Growth-related expression of a 72,000 molecular weight poly(A)+ mRNA binding protein

In this communication, we have studied a 72,000 mol w (p72) host protein which reacts with a mouse monoclonal antibody (PAb6) directed against antigenic determinants on the Simian virus 40 (SV40) large T antigen protein that map 5' of 0.42 map units on the viral genome. The p72 protein is an abundant basic (pI greater than 7) cytoplasmic protein found in both SV40-transformed and untransformed parental cells and in cell lines derived from normal human and tumor tissue. By two-dimensional gel analysis and Western blot analysis the p72 protein identified by PAb6 is indistinguishable from the 72,000 mol w protein PABP associated with the poly(A)+ tract of cytoplasmic messenger RNA molecules. In normal human peripheral blood mononuclear cells stimulated to proliferate with the T-cell-specific mitogenic lectin phytohemagglutinin the synthesis and cytoplasmic accumulation of p72 occurs very early during the G0----G1-phase transition. The p72 protein is also expressed in proliferating and differentiated human promyelocytic HL60 cells indicating that the expression of this protein is not strictly limited to cycling cells.     

Use of a cloned library for the study of abundant poly(A)+RNA during Xenopus laevis development

Over 200 cloned sequences from recombinant DNA libraries prepared from Xenopus laevis embryonic poly(A)⁺RNA have been analyzed by colony hybridization with [³²P]cDNA prepared from poly(A)⁺RNA from several stages of development. The period of early embryogenesis extending through the beginning of gastrulation (stage 10) is marked by the relative constancy of the abundant poly(A)⁺RNA population. Between the gastrula and tailbud stages (stage 24) there is a dramatic change in the pattern of abundant poly(A)⁺RNA species; the new pattern remains fairly constant for at least 2 days of development to the late prefeeding tadpole stages (stage 41). We have also compared nonpolysomal and polysomal poly(A)⁺RNA populations at two different stages. In stage 10 (early gastrula) postribosomal (free ribonucleoprotein) and polysomal poly(A)⁺RNA populations partly overlap; however, many cloned sequences occur in quite different concentrations in one fraction or the other. Among the sequences that are predominantly nonpolysomal at gastrula few become predominantly polysomal at tailbud stages. Thus, we have no evidence for a major recruitment of abundant nonpolysomal RNAs into polysomes with progressing development. We rather observe a general pattern in which a cloned sequence that is nonpolysomal in one stage of development tends to be nonpolysomal (if detectable at all) in other stages as well.     

A study on the steady-state population of poly(A)+RNA during early development of Xenopus laevis

Autoantisera against rabbit testes and rabbit ejaculated spermatozoa have been used to study the appearance of surface autoantigens during spermatogenesis. Two distinct subclasses of autoantigens have been identified: an early subclass which first appears on pachytene spermatocytes and a late subclass which first appears on differentiating spermatids. These spermatids are just beginning to demonstrate migration of the nucleus and overlying acrosomal cap to the cell periphery and changes in nuclear shape. Some autoantigens of the early subclass do not appear on spermatozoa, but those that do are predominantly found over the acrosomal region. Autoantigens of the late subclass are predominantly found over the postacrosomal and middle-piece regions of the spermatozoon. It is suggested that morphological constraints during spermiogenesis may be responsible for the regional localization of the two subclasses.     

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.     

Mex67p, a novel factor for nuclear mRNA export, binds to both poly(A)+ RNA and nuclear pores.

An essential cellular factor for nuclear mRNA export called Mex67p which has homologous proteins in human and Caenorhabditis elegans was identified through its genetic interaction with nucleoporin Nup85p. In the thermosensitive mex67-5 mutant, poly(A)+ RNA accumulates in intranuclear foci shortly after shift to the restrictive temperature, but NLS-mediated nuclear protein import is not inhibited. In vivo, Mex67p tagged with green fluorescent protein (GFP) is found at the nuclear pores, but mutant mex67-5-GFP accumulates in the cytoplasm. Upon purification of poly(A)+ RNA derived from of UV-irradiated yeast cells, Mex67p, but not nucleoporins Nup85p and Nup57p, was crosslinked to mRNA. In a two-hybrid screen, a putative RNA-binding protein with RNP consensus motifs was found to interact with the Mex67p carboxy-terminal domain. Thus, Mex67p is likely to participate directly in the export of mRNA from the nucleus to the cytoplasm.     

Structure and expression of elongation factor 2 gene during development of Dictyostelium discoideum

A cDNA library constructed from poly(A)+ RNA isolated from Dictyostelium discoideum cells at 12 h of development was screened with the hamster elongation factor 2 (EF-2) cDNA. Several different cDNA clones which hybridized were isolated after a second screening. A cDNA clone representing the 5'-end of the mRNA was obtained by primer extension. By comparing the amino acid sequence deduced from the nucleotide sequences of these clones with that of hamster EF-2, we found enough homology between them to conclude that the isolated clones were complementary to the mRNA of D. discoideum EF-2. The N terminus which is the GTP-binding domain and the C-terminal half where it interacts with a ribosome showed a high degree of homology. The amino acid sequence of the carboxyl half includes that it contain a site of ADP-ribosylation by diphtheria toxin. From the Northern blotting analysis, the size of the mRNA was estimated to be 2.6 kilobases. The expression of the mRNA was high in vegetative cells, became maximal at the aggregation stage, and decreased thereafter through development. Upon differentiation of prespore and prestalk cells, the mRNA was highly enriched in the former over the latter. ADP-ribosylation assay of EF-2 protein by diphtheria toxin showed nearly the same developmental changes for the protein as the mRNA. However, prestalk cells were found to contain the same amount of the protein as prespore cells. The Southern blot analyses indicated that the gene encoding EF-2 is unique.     

Fluorescence characterization of the interaction of various transfer RNA species with elongation factor Tu.GTP: evidence for a new functional role for elongation factor Tu in protein biosynthesis

The ubiquity of elongation factor Tu (EF-Tu)-dependent conformational changes in amino-acyl-tRNA (aa-tRNA) and the origin of the binding energy associated with aa-tRNA.EF-Tu.GTP ternary complex formation have been examined spectroscopically. Fluorescein was attached covalently to the 4-thiouridine base at position 8 (s4U-8) in each of four elongator tRNAs (Ala, Met-m, Phe, and Val). Although the probes were chemically identical, their emission intensities in the free aa-tRNAs differed by nearly 3-fold, indicating that the dyes were in different environments and hence that the aa-tRNAs had different tertiary structures near s4U-8. Upon association with EF-Tu.GTP, the emission intensities increased by 244%, 57%, or 15% for three aa-tRNAs due to a change in tRNA conformation; the fourth aa-tRNA exhibited no fluorescence change upon binding to EF-Tu.GTP. Despite the great differences in the emission intensities of the free aa-tRNAs and in the magnitudes of their EF-Tu-dependent intensity increases, the emission intensity per aa-tRNA molecule was nearly the same (within 9% of the average) for the four aa-tRNAs when bound to EF-Tu-GTP. Thus, the binding of EF-Tu.GTP induced or selected a tRNA conformation near s4U-8 that was very similar, and possibly the same, for each aa-tRNA species. It therefore appears that EF-Tu functions, at least in part, by minimizing the conformational diversity in aa-tRNAs prior to their beginning the recognition and binding process at the single decoding site on the ribosome. Since an EF-Tu-dependent fluorescence change was also observed with fluorescein-labeled tRNA(Phe), the protein-dependent structural change is effected by direct interactions between EF-Tu and the tRNA and does not require the aminoacyl group. The Kd of the tRNA(Phe).EF-Tu.GTP ternary complex was determined, at equilibrium, to be 2.6 microM by the ability of the unacylated tRNA to compete with fluorescent Phe-tRNA for binding to the protein. Comparison of this Kd with that of the Phe-tRNA ternary complex showed that in this case the aminoacyl moiety contributed 4.3 kcal/mol toward ternary complex formation at 6 degrees C but that the bulk of the binding energy in the ternary complex was derived from direct protein-tRNA interactions.(ABSTRACT TRUNCATED AT 400 WORDS)     

5'-terminal structures of poly(A)+ cytoplasmic messenger RNA and of poly(A)+ and poly(A)- heterogeneous nuclear RNA of cells of the dipteran Drosophila melanogaster.

Structures at the 5′ terminus of poly (A)-containing cytoplasmic RNA and heterogeneous nuclear RNA containing and lacking poly(A) have been examined in RNA extracted from both normal and heat-shocked Drosophila cells. ³²P-labeled RNA was digested with ribonucleases T₂, T₁ and A and the products fractionated by a fingerprinting procedure which separates both unblocked 5′ phosphorylated termini and the blocked, methylated, “capped” termini, known to be present in the messenger RNA of most eukaryotes.Approximately 80% of the 5′-terminal structures recovered from digests of poly(A)-containing Drosophila mRNA are cap structures of the general form m⁷G^5′ppp^5′X(m)pY(m)pZp. With respect to the extent of ribose methylation and the base distribution, the 5′-terminal sequences of Drosophila capped mRNA appear to be intermediate between those of unicellular eukaryotes and those of mammals. Drosophila is the first organism known in which type 0 (no ribose methylations), type 1 (one ribose methylation), and type 2 (two ribose methylations) caps are all present. In contrast to mammalian cells, the caps of Drosophila never contain the doubly methylated nucleoside N⁶,2′-O-dimethyladenosine. Both purines and pyrimidines can be found as the penultimate nucleoside of Drosophila caps and there is a wide variety of X-Y base combinations. The relative frequencies of these different base combinations, and the extent of ribose methylation, vary with the duration of labeling. The large majority of poly(A)-containing cytoplasmic RNA molecules from heat-shocked Drosophila cells are also capped, but these caps are unusual in having almost exclusively purines as the penultimate X base.Greater than 75% of the 5′ termini of heterogeneous nuclear RNA (hnRNA) containing poly(A) and greater than 50% of the termini of hnRNA lacking poly (A) are also capped. Triphosphorylated nucleotides, common as the 5′ nucleotides of mammalian hnRNA, are rare in the poly(A)-containing hnRNA of Drosophila. The frequency of the various type 0 and type 1 cap sequences of cytoplasmic and nuclear poly (A)-containing RNA are almost identical. The caps of hnRNA lacking poly(A) are also quite similar to those of poly-adenylated hnRNA, but are somewhat lower in their content of penultimate pyrimidine nucleosides, suggesting that these two populations of molecules are not identical.     

Analysis of the membrane-associated poly(A)+RNA in the cytoplasm of dormant Artemia cysts by DNA excess hybridization. Evidence for a nuclear origin

Encysted embryos of Artemia contain latent mRNA, to a large extent associated with a fraction of cytoplasmic membranes. The membranes, purified by EDTA treatment and banding in a sucrose gradient at 1.17 g/cm3, include endoplasmic vesicles and mitochondria. The origin of the membrane-associated poly(A)+RNA was therefore investigated. In gel electrophoresis poly(A)+RNA from the purified membranes of dormant cysts forms two distinct bands at approx. 3 . 10(5) and 5 . 10(5) Da. Later during development the lighter component decreases. Nuclei from dormant cysts are devoid of poly(A)+RNA, while nuclei from developing embryos (50% emergence) contain a predominant poly(A)+RNA component of approx. 5 . 10(5) Da. 125I-labelled preparations of nuclear DNA and of nuclear and membrane-associated poly(A)+RNA were used in reassociation and hybridization experiments with excess nuclear DNA. Poly(A)+RNA from the membranes of dormant cysts hybridized to nuclear DNA to the same extent as the nuclear poly(A)+RNA from developing embryos. The hybridization of labelled, nuclear poly(A)+RNA to nuclear DNA was strongly inhibited by unlabelled membrane RNA from either dormant cysts or developing embryos. It is concluded that the stored, membrane-associated poly(A)+RNA in dormant cysts is essentially of nuclear origin. The 5 . 10(5)-Da component is largely homologous with the corresponding component of nuclear poly(A)+RNA at later stages.     

Regulation of the utilization of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells.

When Friend erythroleukemia cells were allowed to grow to stationary phase (2 X 10(6) to 3 X 10(6) cells per ml), approximately 60% of the mRNA for eucaryotic elongation factor Tu (eEF-Tu) sedimented at less than or equal to 80S, and most of the remaining factor mRNA was associated with small polysomes. Under the same growth conditions, greater than 90% of the mRNA for eucaryotic initiation factor 4A remained associated with polysomes. The association of eEF-Tu mRNA with polysomes changed dramatically when stationary-phase cells were treated with fresh medium. After 1 h in fresh medium, approximately 90% of eEF-Tu mRNA in Friend cells was found in heavy polysomes. Associated with the shift of eEF-Tu mRNA into heavy polysomes, we found at least a 2.6-fold increase in the synthesis of eEF-Tu in vivo as well as a remarkable 40% decrease in the total amount of eEF-Tu mRNA per cell. Our data raise the possibility that eEF-Tu mRNA that has accumulated in ribonucleoprotein particles in stationary-phase cells is degraded rather than reutilized for eEF-Tu synthesis.     

Biosynthesis and utilization of extensively undermethylated poly(A)+ RNA in CHO cells during a cycloleucine treatment.

The role of RNA methylations in the control of mRNA maturation and incorporation into polysomes has been investigated through a study of the effects in vivo of cycloleucine, a specific inhibitor of S-adenosyl-methionine mediated methylation. During the cycloleucine treatment, the rate of biosynthesis of hnRNA and its subsequent polyadenylation were only slightly reduced as compared with untreated cells. However a significant lag-time in the cytoplasmic appearance of poly(A)+ undermethylated molecules was observed, in parallel with a transient shift in the average size of hnRNA towards higher molecular weight. Nevertheless, the total amount of pulse-labelled poly(A)+ mRNA transferred to cytoplasm after a long chase time (3 h.) was approximately the same for both cycloleucine-treated and control cells. Extensively undermethylated poly(A)+ cytoplasmic RNAs, possessing a 5' terminal cap were incorporated into polysomes in proportions very similar to control messenger molecules. These results suggest that a normal level of methylation is not stringently required for the production of the functional mRNA molecules although it appears to be of importance for the kinetics of the maturational process.     

A study of a mutant elongation factor properties of E. coli HAK88 and its mutant elongation factor Tu.

Summary The E. coli chromosome contains two genes for elongation factor Tu, tufA (near the fusidic acid resistance marker) and tufB (near the rifampicin resistance marker). It has been discovered that the mutant E. coli K12 strain HAK88 bears a mutation in the tufB gene, which leads to the synthesis of a protein of increased acidity. To determine whether the mutation has altered the protein's function in peptide chain elongation, we have compared the reactivities of normal tufA EF-Tu and mutant tufB EF-Tu (purified together from HAK88) with the components of the AA-tRNA binding cycle. Normal tufA EF-Tu and mutant tufB EF-Tu are indistinguishable in their affinities for GDP, EF-Ts, and phe-tRNA, and differ only slightly in their affinities for ribosomes. Coupled with the results of a separate study showing the similarity of the normal tufA and tufB gene products, these experiments demonstrate that the mutation has not altered the function of tufB EF-Tu in peptide chain elongation. Contrary to the original report (Kuwano et al., 1974; J. Mol. Biol. 86, 689–698) the HAK88 strains we have examined no longer possess a temperature-sensitive EF-Ts. The growth rates of HAK88 strains resemble the parent HAK8 strain in their lack of tRNA dependence but unlike HAK8 show varying degrees of temperature sensitivity. We conclude that HAK88 contains a physically altered but functionally intact tufB EF-Tu. The mutation in tufB should be valuable for studying in vivo the control of expression of the genes for EF-Tu.     

Changes in hepatic RNA, poly(A)+RNA, and poly(A)-RNA during the acute phase response to inflammation

The steady state changes in total rat hepatic cytoplasmic RNA, poly(A)+ RNA and poly(A)-RNA were assessed in response to turpentine induced inflammation. From 18 to 24 h after injury, cytoplasmic RNA doubled, while poly(A)+ RNA peaked at 24 h, 3.5 times over control animals. Cell-free translation showed significant increases in messenger RNA levels beginning at 18 h. Gel electrophoresis of translation products revealed significant increases in several polypeptides and a decrease in others. Poly(A)-RNA from control and injured rats translated to an insignificant level and the electrophoretic gel patterns of their proteins were similar. Furthermore, no change had occurred in the 3' poly(A)-sequences during the course of inflammation.     

Isolation,expression, and evolution of the gene encoding mitochondrial elongation factor Tu in Arabidopsis thaliana

We have characterized a second nuclear gene (tufM) in Arabidopsis thaliana that encodes a eubacterial-like protein synthesis elongation factor Tu (EF-Tu). This gene does not closely resemble the previously described Arabidopsis nuclear tufA gene, which encodes the plastid EF-Tu, and does not contain sequence elements found in all cyanobacterial and plastid tufA genes. However, the predicted amino acid sequence includes an N-terminal extension which resembles an organellar targeting sequence and shares three unique sequence elements with mitochondrial EF-Tu's, from Saccharomyces cerevisiae and Homo sapiens, suggesting that this gene encodes the Arabidopsis mitochondrial EF-Tu. Consistent with this interpretation, the gene is expressed at a higher level in flowers than in leaves. Phylogenetic analysis confirms the mitochondrial character of the sequence and indicates that the human, yeast, and Arabidopsis tufM genes have undergone considerably more sequence divergence than their cytoplasmic counterparts, perhaps reflecting a cross-compartmental acceleration of gene evolution for components of the mitochondrial translation apparatus. As previously observed for tufA, the tufM gene is present in one copy in Arabidopsis but in several copies in other species of crucifers.     

Role of calcium in the localization of maternal poly(A)+RNA and tubulin mRNA in Xenopus oocytes

Summary Poly(A)⁺RNA and tubulin mRNA are localized in the periphery of Xenopus oocytes and become delocalized during meiotic maturation. Delocalization of this RNA can be triggered by incubation in agents which reduce entry of calcium ions into the cell (e.g. lanthanum chloride and verapamil). Although these agents ordinarily promote meiotic maturation, addition of theophylline to the medium will inhibit maturation but not delocalization. Manipulations which prevent calcium entry without inducing meiotic maturation (e.g. calcium-free buffer) are also shown to trigger disruption of the RNA localization. In addition, manipulations which reduce chloride efflux from the cell (e.g. increasing the external chloride ion concentration with choline chloride) result in disruption of the localization of poly (A)⁺ RNA and tubulin mRNA without inducing meiotic maturation. The calcium-dependent chloride efflux present in Xenopus oocytes disappears after the oocyte has been stimulated to proceed through meiotic maturation. We show that reduction of the influx of calcium ions or efflux of chloride ions induces the delocalization of poly (A)⁺RNA and tubulin mRNA without inducing meiotic maturation. We suggest, therefore, that reducing the transmembrane movement of these ions is likely to be the natural trigger for the delocalization of poly(A)⁺RNA and tubulin mRNA.     

Characterization of the export of bulk poly(A)+ mRNA in Saccharomyces cerevisiae during the wine-making process

Ethanol stress affects the nuclear export of mRNA similarly to heat shock in Saccharomyces cerevisiae. However, we have little information about mRNA transport in actual alcoholic fermentation. Here we characterized the transport of mRNA during wine making and found that bulk poly(A)+ mRNA accumulated in the nucleus as fermentation progressed.