competition for epidermal growth factor binding to its receptor. The data indicate that at least a major portion of the transforming growth factors produced by the chemically transformed cells is different from those described previously in murine sarcoma virus-transformed mouse cells and human tumor cells.

Animals↗

PubMed1981

Photodynamic effect of hematoporphyrin derivative as a function of optical spectrum and incident energy density.

Tumor cell killing effect of hematoporphyrin derivative (HPD) and light was studied in culture to determine the dependence of this effect on treatment variables. Particular attention has been given to the spectral characteristics of the light and the absorption properties of hematoporphyrin. A human tumor cell line was treated using HPD and three broad bands of light ranging from the short- to the long-wavelength end of the visible spectrum. Cell killing was assessed by trypan blue exclusion. A transformed mouse embryo cell line was treated in a similar manner, and its reproductive efficiency was determined following treatment. Results of both studies are consistent with the hypothesis that the photocytotoxic action of HPD plus light is directly proportional to the number of light quanta absorbed by the HPD in each cell. For thin layers of cells, such as in situ carcinoma, it appears that short-wavelength radiation falling in the porphyrin Soret band around 400 nm may have from 12 to 30 times the killing power as does red light.

Animals↗

PubMed1980

Variation of microsomal mixed function oxidase(s) and human lung cancer.

Animal and human studies on aryl hydrocarbon hydroxylase (AHH) have demonstrated wide inter-individual variation. First attempts to link this variation to the susceptibility to certain cancers have been successful in mice but remained inconclusive in man. In a new approach, saliva antipyrine half-lives and metabolic clearance rates have been used to assess individual rates of benzo]a]pyrene metabolism in human subjects. Saliva antipyrine half-lives and metabolic clearance rates have been measured in 57 patients with lung cancer, 90% of whom had quit smoking more than three months prior to the test, 57 cancer-free matched controls, and 59 healthy smoking controls. The mean antipyrine half-life was significantly shorter (P less than 0.001) in lung cancer patients when compared with the cancer-free matched control group, but differed little from that of the smoking group (P less than 0.05). The data support the previous observation than lung cancer patients have increased oxidation rates which, in addition to smoking, might have predisposed them to developing lung cancer.

Aged↗

PubMed1980

Growth arrest of AKR-2B cells maintained in the presence of epidermal growth factor or 12-O-tetradecanoylphorbol-13 acetate: evidence for two separate G1 arrest points.

Nontransformed mouse embryo derived AKR-2B cells stop growing in the G1 phase of the cell cycle at saturation density due to depletion of serum growth factors, whereas a chemically transformed derivative line (AKR-MCA) arrests growth in G1 at a higher saturation density due to depletion of amino acids and glucose. Stimulation of DNA synthesis is inhibited in the AKR-2B cells, but not in the AKR-MCA cells, by two inhibitors of RNA metabolism, alpha-amanitin and 5-fluorouridine (5-FU). To determine whether the AKR-MCA cells growth arrest at a unique point in G1 or whether they arrest in a physiologic state which can also be achieved by the nontransformed cells, AKR-2B cells were maintained in medium with 10% serum containing the mitogens, epidermal growth factor (EGF) or 12-O-tetradecanoylphorbol-13-acetate (TPA), until they reached saturation density or were arrested at subconfluence by artificial deletion of amino acids from the medium. The AKR-2B cells maintained in EGF or TPA stopped growing in G1 at a higher saturation density, due to depletion of amino acids. Cells arrested in EGF or TPA or in amino acid deficient medium had a shortened interval between stimulation and the onset of DNA synthesis, and the stimulation of DNA synthesis was not inhibited by alpha-amanitin or 5-FU. The data show that the nontransformed AKR-2B cells have two different arrest states which may represent two separate and distinct G1 arrest points--a growth factor deficiency arrest point and a nutrient deficiency arrest point. The nutrient deficiency arrested cells were very similar to the G1 arrested transformed AKR-MCA cells.

Amanitins↗

PubMed1979

Frequency difference between nuclear and polysomal sequences of poly(A)-containing RNA from cultured AKR mouse embryo cells.

The involvement of post-transcriptional mechanisms in the determination of the frequency distribution of messenger RNA sequences has been studied in cloned mouse embryo cells in culture. Hybridization kinetic experiments between poly(A)-containing RNA and complementary DNA have been used to study the alterations in frequencies which occur in those nuclear poly(A)-containing RNA sequences which are conserved in cytoplasmic poly-ribosomes. Sequences adjacent to nuclear poly(A) tracts are present in a much narrower frequency distribution in the nucleus than in polysomes, with a large proportion of the nuclear sequences present in an average frequency of about one molecule per cell. Few nuclear sequences appear to be present in more than ten copies per cell. A minimum of 70% of these sequences are also found in poly-ribosomal RNA but in greatly altered frequencies. Abundant sequences which comprise a major fraction of the poly(A)-containing polyribosomal RNA are derived from a small fraction of the nuclear poly(A)-adjacent RNA sequences. Very few nuclear poly(A)-adjacent sequences are present in a frequency characteristic of high abundance polysomal sequences. Conversely, poly(A)-containing polyribosomal RNA appears to contain few sequences which are present in as low a frequency as the majority of nuclear poly(A)-adjacent sequences. These observations suggest that post-transcriptional mechanisms play a major role in determining the steady-state frequency of polyribosome-associated messenger RNAs.

Animals↗