Alterations of activities of ribonucleases and polyadenylate polymerase in synchronized mouse L cells

Nocturnin in the demosponge Suberites domuncula: a potential circadian clock protein controlling glycogenin synthesis in sponges

Sponges are filter feeders that consume a large amount of energy to allow a controlled filtration of water through their aquiferous canal systems. It has been shown that primmorphs, three-dimensional cell aggregates prepared from the demosponge Suberites domuncula and cultured in vitro, change their morphology depending on the light supply. Upon exposure to light, primmorphs show a faster and stronger increase in DNA, protein and glycogen content compared with primmorphs that remain in the dark. The sponge genome contains nocturnin, a light/dark-controlled clock gene, the protein of which shares a high sequence similarity with the related molecule of higher metazoans. The sponge nocturnin protein was found showing a poly(A)-specific 3'-exoribonuclease activity. In addition, the cDNA of the glycogenin gene was identified for subsequent expression studies. Antibodies against nocturnin were raised and used in parallel with the cDNA to determine the regional expression of nocturnin in intact sponge specimens; the highest expression of nocturnin was seen in the epithelial layer around the aquiferous canals. Quantitative PCR analyses revealed that primmorphs after transfer from light to dark show a 10-fold increased expression in the nocturnin gene. In contrast, the expression level of glycogenin decreases in the dark by 3-4-fold. Exposure of primmorphs to light causes a decrease in nocturnin transcripts and a concurrent increase in glycogenin transcripts. It was concluded that sponges are provided with the molecular circadian clock protein nocturnin that is highly expressed in the dark where it controls the stability of a key metabolic enzyme, glycogenin.

Brain non-adenylated mRNAs

Most eukaryotic messenger RNA (mRNA) species contain a 3'-poly(A) tract. The histone mRNAs are a notable exception although a subclass of histone-encoding mRNAs is polyadenylated. A class of mRNAs lacking a poly(A) tail would be expected to be less stable than poly(A)+ mRNAs and might, like the histones, have a half-life that varied in response to changes in the intracellular milieu. Brain mRNA exhibits an unusually high degree of sequence complexity; studies published ten years ago suggested that a large component of this complexity might be present in a poly(A)- mRNA population that was expressed postnatally. The question of the existence of a complex class of poly(A)- brain mRNAs is particularly tantalizing in light of the heterogeneity of brain cells and the possibility that the stability of these poly(A)- mRNAs might vary with changes in synaptic function, changing hormonal stimulation or with other modulations of neuronal function. The mRNA complexity analyses, although intriguing, did not prove the existence of the complex class of poly(A)- brain mRNAs. The observed mRNA complexity could have resulted from a variety of artifacts, discussed in more detail below. Several attempts have been made to clone members of this class of mRNA. This search for specific poly(A)- brain mRNAs has met with only limited success. Changes in mRNA polyadenylation state do occur in brain in response to specific physiologic stimuli; however, both the role of polyadenylation and de-adenylation in specific neuronal activities and the existence and significance of poly(A)- mRNAs in brain remain unclear.

Dramatic increase in poly(A) synthesis after infection of Molt-3 cells with HIV

Infection of Molt-3 cells with human immunodeficiency virus-1 (HIV-1) was found to cause a rapid increase in extractable poly(A) polymerase activity, while the activity of poly(A) degrading endoribonuclease IV strongly decreased at the same time. The increase in poly(A) polymerase activity seems not to be due to a change in the actual number of enzyme molecules, but rather to posttranslational enzyme modification, most likely caused by phosphorylation by nuclear protein kinase NI or protein kinase C. Both kinases were found to be able to phosphorylate poly(A) polymerase in vitro [homogeneous enzyme as well as poly(A) polymerase in intact nuclei]. Phosphoamino acid analysis revealed an incorporation of phosphate into serine and, to a lower extent, into threonine residues of the enzyme protein; no phosphotyrosine could be detected. In the nucleus, the poly(A) polymerase and the endoribonuclease IV are bound to the nuclear matrix. The phosphorylation related enhancement of nuclear poly(A) polymerase activity could be abolished by addition of the zinc and copper chelator o-phenanthroline, which inhibited zinc-containing purified poly(A) polymerase and destroyed the poly(A) polymerase containing nuclear matrix structure, resulting in a solubilization of the enzyme.

Polyadenylic acid polymerase activity in chronic myelogenous leukemia

Poly(A) polymerase activity was markedly elevated in CML in the blastic phase, moderately high in the accelerated phase and low in the chronic phase. The activity was significantly higher in the myeloid crisis than in the lymphoid crisis and elevated with increasing ratio of blasts in leukemia cases. In TPA or retinoic acid-treated leukemia cells poly(A) polymerase activity was decreased. These results suggest that poly(A) polymerase activity changes, depending on the maturation of leukemic cells and the assay of this enzyme activity may be useful for the early detection of the exacerbation of CML cases.

Poly(A) polymerase from Vigna unguiculata seedlings. A bifunctional enzyme responsible for both poly(A)-polymerizing and poly(A)-hydrolyzing activities

Poly(A)-specific ribonuclease was co-purified with poly(A) polymerase from Vigna unguiculata seedlings. Both activities were separated into two forms (enzymes I and II) by a final hydrophobic column chromatography. The enzyme I preparation, which was homogeneous as examined by SDS/PAGE, had both poly(A) polymerase and poly(A)-specific ribonuclease activities. The antibody raised to the enzyme I preparation precipitated both enzyme activities. These indicate that a single polypeptide (Mr 63,000) is responsible for both poly(A)-polymerizing and poly(A)-hydrolyzing activities. The poly(A)-specific ribonuclease was a 3'-exonuclease specific to single-stranded poly(A), forming 5'AMP as the sole reaction product. The hydrolytic activity required either Mn2+ or Mg2+ with different optimum concentrations, whereas the polymerizing activity required Mn2+ but not Mg2+. ATP and PPi had little or no effect on the poly(A)-specific ribonuclease activity.

Poly(A) metabolism and aging: a current view

Polyadenylation of mRNA is a key step in post-transcriptional control of gene expression. Therefore, age-dependent changes in poly(A) synthesis have to play a crucial role in the course of cellular aging. In this review, the importance of the signal sequence, poly(A), in determining mRNA stability and intracellular distribution of mRNA during aging is discussed.

Simultaneous isolation of cytoplasmic endoribonuclease and exoribonuclease of Trypanosoma brucei

An endoribonuclease and an exoribonuclease have been isolated simultaneously from the cytoplasm of Trypanosoma brucei by hydroxyapatite column chromatography. The endoribonuclease produced oligonucleotides from poly(adenylic acid) with 5'-phosphate and 3'-OH termini. The exoribonuclease produced only ribonucleoside 5'-phosphates from poly(adenylic acid). The relative rates of degradation of synthetic homopolynucleotides by the endoribonuclease under standard conditions were in the order poly(adenylic acid) greater than poly(uridylic acid) poly(cytidylic acid); for the exoribonuclease the order was poly(adenylic acid) poly(uridylic acid) greater than poly(cytidylic acid). Natural transfer and ribosomal RNAs were also degraded by both enzymes, while DNA was resistant to them. The optimal pH of activity for each enzyme was 7.5-8.0. Both ribonucleases require Ca2+ for maximum enzymatic activity.

Dosage compensation in mammals: why does a gene on the inactive X yield less product than one on the active X?

An expressed gene on the inactive mammalian X chromosome yields less product than the same gene on the active X. Characteristics of the inactive X which might be responsible for this are late replication, chromatin clumping, and altered patterns of DNA methylation. If an expressed gene on the inactive X is not replicated until late in S, it will be present in two copies for a shorter fraction of the cell cycle than its early replicating homologue and therefore yield less product. Alternatively, transcription may be slowed by a microenvironment of highly condensed chromatin or by an abnormal pattern of methylation of the DNA template. Experiments are proposed by which to test these and related hypotheses.

Influence of the xyloadenosine analogue of 2',5'-oligoriboadenylate on poly(A)-specific, 2',5'-oligoriboadenylate degrading 2',3'-exoribonuclease and further enzymes involved in poly(A)(+)mRNA metabolism

The homogeneous poly(A)-specific 2',3'-exoribonuclease from calf thymus gland, which cleaves both 3',5'- and 2',5'-linked oligoriboadenylates, does not degrade (xyloA2'p)2 xyloA, the xylofuranosyladenosine analogue of the 2-5A core. This oligonucleotide, which is supposed to enter intact cells rapidly, was found to possess an increased stability and an enhanced antiherpesvirus activity compared to the natural (A2'p)2A (Eppstein, D.A., Barnett, J.W., Marsh, Y.V., Gosselin, G. and Imbach, J.-L. (1983) Nature 302, 723-724). The poly(A) anabolic enzyme, poly(A) polymerase (Mn2+-dependent), from the same source, which is initiated by (A3'p)2A and its higher oligomers, does not accept 2-5A core and its xyloadenosine analogue as primer. Both oligonucleotides exert no influence on endoribonuclease IV and on the integrity of the poly(A)-ribonucleoprotein complex.

Base-specific ribonucleases potentially involved in heterogeneous nuclear RNA processing and poly(A) metabolism

Polyadenylation and splicing of heterogeneous nuclear RNA, two crucial steps in mRNA processing, are apparently enzymatically mediated processes. This contribution summarizes the properties and the presumed functions of the known poly(A) catabolic enzymes (endoribonuclease IV and V, 2',3'- exoribonuclease ) as well as those of the pyrimidine-specific endoribonucleases associated with snRNP -hnRNP complexes (endoribonuclease VII, acidic pI 4.1 endoribonuclease and poly(U)-specific U1 snRNP -nuclease).

Occurrence of short-sized oligo(A) fragments during course of cell cycle and ageing

Affinity chromatography of nucleic acids precipitated by N-cetyl-N,N,N-trimethyl ammonium bromide on poly(U)-Sepharose has proved to be a suitable method for a nearly quantitative isolation of oligo(A) sequences down to a chain length of 4 nucleotide units. Analysis of short oligo(A) fragments in synchronized L5178y mouse lymphoma cells after labeling with [3H]Ado revealed that the percentage of A2-6 sequences on the total radioactivity amounted in S-phase cells to 1.6%, while the value obtained for the stationary L-cell system was 8.0%. The alterations of occurrence and chain length distribution of short oligo(A) fragments during ageing were studied in two age groups of female quails: mature (250-320 days old) and senescent animals (3-3.5 yr old). It was found that the amount of low molecular weight oligo(A) fragments gradually decreases during ageing of the animals; the amount in the mature animal group was significantly higher (6-fold) than in the old animal group. The decreased amounts of oligo(A) during S phase and ageing could in part be due to posttranslational modification of enzymes involved in poly(A) metabolism. It could be demonstrated that both homogeneous poly(A) anabolic poly(A) polymerase and homogeneous poly(A) catabolic endoribonuclease IV are phosphorylated by nuclear protein kinase NI.

Age-dependence of polyadenylate stimulation of nuclear-envelope nucleoside triphosphatase

Nuclear envelopes of mammalian cells contain a nucleoside triphosphatase which is probably involved in mRNA transport through the nuclear membrane. The activity of the enzyme, studied in RNA-depleted nuclear ghosts, can be stimulated by poly(A) or by poly(A) (+)mRNA. Using nuclear ghost preparations from mature (8-10 months' old) and old (40-42 months' old) Wistar rats, it was shown that in "old" preparations the basal activity of the enzyme is significantly reduced (by 15%). In addition, the enzyme from old animals responds only very little to poly(A) or poly(A) (+)mRNA, compared to preparations from mature animals. Using a concentration of 6.8 X 10(11) poly(A) (+) mRNA molecules per microgram of enzyme preparation, the nucleoside triphosphatase from mature animals is stimulated by 77% and the enzyme from old animals by only 26%. Binding studies of poly(A) to pore laminae revealed that the number of binding sites in unphosphorylated preparations from old animals is significantly reduced (by 24%) compared to "mature" preparations. As a consequence of in vitro phosphorylation, no difference is observable in the number of binding sites between the two age groups. The values for half-maximal saturation binding constants for poly(A) are identical in unphosphorylated and phosphorylated pore-laminae preparations, irrespective of the age group studied. The results presented indicate that in old animals the pathway from the phosphorylated to the dephosphorylated nuclear-envelope protein which is controlled by poly(A) is impaired in the proposed cycle for mRNA efflux from nuclei.

Nonadenylylated mRNA is present as polyadenylylated RNA in nuclei of Drosophila

The sequence complexity of nuclear total RNA and nuclear poly(A)+RNA from Drosophila third-instar larvae was determined by hybridization of these RNAs to labeled single-copy DNA. At saturation, the nuclear poly(A)+ - and total RNA hybridized to 11% and 22.5% of the single-copy DNA, respectively. The increase in complexity of nuclear total RNA over that observed for nuclear poly(A)+RNA indicates the presence of a discrete class of nonoadenylylated nuclear RNA molecules. The relationship between DNA sequences coding for nuclear RNA and mRNA was then determined by hybridization of nuclear total and poly(A)+RNA to DNA enriched for mRNA coding sequences. The results of these studies show that those single-copy DNA sequences that are represented in either the poly(A)+ - or poly(A)- mRNA population are transcribed into RNA molecules that appear in the nuclear poly(A)+RNA population.

Poly(adenylate) polymerase activity of rat brain

The poly(adenylate)[poly(A)] polymerase of rat brain, as in rat liver, is located primarily in the nuclear sap when nuclei are prepared under hypertonic conditions. The enzyme can be released from nuclei in two forms. Form I is prepared by gentle incubation of nuclei at 0 degrees C in hypotonic buffer. It has a Mn optimum of 0.6 mM and a pH optimum between 8 and 9. The ATP concentration curve plateaus at 0.2 mM. The optimal poly(A) primer concentration is 600 micrograms/ml, which is three times higher than that for the enzyme similarly prepared from liver. The time course of the reaction for the form I enzyme is increasing over the first 40 min and becomes nearly linear thereafter. Form I is not stimulated by either calcium or cyclic nucleotides, but is inhibited by polyamines, pyrophosphate, and high concentrations of GTP. Form II enzyme is prepared by homogenization of nuclei in hypotonic buffer. It has the same ATP and poly(A) optima as the form I enzyme but displays linear kinetics over a 60-min time course. It is slightly stimulated by cGMP and cAMP and strongly inhibited by spermine, sodium pyrophosphate, and high concentrations of GTP.

Arabinosyl nucleosides. XXXII. Mechanism of inhibition of L5178y mouse lymphoma cells by 9-beta-D-arabinofuranosylguanine

The mechanism of the growth-inhibitory action of 9-beta-D-arabinofuranosylguanine (ara-G) on mouse lymphoma cells (L5178y) at the level of nucleic acid synthesis was studied. Ara-G inhibits the cell proliferation at an ED50 concentration of 12.3 microM. The cells die because of unbalanced growth. At cytostatic concentrations ara-G inhibits the incorporation rate of dThd into DNA to a much higher degree compared to those of the precursors into RNA or proteins. Ara-G has no effect on the induction of DNA polymerase alpha occurring at the beginning of the S-phase; this compound exerts its inhibitory activity during the S-phase. Ara-G is intracellularly phosphorylated to ara-GTP. It was demonstrated that ara-G is incorporated into DNA but not into RNA; one molecule of ara-G is incorporated per 7050 molecules of deoxyguanosine. The ara-GMP moieties, incorporated into DNA of L5178y cells are not removed during a culture period of 7.7 doubling steps; compared to the controls no reduction of the viability and growth rate was observed.

Age-associated changes in poly(adenylate) polymerase of rat liver and brain

The activity of poly(adenylate) polymerase was measured in nuclear lysates of rat brain and liver of animals ranging from 25 to 300 days. The activity was 26 nmoles AMP incorporated per h per mg of protein in liver and 8 moles per h mg in brain for 25-day-old rats, and declined to 1.0 and 3.3 for liver and brain, respectively, in 300-day-old rats. At all ages the liver showed a linear time course and had a Km for ATP of 0.5 mM. The brain enzyme gave a non-linear time course at all ages with an ATP optimum of 0.2 mM. Mixing enzyme preparations from young and old animals produced additive results. Mixing brain and liver enzyme produced greater than additive results. No substantial increase in activity was observed when enzyme was prepared from old or young partially hepatectomized rats as compared to sham-operated controls.

Age-dependent gene induction in quail oviduct. XI. Alterations on the post-transcriptional level (analytical aspect)

For the elucidation of the age-dependent reduction of avidin synthesis in the oviduct of quails studies on on the level of post-transcriptional modification of mRNA were performed. Incorporation of [3H] adenosine into mRNA is drastically reduced in senescent animals compared to adult animals. Double labeling of mRNA with [3H]-adenosine and [3H] uridine revealed that in the presence of actinomycin D the incorporation ratio adenosine/uridine into mRNA increases to higher values in the case of adult animals compared with the ratio determined for mRNA from senescent animals. These data support the assumption that in senescent animals the length of the poly(A) sequence on mRNA might be shorter. Because of the low efficiency of label incorporation into poly(A) in whole animals, a method for directly introducing a radioactive label in vitro into the isolated poly(A) was developed for the estimation of the length of the poly(A) segments. A randomly labeled product with no apparent degradation is achieved by methylation of poly(A) with [3H] dimethyl sulphate. Therefore the method is a powerful tool for the detection and size determination of small amounts of poly(A). Size determination of poly(A) from oviducts of adult and senescent quails by the methylation method revealed that in the case of senescent animals an increase in shorter poly(A) tracts can be detected. These results support the assumption that the age-dependent reduction of avidin synthesis is also caused by changes in the control mechanisms on the post-transcriptional level.

Age-dependent gene induction in quail oviduct X. Alterations on the post-transcriptional level (enzymic aspect)

In quail oviducts the rate of synthesis of avidin, the biological end-point marker for the molecular events caused by progesterone, decreases with age. The cause of the reduced capacity of avidin induction has been studied on the molecular biological level polyadenylation of RNA which is one step in the process of post-transcriptional modification of heterogeneous nuclear RNA resulting in the formation of functional mRNA molecules. This novel approach was biochemically possible after the discovery of the poly(A) anabolic enzyme (poly(A) polymerase) and the two poly(A) catabolic enzymes (endoribonuclease IV and 5'-exoribonuclease). These enzymes are involved in the synthesis and degradation of the poly(A) segment of mRNA in vitro and most likely also in poly(A) metabolism in intact cell systems. Enzymatically controlled poly(A) metabolism of mRNA is regulated by the following interrelations: poly(A)-associated proteins and endoribonuclease IV; labilizing factor and poly(A)-associated proteins; 5'-exoribonuclease in cooperation with endoribonuclease IV and poly(A) polymerase. A close correlation between high levels of poly(A) catabolic enzymes and low rate of protein synthesis which was established in cell culture systems, seems also to be partially the biochemical cause for the reduced avidin synthesis in aging quail oviduct.

A microsomal endoribonuclease from rat liver

An endoribonuclease has been purified about 320-fold from the microsomes of rat liver. The enzyme had an apparent molecular weight of 54 000-58 000 and produced oligonucleotides, each consisting of 3-7 nucleotides from poly(A) and poly(U). No mononucleotide was obtained by the enzymatic hydrolysis of poly(A) and poly(U) under standard coditions. The relative rates of breakdown of synthetic polynucleotides by the enzyme under standard conditions were in the order poly(U) = poly(A) > poly(C). Divalent cations (Mg2+ or Mn2+) was required for the enzymatic activity, but monovalent cations (Na+, K+ or NH4+) inhibited the enzyme. The breakdown of poly(C) and poly(U) by the enzyme was inhibited by spermine, but that of poly(A) was not influenced by spermine. The enzyme was inhibited by p-chloromercuribenzoate and poly(G), but not by rat-liver ribonuclease-inhibitor and anti-RNase A serum.

Ribonuclease activity during G1 arrest of the yeast Saccharomyces cerevisiae

When cells of the yeast, Saccharomyces cerevisiae, were deprived of nitrogen, a condition leading to G1 arrest, there was an immediate increase in the levels of total ribonuclease (RNase) activity within these cells. During starvation, only the cells arrested in G1 showed increased RNase activity. Although the RNase activities of extracts of starved and actively growing cells were similarly influenced by pH, the activities of starved cells were less stable on both storage and heating. Differences were also noted in substrate specificity. The results of this study suggest that arrest within G1 may increase RNase activity. However, all RNases did not appear to be influenced equally, since the total pool of RNase activity from log phase and G1 arrested cells showed differences in stability and substrate specificity.

Two distinct poly(A) polymerases isolated from the cytoplasm of Ehrlich ascites tumour cells

The poly(A) polymerases from the cytosol and ribosomal fractions of Ehrlich ascites tumour cells are isolated and partially purified by DEAE-cellulose and phosphocellulose column chromatography. Two distinct enzymes are identified: (a) a cytosol Mn2+-dependent poly(A) polymerase (ATP:RNA adenylyltransferase) and (b) a ribosome-associated enzyme defined tentatively as ATP(UTP): RNA nucleotidyltransferase. The cytosol poly(A) polymerase is strictly Mn2+-dependent (optimum at 1 mM Mn2+) and uses only ATP as substrate, poly(A) is a better primer than ribosomal RNA. The purified enzyme is free of poly(A) hydrolase activity, but degradation of [3H]poly(A) takes place in the presence of inorganic pyrophosphate. Most likely this enzyme is of nuclear origin. The ribosomal enzyme is associated with the ribosomes but it is found also in free state in the cytosol. The purified enzyme uses both ATP and UTP as substrates. The substrate specificity varies depending on ionic conditions: the optimal enzyme activity with ATP as substrate is at 1 mM Mn2+, while that with UTP as substrate is at 10--20 mM Mg2+. The enzymes uses both ribosomal RNA and poly(A) [but not poly(U)] as primers. The purified enzyme is free of poly(A) hydrolase activity.

Age-dependent gene induction in quail oviduct. VII. Alteration of DNA polymerase activities in response to progesterone treatment

The changes of DNA synthesis as well as the alterations of DNA polymerase alpha and beta have been determined in oviducts from immature (35 days old), mature (about 300 days old) and senescent (about 3 years old) quails in response to diethylstilbestrol (DES). The DNA synthesis in oviducts from immature quails is strongly stimulated by DES; after 12 days, values equivalent to those determined in mature and senescent animals are reached. The DNA synthesis in immature oviducts can be reversibly blocked by progesterone administration; no influence is observed in the case of DNA synthesis in mature as well as senescent animals. During DNA synthesis in immature oviducts, stimulated with DES, DNA polymerase alpha is strongly induced. Co-administration of progesterone blocks this enzyme induction in a reversible and strong way. The activity of DNA polymerase alpha is identical in mature and in immature animals (after DES treatment for 15 days); the activity of the same enzyme in senescent animals is about 40% lower than the values found in the younger quails. The activity of DNA polymerase beta is not altered if the animals are treated with DES or with DES and progersterone; however, the basic level of the enzyme in senescent animals in 50% lower than in immature or in mature animals.