Effects of cordycepin on RNA metabolism in germinating seedlings

Histone H3 transcript stability in alfalfa

The stability of histone H3 transcripts in alfalfa for replication-dependent and -independent gene variants was measured by northern analysis under conditions of inhibition of transcription and/or translation. Replication-dependent histone H3.1 transcripts were about three-fold less stable than the equally polyadenylated mRNA for replacement variant H3.2 histone. In actively growing suspension cultures treated with dactinomycin half-lives of 2 and 7 h were observed for H3.1 and H3.2 mRNAs, respectively. mRNA stabilities were also measured indirectly by histone protein synthesis. The translation inhibitor cycloheximide strongly increased mRNA levels for both histone H3 variants. The dependence of histone mRNA turnover on translation in animals also appears to exist in plants. The combination of inhibition of transcription and translation by dactinomycin and cycloheximide was used in an indirect assessment of H3 mRNA stability throughout the cell cycle in partially synchronized and cycle-arrested cultures. Destabilization of replication-dependent histone H3.1 mRNA was detected in non-S phase cells.

Reduced steady-state levels of rbcS mRNA in plants kept in the dark are due to differential degradation

When plants are placed in the dark, the level of the abundant mRNA encoding the small subunit of ribulose-1,5-bisphosphate carboxylase (rbcS) declines rapidly. We present evidence demonstrating an active degradation of rbcS mRNA in the dark. Detailed analysis shows that transcripts originating from different members of the rbcS gene family are differentially affected by this degradation. This phenomenon is not common to all light-regulated plant genes since the mRNA for ST-LS1, another leaf-specific and light-induced gene, is not degraded in the dark within the same time scale.

Role of protein and RNA synthesis in the initiation of auxin-mediated growth in coleoptiles of Zea mays L

The kinetics of inhibition by protein- and RNA-synthesis inhibitors (cycloheximide and cordycepin, respectively) of indole-3-acetic acid (IAA)-induced elongation growth were investigated using abraded coleoptile segments of Zea mays L. Removal of the cuticle - a diffusion barrier for solutes - by mechanical abrasion of the outer epidermal cell wall increased the effectiveness of inhibitors tremendously. In an attempt to elucidate the role of 'growth-limiting protein(s)' (GLP) in the growth mechanism the following results were obtained. The elongation induced by IAA was completely inhibited when cycloheximide (10 μmol·l(-1)) was applied to abraded coleoptile segments as shortly as 10 min before the onset of the growth response (=5 min after administration of IAA). However, when cycloheximide was applied after 60 min of IAA treatment (when a steady-state growth rate is reached), the time required for complete cessation of growth was much longer (about 40 min). Cycloheximide inhibited the incorporation of [(3)H]leucine into protein within about 5 min. Cordycepin (400 μmol·l(-1)) prevented IAA-induced growth when applied as shortly as 25 min before the onset of the growth response (=10 min before administration of IAA) but required more than 60 min for a full inhibition of steady-state growth. The incorporation of [(3)H]adenosine into RNA was inhibited by cordycepin within 10 min. It is concluded that, contrary to previous investigations with nonabraded organ segments, the initiation of growth by IAA depends directly on the synthesis of GLP. Moreover, the apparent lifetime of GLP is at least four times longer than the time required by cycloheximide to inhibit the initiation of growth by IAA. This is interpreted to mean that GLP is not present before IAA starts to act but is synthesized as a consequence of IAA action starting a few minutes before the initiation of growth. Interpreting the kinetics of growth inhibition by cordycepin in a similar way, we further conclude that GLP synthesis is mediated by IAA-induced synthesis of the corresponding mRNA which starts about 10 min before the onset of GLP synthesis. Inhibition by cycloheximide and cordycepin of IAA-induced growth cannot be alleviated by acidifying the cell wall to pH 4-5, indicating that these inhibitors do not act on growth via an inhibition of auxin-mediated proton excretion.

Transient gene expression in tobacco protoplasts: I. Time course of CAT appearance

The early events of transient gene expression have been investigated monitoring CAT activity in tobacco protoplasts encoded by the recombinant plasmid pRT101cat. The first appearance of CAT activity was observed within 30 minutes after the outset of cultivation, and maximal values were obtained between four and 24 hours. CAT expression, at the level of RNA synthesis, could not be inhibited by cordycepin (3'deoxyadenine) added one hour after protoplast plating, whereas cycloheximide, an inhibitor of protein synthesis, showed an influence during the first four hours. This indicates a rapid decay of biologically active forms of both the DNA transferred and the CAT-mRNA synthesized within the first hours. These results suggest that in the tobacco protoplast system CAT protein stability lasts up to two weeks rather than a continuous synthesis of new enzyme.

Synthesis of the peptide-transport carrier of barley scutellum during the early stages of germination

Development of peptide-transport activity in the scutella of isolated barley (Hordeum vulgare l. cv. Maris Otter, Winter) embryos is shown to increase rapidly after about 15 h of imbibition, with the bulk of the transport activity being expressed by 24 h. This development is prevented by treatment of 15 h embryos with cycloheximide. Protein synthesis is found to increase in a closely related manner and also to be abolished by cycloheximide. Measurement of the incorporation of bound [(35)S]methionine by 15 to 21-h embryos indicates that de-novo protein synthesis during this period is greater in the scutellum than in the embryonic axis. Previous studies have shown that the peptide-transport system possesses essential dithiol groups, probably located at the substrate-binding site (Walker-Smith and Payne 1983 b, 1984b). Treatment of 15-h embryos with the non-penetrant thiol reagent p-chloromercuribenzene sulphonic acid did not affect development of peptide-transport activity during the following 6 h, whereas with 3-d embryos identical treatment inhibited uptake almost completely during a subsequent 6-h period. Radioautography revealed that amongst the proteins synthesised during this early phase of germination and labelled in vitro with [(35)S]methionine some are found within the epithelial plasmalemmae of the scutellum, which is the location of the peptide-transport carrier identified previously by externally labelling with a radioactive thiol reagent. The results provide evidence that protein(s) of the peptide-transport system are synthesised and inserted into the scutellum during early germination, allowing the system to play a major role in the nitrogen nutrition of the embryo.

Polysome assembly and RNA synthesis during phytochrome-mediated photomorphogenesis in mustard cotyledons

The effect of phytochrome (high irradiance reaction, elicited by continuous far-red light) on cellular polysome levels was investigated using ribosome-isolation procedures which prevent the methodological artifacts inherent in previous studies on polysomes. By including the large pool of ribosomal subunits in the analysis and using the ratio (polysomes: monomers + subunits) as a quantitative estimate of the translational capacity of the ribosomes in mustard (Sinapis alba L.) cotyledons, we found the following results: 1) After a lag-phase of less than 30 min, phytochrome induces a massive increase in the relative amount of cytosolic (free) polysomes at the expense of ribosomal subunits. 2) Cytosolic and membrane-bound polysomes are increased by phytochrome in constant proportions (constant ratio of 65:35 in light and darkness). 3) Simultaneously with the light-mediated increase of the polysome level there is an increased incorporation of newly synthesized (labeled) non-ribosomal RNA, presumably mRNA, into the polysomes which can be kinetically discriminated from the slower incorporation of newly synthesized (labeled) rRNA. 4) Cordycepin strongly inhibits the synthesis of RNA and completely prevents the light-mediated increase of polysomes. 5) The electrophoretic patterns of the in-vitro translation products obtained with polysomal polyadenylated RNA from dark-grown and light-grown cotyledons showed no significant qualitative differences. We conclude from these results that photomorphogenesis of mustard cotyledons is related to a massive increase of newly synthesized mRNA leading to a correspondingly increased recruitment of ribosomal subunits into polysomes. The phytochrome-induced increase of translatable mRNA involves mainly quantitative changes in the production of mRNA species which are also present in the dark-grown cotyledons.

Stability of polyadenylic and polyadenylated ribonucleic acids in radish (Raphanus sativus) seedlings

The stability of polyadenylic acid and polyadenylated RNA was investigated in young radish (Raphanus sativus) seedlings. We first studied the decay of poly(A) content, using a [3H]poly(U) assay, following a complete block of transcription by cordycepin (200 microgram/ml). Two lifetime classes of polyadenylic acid have been determined in these seedlings: a short-lived component with a half-life of 30 min which represents 60% of poly(A) and a more stable component with varying half-lives of which the majority range from 4-10 h and a few are considerably longer. During this period rRNA was shown to decay linearly, taking about 41 h for half of this RNA to disappear. The life-time of the other moiety of polyadenylated-RNA was analysed by continuous labelling with [3H]uridine. We have been able to demonstrate that a significant part of the mRNA molecules turns over with a half-life similar to that of the more slowly turning-over poly(A). No evidence could be obtained for rapidly turning-over messenger RNA. Thus the rapidly turning over poly(A) could correspond to a poly(A) turn-over independent of the remainder of the sequence. When labelling was very long, an apparent steady-state was reached and we determined the polyadenylated RNA content of seedlings to be 2.2% of whole cell RNA. Finally, these results were compared with those previously obtained in studying early germination of radish embryo axes. In contrast with stored mRNA which is rapidly degraded following imbibition, part of the mRNA present in 22 h old seedlings is stable for several hours.

L-Phenylalanine ammonia-lyase and pisatin induction by 5-bromodeoxyuridine in Pisum sativum

The substitution of the base analogue 5-bromodeoxyuridine (BrdU) for thymidine in the DNA of pea pods (Pisum sativum) induces or enhances the level of phenylalanine ammonia-lyase (PAL) and also induces the phytoalexin, pisatin, a product of the same metabolic pathway. Cordycepin, a polyadenylate inhibitor at the RNA level, is a potent inhibitor of pisatin synthesis. Kinetic studies on the inhibition of the PAL-pisatin production by hydroxyurea indicate that BrdU must be incorporated into DNA before any induction takes place. 5-Iododeoxyuridine is also an inducer while 5-fluorodeoxyuridine is ineffective when applied alone. BrdU is incorporated into the DNA of pea cells and the nuclei undergoes condensation just prior to the detection of the induced responses.

RNA metabolism and membrane-bound polysomes in relation to globulin biosynthesis in cotyledons of developing field beans (Vicia faba L.)

In cotyledon cells of developing field beans the RNA content per cell does not change in the second half of developmental period 2, whereas globulin biosynthesis continues. The constant RNA content per cell results from an equilibrium between RNA synthesis and degradation. All types of RNA are synthesized until the end of globulin biosynthesis, but poly(A)-containing RNA was preferentially labelled during maximum globulin formation. During stage 2 of seed development of poly(A)-containing RNA fraction represents a discrete peak in the 12--18-S region on agarose gels and corresponds to the peak of poly(A)-containing RNA isolated from polysomes. alpha-Amanitin inhibits selectively the labelling of poly(A)-containing RNA and concomitantly globulin formation. Translation of total poly(A)-containing RNA, free and membrane-bound polysomes in a cell-free wheat germs demonstrates that the globulins are preferentially produced on membrane-bound polysomes and that poly(A)-containing RNA includes the mRNA for both vicilin and legumin.

Preformed and newly synthesized messenger RNA in germinating wheat embryos

In 6 h germinated wheat (Triticum aestivum L. cv. Cama) embryos, more than half of the messenger RNAs are actively involved in translation. Neither preformed nor newly synthesized poly A(+)-RNA is translated preferentially. Germination in the presence of cordycepin showed that the half-life of the templates is about 2 h and that the newly synthesized messengers are essential to support protein synthesis in the embryo from the first hours of germination. Most of the messenger RNAs in 6 h germinated embryos are newly synthesized. The polypeptides coded for by either the endogenous messenger ribonucleoproteins or purified poly A(+)-RNA from both dry and germinated embryos are qualitatively identical; minor quantitative differences can however be observed.

Effect of the protein-synthesis-initiation inhibitor 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide on ribonucleic acid synthesis in radish seedlings

2-(4-Methyl-2,6-dinitroanilino)-N-methylpropionamide, an inhibitor of initiation of protein synthesis in plants, selectively alters rRNA synthesis. Ribosomal genes are transcribed in the absence of protein synthesis, but processing of the transcripts is impaired. This results in an accumulation of the 2.3 X 10(6)-dalton pre-rRNA. Synthesis of polyadenylated RNA is not affected by the drug.

Effect of the protein synthesis inhibitor cycloheximide on RNA synthesis in radish seedlings

The effect of cycloheximide on rRNA and poly (A) (+) RNA synthesis is studied in radish seedlings. When used at a concentration of 50 microng/ml cycloheximide selectively blocks rRNA synthesis without altering poly (A) (+) RNA synthesis. Processing of the 2.3 X 10(6) dalton pre-rRNA is severely impaired. This observation should help in studies of mRNA metabolism in plants. When the concentration of cycloheximide is lowered, it is possible to completely block protein synthesis without preventing RNA synthesis. This implies that the effect of cycloheximide on RNA synthesis is probably more complex than usually assumed.

Disappearance of stored polyadenylic acid and mRNA during early germination of radish (Raphanus sativus L.) embryo axes

Polyadenylic acid [poly (A)] is detected, characterized and quantitated in dry radish embryo axis RNA using a (3)H poly (U) probe. The amount of poly (A) gradually decreases after the onset of soaking, and, after a few hours, recovers to the initial level. This variation is shown to result from the addition of two opposed phenomena: the decay of stored poly (A) and the accumulation of newly synthesized poly (A). Stored poly (A), as well as the "in vivo" protein synthesis coded for by preformed mRNA, decreases during early germination with a half-life of two hours. As a whole, these results demonstrate that at least a fraction of the stored mRNA is translated as soon as the seed is soaked and that its role is rapidly taken over by newly-made mRNA.

Occurence of nucleoside polyphosphates in rapidly labelled RNA preparations from radish seedlings (Raphanus sativus.)

Preparations of rapidly labelled RNA are usually heavily contaminated by rapidly labelled low molecular weight components. This contaminating fraction mainly contains nucleoside triphosphates, as previously reported, but also nucleoside tetra, penta and hexaphosphates. These highly phosphorylated nucleosides are, according to recent literature, likely candidates for an important regulatory role in the coordination of a variety of biochemical reactions. This is the first demonstration of their occurence in higher plants and a method is presented to separate them from other usual nucleotides.