mRNA methylation and protein synthesis in extracts from embryos of brine shrimp, Artemia salina

Essential roles of RNA cap-proximal ribose methylation in mammalian embryonic development and fertility

Eukaryotic RNA pol II transcripts are capped at the 5' end by the methylated guanosine (m⁷G) moiety. In higher eukaryotes, CMTR1 and CMTR2 catalyze cap-proximal ribose methylations on the first (cap1) and second (cap2) nucleotides, respectively. These modifications mark RNAs as "self," blocking the activation of the innate immune response pathway. Here, we show that loss of mouse Cmtr1 or Cmtr2 leads to embryonic lethality, with non-overlapping sets of transcripts being misregulated, but without activation of the interferon pathway. In contrast, Cmtr1 mutant adult mouse livers exhibit chronic activation of the interferon pathway, with multiple interferon-stimulated genes being expressed. Conditional deletion of Cmtr1 in the germline leads to infertility, while global translation is unaffected in the Cmtr1 mutant mouse liver and human cells. Thus, mammalian cap1 and cap2 modifications have essential roles in gene regulation beyond their role in helping cellular transcripts to evade the innate immune system.

The Mammalian Cap-Specific m6Am RNA Methyltransferase PCIF1 Regulates Transcript Levels in Mouse Tissues

The 5' end of eukaryotic mRNAs is protected by the m⁷G-cap structure. The transcription start site nucleotide is ribose methylated (Nm) in many eukaryotes, whereas an adenosine at this position is further methylated at the N⁶ position (m⁶A) by the mammalian Phosphorylated C-terminal domain (CTD)-interacting Factor 1 (PCIF1) to generate m⁶Am. Here, we show that although the loss of cap-specific m⁶Am in mice does not affect viability or fertility, the Pcif1 mutants display reduced body weight. Transcriptome analyses of mutant mouse tissues support a role for the cap-specific m⁶Am modification in stabilizing transcripts. In contrast, the Drosophila Pcif1 is catalytically dead, but like its mammalian counterpart, it retains the ability to associate with the Ser5-phosphorylated CTD of RNA polymerase II (RNA Pol II). Finally, we show that the Trypanosoma Pcif1 is an m⁶Am methylase that contributes to the N⁶,N⁶,2'-O-trimethyladenosine (m⁶₂Am) in the hypermethylated cap4 structure of trypanosomatids. Thus, PCIF1 has evolved to function in catalytic and non-catalytic roles.

Influenza Virus RNA-Dependent RNA Polymerase and the Host Transcriptional Apparatus

Influenza virus RNA-dependent RNA polymerase (FluPol) transcribes the viral RNA genome in the infected cell nucleus. In the 1970s, researchers showed that viral transcription depends on host RNA polymerase II (RNAP II) activity and subsequently that FluPol snatches capped oligomers from nascent RNAP II transcripts to prime its own transcription. Exactly how this occurs remains elusive. Here, we review recent advances in the mechanistic understanding of FluPol transcription and early events in RNAP II transcription that are relevant to cap-snatching. We describe the known direct interactions between FluPol and the RNAP II C-terminal domain and summarize the transcription-related host factors that have been found to interact with FluPol. We also discuss open questions regarding how FluPol may be targeted to actively transcribing RNAP II and the exact context and timing of cap-snatching, which is presumed to occur after cap completion but before the cap is sequestered by the nuclear cap-binding complex.

Roles for Drosophila cap1 2'-O-ribose methyltransferase in the small RNA silencing pathway associated with Argonaute 2

Cap1 2'-O-ribose methyltransferase (CMTR1) modifies RNA transcripts containing the 7-methylguanosine cap via 2'-O-ribose methylation of the first transcribed nucleotide, yielding cap1 structures. However, the role of CMTR1 in small RNA-mediated gene silencing remains unknown. Here, we identified and characterized a Drosophila CMTR1 gene (dCMTR1) mutation. We found that the catalytic activity of dCMTR1 was involved in the biogenesis of small interfering RNAs (siRNAs) but not microRNAs. Additionally, dCMTR1 interacted with R2D2, a key component for the assembly of the RNA-induced silencing complex (RISC) containing Argonaute 2 (Ago2). Consistent with this finding, loss of dCMTR1 function impaired RISC assembly by inhibiting the unwinding of Ago2-bound siRNA duplexes, thus preventing the retention of the guide strand. Moreover, dCMTR1 is unlikely to modify siRNAs during RISC assembly. Collectively, our data indicate that dCMTR1 is a positive regulator of the small RNA pathway associated with Ago2 with roles in both siRNA biogenesis and RISC assembly.

Above the Epitranscriptome: RNA Modifications and Stem Cell Identity

Sequence databases and transcriptome-wide mapping have revealed different reversible and dynamic chemical modifications of the nitrogen bases of RNA molecules. Modifications occur in coding RNAs and noncoding-RNAs post-transcriptionally and they can influence the RNA structure, metabolism, and function. The result is the expansion of the variety of the transcriptome. In fact, depending on the type of modification, RNA molecules enter into a specific program exerting the role of the player or/and the target in biological and pathological processes. Many research groups are exploring the role of RNA modifications (alias epitranscriptome) in cell proliferation, survival, and in more specialized activities. More recently, the role of RNA modifications has been also explored in stem cell biology. Our understanding in this context is still in its infancy. Available evidence addresses the role of RNA modifications in self-renewal, commitment, and differentiation processes of stem cells. In this review, we will focus on five epitranscriptomic marks: N6-methyladenosine, N1-methyladenosine, 5-methylcytosine, Pseudouridine (Ψ) and Adenosine-to-Inosine editing. We will provide insights into the function and the distribution of these chemical modifications in coding RNAs and noncoding-RNAs. Mainly, we will emphasize the role of epitranscriptomic mechanisms in the biology of naïve, primed, embryonic, adult, and cancer stem cells.

5' End Nicotinamide Adenine Dinucleotide Cap in Human Cells Promotes RNA Decay through DXO-Mediated deNADding

Eukaryotic mRNAs generally possess a 5' end N7 methyl guanosine (m⁷G) cap that promotes their translation and stability. However, mammalian mRNAs can also carry a 5' end nicotinamide adenine dinucleotide (NAD⁺) cap that, in contrast to the m⁷G cap, does not support translation but instead promotes mRNA decay. The mammalian and fungal noncanonical DXO/Rai1 decapping enzymes efficiently remove NAD⁺ caps, and cocrystal structures of DXO/Rai1 with 3'-NADP⁺ illuminate the molecular mechanism for how the "deNADding" reaction produces NAD⁺ and 5' phosphate RNA. Removal of DXO from cells increases NAD⁺-capped mRNA levels and enables detection of NAD⁺-capped intronic small nucleolar RNAs (snoRNAs), suggesting NAD⁺ caps can be added to 5'-processed termini. Our findings establish NAD⁺ as an alternative mammalian RNA cap and DXO as a deNADding enzyme modulating cellular levels of NAD⁺-capped RNAs. Collectively, these data reveal that mammalian RNAs can harbor a 5' end modification distinct from the classical m⁷G cap that promotes rather than inhibits RNA decay.

Traversing the RNA world

An invitation to write a "Reflections" type of article creates a certain ambivalence: it is a great honor, but it also infers the end of your professional career. Before you vanish for good, your colleagues look forward to an interesting but entertaining account of the ups-and-downs of your past research and your views on science in general, peppered with indiscrete anecdotes about your former competitors and collaborators. What follows will disappoint those who await complaint and criticism, for example, about the difficulties of doing research in the 1960s and 1970s in Eastern Europe, or those seeking very personal revelations. My scientific life has in fact seen many happy coincidences, much good fortune, and several lucky escapes from situations that at the time were quite scary. I have also been fortunate with regard to competitors and collaborators, particularly because, whenever possible, I tried to "neutralize" my rivals by collaborating with them - to the benefit of all. I recommend this strategy to young researchers to dispel the nightmares that can occur when competing against powerful contenders. I have been blessed with the selection of my research topic: RNA biology. Over the last five decades, new and unexpected RNA-related phenomena emerged almost yearly. I experienced them very personally while studying transcription, translation, RNA splicing, ribosome biogenesis, and more recently, different classes of regulatory non-coding RNAs, including microRNAs. Some selected research and para-research stories, also covering many wonderful people I had a privilege to work with, are summarized below.

Structural analysis of human 2'-O-ribose methyltransferases involved in mRNA cap structure formation

The 5′ cap of human messenger RNA contains 2′-O-methylation of the first and often second transcribed nucleotide that is important for its processing, translation and stability. Human enzymes that methylate these nucleotides, termed CMTr1 and CMTr2, respectively, have recently been identified. However, the structures of these enzymes and their mechanisms of action remain unknown. In the present study, we solve the crystal structures of the active CMTr1 catalytic domain in complex with a methyl group donor and a capped oligoribonucleotide, thereby revealing the mechanism of specific recognition of capped RNA. This mechanism differs significantly from viral enzymes, thus providing a framework for their specific targeting. Based on the crystal structure of CMTr1, a comparative model of the CMTr2 catalytic domain is generated. This model, together with mutational analysis, leads to the identification of residues involved in RNA and methyl group donor binding.

Human mRNA transcripts possess a 5' cap structure that is modified by methylation. Here, Smietanski et al. present the structures of human methyltransferases responsible for this reaction, revealing key differences to their viral counterparts and thereby providing a framework for targeted drug design.

RNA methyltransferases involved in 5' cap biosynthesis

In eukaryotes and viruses that infect them, the 5′ end of mRNA molecules, and also many other functionally important RNAs, are modified to form a so-called cap structure that is important for interactions of these RNAs with many nuclear and cytoplasmic proteins. The RNA cap has multiple roles in gene expression, including enhancement of RNA stability, splicing, nucleocytoplasmic transport, and translation initiation. Apart from guanosine addition to the 5′ end in the most typical cap structure common to transcripts produced by RNA polymerase II (in particular mRNA), essentially all cap modifications are due to methylation. The complexity of the cap structure and its formation can range from just a single methylation of the unprocessed 5′ end of the primary transcript, as in mammalian U6 and 7SK, mouse B2, and plant U3 RNAs, to an elaborate m⁷Gpppm^6,6AmpAmpCmpm³Um structure at the 5′ end of processed RNA in trypanosomes, which are formed by as many as 8 methylation reactions. While all enzymes responsible for methylation of the cap structure characterized to date were found to belong to the same evolutionarily related and structurally similar Rossmann Fold Methyltransferase superfamily, that uses the same methyl group donor, S-adenosylmethionine; the enzymes also exhibit interesting differences that are responsible for their distinct functions. This review focuses on the evolutionary classification of enzymes responsible for cap methylation in RNA, with a focus on the sequence relationships and structural similarities and dissimilarities that provide the basis for understanding the mechanism of biosynthesis of different caps in cellular and viral RNAs. Particular attention is paid to the similarities and differences between methyltransferases from human cells and from human pathogens that may be helpful in the development of antiviral and antiparasitic drugs.

Posttranscriptional control over rapid development and ciliogenesis in Marsilea

Marsilea vestita is a semiaquatic fern that produces its spores (meiotic products) as it undergoes a process of natural desiccation. During the period of desiccation, the spores mature, and produce large quantities of pre-mRNA, which is partially processed and stored in nuclear speckles and can remain stable during a period of extended quiescence in the dry spore. Rehydration of the spores initiates a highly coordinated developmental program, featuring nine successive mitotic division cycles that occur at precise times and in precise planes within the spore wall to produce 39 cells, 32 of which are spermatids. The spermatids then undergo de novo basal body formation, the assembly of a massive cytoskeleton, nuclear and cell elongation, and finally ciliogenesis, before being released from the spore wall. The entire developmental program requires only 11 h to reach completion, and is synchronous in a population of spores rehydrated at the same time. Rapid development in this endosporic gametophyte is controlled posttranscriptionally, where stored pre-mRNAs, many of which are intron-retaining transcripts, are unmasked, processed, and translated under tight spatial and temporal control. Here, we describe posttranscriptional mechanisms that exert temporal and spatial control over this developmental program, which culminates in the production of ∼140 ciliary axonemes in each spermatozoid.

Removal of retained introns regulates translation in the rapidly developing gametophyte of Marsilea vestita

The utilization of stored RNA is a driving force in rapid development. Here, we show that retention and subsequent removal of introns from pre-mRNAs regulate temporal patterns of translation during rapid and posttranscriptionally controlled spermatogenesis of the fern Marsilea vestita. Analysis of RNAseq-derived transcriptomes revealed a large subset of intron-retaining transcripts (IRTs) that encode proteins essential for gamete development. Genomic and IRT sequence comparisons show that other introns have been previously removed from the IRT pre-mRNAs. Fully spliced isoforms appear at distinct times during development in a spliceosome-dependent and transcription-independent manner. RNA interference knockdowns of 17/17 IRTs produced anomalies after the time points when those transcripts would normally be spliced. Intron retention is a functional mechanism for forestalling precocious translation of transcripts in the male gametophyte of M. vestita. These results have broad implications for plant gene regulation, where intron retention is widespread.

Extremes in rapid cellular morphogenesis: post-transcriptional regulation of spermatogenesis in Marsilea vestita

The endosporic male gametophyte of the water fern, Marsilea vestita, provides a unique opportunity to study the mechanisms that control cell fate determination during a burst of rapid development. In this review, we show how the spatial and temporal control of development in this simple gametophyte involves several distinct modes of RNA processing that allow the translation of specific mRNAs at distinct stages during gametogenesis. During the early part of development, nine successive cell division cycles occur in precise planes within a closed volume to produce seven sterile cells and 32 spermatids. There is no cell movement in the gametophyte; so, cell position and size within the spore wall define cell fate. After the division cycles have been completed, the spermatids become sites for the de novo formation of basal bodies, for the assembly of a complex cytoskeleton, for nuclear and cell elongation, and for ciliogenesis. In contrast, the adjacent sterile cells exhibit none of these changes. The spermatids differentiate into multiciliated, corkscrew-shaped gametes that resemble no other cells in the entire plant. Development is controlled post-transcriptionally. The transcripts stored in the microspore are released (unmasked) in the gametophyte at different times during development. At the start of these studies, we identified several key mRNAs that undergo translation at specific stages of gametophyte development. We developed RNA silencing protocols that enabled us to block the translation of these proteins and thereby establish their necessity and sufficiency for the completion of specific stages of gametogenesis. In addition, RNAi enabled us to identify additional proteins that are essential for other phases of development. Since the distributions of mRNAs and the proteins they encode are not identical in the gametophyte, transcript processing is apparently important in allowing translation to occur under strict temporal and spatial control. Transcript polyadenylation occurs in the spermatogenous cells in ways that match the translation of specific mRNAs. We have found that the exon junction complex plays key roles in transcript regulation and modifications that underlie cell specification in the gametophyte. We have recently become interested in the mechanisms that control the unmasking of the stored transcripts and have linked the synthesis and redistribution of spermidine in the gametophyte to the control of mRNA release from storage during early development and later to basal body formation, cytoskeletal assembly, and nuclear and cell elongation in the differentiating spermatids.

2'-O-ribose methylation of cap2 in human: function and evolution in a horizontally mobile family

The 5′ cap of human messenger RNA consists of an inverted 7-methylguanosine linked to the first transcribed nucleotide by a unique 5′–5′ triphosphate bond followed by 2′-O-ribose methylation of the first and often the second transcribed nucleotides, likely serving to modify efficiency of transcript processing, translation and stability. We report the validation of a human enzyme that methylates the ribose of the second transcribed nucleotide encoded by FTSJD1, henceforth renamed HMTR2 to reflect function. Purified recombinant hMTr2 protein transfers a methyl group from S-adenosylmethionine to the 2′-O-ribose of the second nucleotide of messenger RNA and small nuclear RNA. Neither N⁷ methylation of the guanosine cap nor 2′-O-ribose methylation of the first transcribed nucleotide are required for hMTr2, but the presence of cap1 methylation increases hMTr2 activity. The hMTr2 protein is distributed throughout the nucleus and cytosol, in contrast to the nuclear hMTr1. The details of how and why specific transcripts undergo modification with these ribose methylations remains to be elucidated. The 2′-O-ribose RNA cap methyltransferases are present in varying combinations in most eukaryotic and many viral genomes. With the capping enzymes in hand their biological purpose can be ascertained.

Effects of cap analogue or cap removal on the translation of rat brain mRNA in vitro

The role of cap structures in the translation of brain mRNA was examined by measuring protein biosynthesis in vitro in wheat germ and reticulocyte systems programmed by mRNA that was either untreated or oxidized by periodate or from which 5'-terminal 7-methylguanosine (m7G) was removed by oxidation and beta-elimination. In another series of reactions, amino acid incorporation into polypeptides was measured in the absence and in the presence of varying concentrations of the cap analogue 7-methylguanosine 5'-triphosphate (pppm7G). The results indicated that any of the above treatments interfered with brain mRNA translation, the degree of inhibition depending on the translation system used, the concentration of mRNA, and the source of initiation factors. Homologous brain initiation factors were superior to reticulocyte factors in providing a partial relief from inhibition of translation caused by these treatments. It was also found that synthesis of the brain-specific protein S-100 was inhibited by beta-elimination of mRNA, by pppm7G, or by the presence of capped globin mRNA, indicating that the mRNA for this protein was probably capped.

5'-Terminal 7-methylguanosine and mRNA function. The effect of enzymatic decapping and of cap analogs on translation of tobacco-mosaic-virus RNA and globin mRNA in vitro

1. Decapped tobacco mosaic virus (TMV) RNA and rabbit globin mRNA were prepared by enzymic treatment of RNAs with nucleotide pyrophosphatase purified from potato. The extent of removal of 5'-terminal 7-methylguanosine 5'-monophosphate (m7GMP) from TMV RNA was at least 97% as estimated by labeling of the 5' termini in vitro with S-adenosyl[methyl-3H]methionine catalysed by vaccinia virus methyltransferases. 2. The effect of enzymic decapping was compared with the effect of cap analogs on mRNAs translation in a nuclease-treated rabbit reticulocyte lysate and in a wheat germ extract. When translation was studied at low K+ concentration, little or no dependence on 5'-terminal 7-methylguanosine was found with either cell-free system. The importance of the 5'-terminal cap for the efficient translation of TMV RNA and globin mRNA increased as the concentration of K+ in a protein-synthesis system was raised. In a reticulocyte lysate analogs and enzymic decapping had a similar effect on translation. In a wheat germ extract, mRNA decapping resulted in a more pronounced decrease of mRNA activity, presumably due to the increased susceptibility of decapped mRNAs to the nucleases present in this protein synthesis system. 3. The requirement for a 5'-terminal cap was similar for the synthesis of 130,000-Mr and 165,000-Mr polypeptides coded by TMV RNA. This indicates that both proteins may be initiated at the common site close to the 5' terminus.

The effect of the messenger RNA concentration on the competitive inhibition of translation by cap-analogues

Inhibition of translation of several mRNA species in a micrococcal nuclease treated reticulocyte lysate by cap analogues was compared with the competition between two mRNAs. Inhibition characteristics were very similar, only complete mRNA molecules inhibited at concentrations 150 times lower than m7 G5'ppp5'G. The inhibition of mRNA translation by cap analogues could be neutralized by the addition of extra mRNA in a manner predicted from the competitive nature of the inhibition by cap analogues.

Translational recognition of the 5'-terminal 7-methylguanosine of globin messenger RNA as a function of ionic strength

The translation of rabbit globin mRNA in cell-free systems derived from either wheat germ or rabbit reticulocyte was studied in the presence of various analogues of the methylated 5' terminus (cap) as a function of ionic strength. Inhibition by these analogues was strongly enhanced by increasing concentrations of KCl, K(OAc), Na(OAc), or NH4(OAc). At appropriate concentrations of K(OAc), both cell-free systems were equally sensitive to inhibition by m7GTP. At 50 mM K(OAc), the reticulocyte system was not sensitive to m7GMP or m7GTP, but at higher concentrations up to 200 mM K(OAc), both nucleotides caused strong inhibition. The compound in m7G5'ppp5'Am was inhibitory at all concentrations of K(OAc) ranging from 50 to 200 mM, although more strongly so at the higher concentrations. Over the same range of nucleotide concentrations, the compounds GMP, GTP, and G5'ppp5'Am were not inhibitors. The mobility on sodium dodecyl sulfate-polyacrylamide electrophoresis of the translation product was that of globin at all K(OAc) concentrations in the presence of m7GTP. Globin mRNA from which the terminal m7GTP group had been removed by chemical treatment (periodate-cyclohexylamine-alkaline phosphatase) or enzymatic treatment (tobacco acid pyrophosphatase-alkaline phosphatase) was translated less efficiently than untreated globin mRNA at higher K(OAc) concentrations, but retained appreciable activity at low K(OAc) concentrations.

Cleavage of pm7G from mRNA 5' terminal cap structures by pyrophosphatase activity in embryonic chick lens cells

The presence of pyrophosphatase activity in embryonic lens cells which cleaves pm7G and ppGm from m7G(5')pppGm was demonstrated. It was also found that m7G(5')pppG, but not G(5')pppG, was hydrolyzed, and conversion of m7GpppG to m7G*pppG, in which the 5-membered ring of the m7G moiety is open, abolished its hydrolysis. For the caps hydrolyzed, pm7G was released only in the presence of lens cellular fraction; pm7G inhibited cap hydrolysis.

Translation of enzymically decapped messenger RNA

An enzymic procedure was used to remove the 7-methylguanosine diphosphate moiety at the 5' ends of rabbit hemoglobin mRNA and mouse immunoglobulin light-chain mRNA. Evidence was obtained that the procedure, which involves the use of polynucleotide kinase, does not result in any further degradation of the mRNA. The enzymically decapped mRNA was as effective as untreated mRNA in supporting protein synthesis in a wheat germ system. This was the case over a wide range of mRNA concentrations and over a considerable period of time. The presence in the incubation mixture of S-adenosylhomocysteine, an inhibitor of methylation, did not affect the results. The data indicate that the presence of a 7-methylguanosine diphosphate residue at the 5' end of mRNAs is not an obligatory requirement for translation in eucaryotic systems.

Comparative studies on the '5'-cap' and in vitro translational activity of cytoplasmic and nuclear poly A(+) RNA1

The translational activities of cytoplasmic poly A(+)RNA of normal rat liver and Novikoff hepatoma cells in the wheat germ cell free system were found to be approximately 15-20 times greater than tose of the corresponding nuclear poly A(+) RNA. The translationsl activities were 85 and 62 pmoles 3H-leucine incorporated/micron g cytoglasmic poly A(+) RNA for the liver and tumor respectively and 3-4 pmoles 3H-leucine incoporated/micron g nuclear poly A(+)RNA. Inasmuch as intergity of the '5'-cap' of mRNA is essential for its translational activity, quantitative comparisons were made of its content in these RNA fractions. Of the total 32P incorporated into the tumor cytoplasmic poly A(+) RNA, 0.41% was in the '5'-cap'; in nuclear poly A(+) RNA, the '5'-cap' contained 0.11%. After periodate oxidation and labeling with KB3H4, m7 guanosine, the 5'-terminal nucleoside in both liver and Novikoff hepatoma nuclear poly A(+) RNA contained approximately 20% as much isotope as in the cytoplasmic poly A(+) RNA. These results suggest the lower translational activity of nuclear poly A(+) RNA is partly related to its lower content of the '5'-cap'. Molecular selection of poly A(+) RNA for transport out of the nucleus or further cytoplasmic processing may account for the higher percentage of the '5-cap' and the greater translational activity of the cytoplasmic poly A(+) RNA. During these studies, it was also found that the m7 guanosine of the '5'-cap' was not removed during translation of the mRNA in the wheat germ system; this result suggests that the '5'-cap' may associate with allosteric binding sites of initiation factor(s).

5'-Terminal structure and mRNA stability

Reovirus mRNAs with 5'terminal m7GpppGm or GpppG are more stable than mRNA containing unblocked ppG 5'-ends when injected into Xenopus laevis oocytes or incubated in cell-free protein synthesising extracts of wheat germ and mouse L cells. The greater stability of mRNA with blocked 5' termini is not dependent upon translation but seems to result from protection against 5'-exonucleolytic degradation.

Studies on the structure and possible function of the RNA "cap" in developing sea urchins

Paracentrotus lividus embryos at the hatching blastula stage very quickly incorporate radioactivity from labeled nucleosides (except uridine) or 32P- or methyl-labeled methionine into a portion of the RNA that has been identified as a "cap". The most probable sequence of this cap is m7G (5') ppp (5')mAmpCp. A very active "capping" and methylation of the "cap" of preexisting RNA molecules was shown to occur at the blastula stage.

The RNA of unfertilized sea urchin eggs is "capped"

It is shown that the RNA of unfertilized sea urchin eggs is active in stimulating protein synthesis in a wheat germ cell free system. This activity is not lowered by conditions that inhibit the methylation processes but is inhibited by a treatment that damages the "cap", A difference in activity in a wheat germ cell free system between the RNA of unfertilized eggs and the RNA from early embryos is described.

5'-Capping structures of Artemia salina mRNA and the translational inhibition by cap analogs

The mRNA of the brain shrimp Artemia salina has two types of blocked methylated 5'-terminal structures (caps). About 75% of the mRNA molecules have the 5'-end structure of m7G5'ppp5'-AmpGp and about 25% have the structure of m7G5'ppp5'GmpGp. The only other type of methylated residue found in Artemia mRNA is N6-methyladenosine and which is located at internal positions along the mRNA chain. Translation of Artemia cyst or nauplius poly(A)-rich mRNA in wheat-germ extracts was found to be inhibited by 7-methylguanosine 5'-monophosphate, a chemical analog of the cap, as well as by snythetic caps such as m7G5'ppp5'Gm. On the other hand, the elongation activity on endonegous mRNA in an Artemia cell-free system was not sensitive to 7-methylguanosine 5'-monophosphate.

The translation in vitro of mRNA from developing cysts of Artemia salina

Successive stages in the development of the brine shrimp cyst were used as a model for studying differentiation at the level of mRNA transcription and translation. The poly (A)-containing mRNA from dormant cysts and free-swimming larvae (nauplii) was found to be efficiently translated in a wheat-germ cell-free system, and electrophoretic patterns of translation products in vitro resembled those of the endogenous proteins extracted from the equivalent developmental stages. Each stage, however, exhibits a characteristic protein pattern. Two low-molecular-weight proteins prominent in the cyst disappeared almost completely in the nauplius stage, whereas the proportion of actin increased 3-fold. Parallel patterns were observed upon translation in vitro of the respective mRNA preparations. The percentage of the acidic protein, tubulin, decreased somewhat during development.

7-Methyl-guanosine and efficiency of RNA translation

Brome mosaic virus RNAs 3 and 4 were chemically modified to remove the terminal 7-methyl-guanosine (m7G) structure, and the modified RNAs were tested for their messenger activity in a cell-free system derived from wheat embryo. Amino acid incorporation and ribosome-binding data show that removal of m7G results in reduction, but not complete abolition, of the messenger activity of the RNA. This suggests that the function of m7G may be related to efficient translation of messenger RNA. Possible involvement of other structural factors in RNA translation is discussed.