Storage of messenger RNA in eukaryotes: envelopment with protein, translational barrier at 5' side, or conformational masking by 3' side?

Y-box Binding Protein 1: Looking Back to the Future

Y-box binding protein 1 is a member of the cold shock domain (CSD) protein family and one of the most studied proteins associated with a large number of human diseases. This review aims to critically reassess the growing number of pathological functions ascribed to YB-1 in the past decades. The focus is given on the important role of YB-1 and related CSD proteins in the physiology of normal cells. The functional significance of these proteins is highlighted by their high evolutionary conservation from bacteria to men, where they are ubiquitously expressed and involved in coordinating all steps of mRNA biogenesis, including transcription, translation, storage, and degradation. Their activities are especially important under conditions requiring rapid change in the gene expression programs, such as early embryonic development, differentiation, stress, and adaptation to new environments. Therefore, to define a precise role of YB-1 in tumorigenic transformation and in other pathological conditions, it is important to understand its basic properties and functions in normal cells, and how they are interrupted in complex diseases including cancer.

Role of Y-Box Binding Proteins in Ontogenesis

Y-box binding proteins (YB proteins) are multifunctional DNA/RNA-binding proteins capable of regulating gene expression at multiple levels. At present, the most studied function of these proteins is the regulation of protein synthesis. Special attention in this review has been paid to the role of YB proteins in the control of mRNA translation and stability at the earliest stages of organism formation, from fertilization to gastrulation. Furthermore, the functions of YB proteins in the formation of germ cells, in which they accumulate in large amounts, are summarized. The review then discusses the contribution of YB proteins to the regulation of gene expression during the differentiation of various types of somatic cells. Finally, future directions in the study of YB proteins and their role in ontogenesis are considered.

Informosomes Travel in Time: An Early mRNA Concept in the Current mRNP Landscape

Messenger RNA is complexed with proteins throughout its life cycle. The first mRNA-containing particles of non-ribosomal nature, named informosomes, were discovered in cytoplasmic extracts of fish embryos by the laboratory of Alexander Spirin, and later described in live cells. Over time, various other nuclear and cytoplasmic mRNA-containing ribonucleoproteins (mRNPs) have been found and characterized. Although these mRNPs are very diverse in their subcellular localization, structure and functions, they share many common characteristics with informosomes. In this mini-review, I will discuss the discovery of informosomes, their characteristics and proposed functions, and their potential relationship to other mRNPs.

To the Academician Alexander Spirin 90th Anniversary from Informosomes to the World of mRNP

The main stages and achievements in the investigation of the fate of mRNA in eukaryotic cells, which began half a century ago with the discovery of informosomes by A.S. Spirin, there are discussed. The results of even the most recent studies confirm Spirin's hypotheses about the structure and function of the informosomes, now called mRNPs. In the bibliography, we present only small part of the published articles in this field. The full list is too large.

Translational repression of the Drosophila nanos mRNA involves the RNA helicase Belle and RNA coating by Me31B and Trailer hitch

Translational repression of maternal mRNAs is an essential regulatory mechanism during early embryonic development. Repression of the Drosophila nanos mRNA, required for the formation of the anterior-posterior body axis, depends on the protein Smaug binding to two Smaug recognition elements (SREs) in the nanos 3' UTR. In a comprehensive mass spectrometric analysis of the SRE-dependent repressor complex, we identified Smaug, Cup, Me31B, Trailer hitch, eIF4E, and PABPC, in agreement with earlier data. As a novel component, the RNA-dependent ATPase Belle (DDX3) was found, and its involvement in deadenylation and repression of nanos was confirmed in vivo. Smaug, Cup, and Belle bound stoichiometrically to the SREs, independently of RNA length. Binding of Me31B and Tral was also SRE-dependent, but their amounts were proportional to the length of the RNA and equimolar to each other. We suggest that "coating" of the RNA by a Me31B•Tral complex may be at the core of repression.

Understanding the molecular pathways associated with seed vigor

Farmers and growers are constantly looking for high quality seeds able to ensure uniform field establishment and increased production. Seed priming is used to induce pre-germinative metabolism and then enhance germination efficiency and crop yields. It has been hypothesized that priming treatments might also improve stress tolerance in germinating seeds, leaving a sort of 'stress memory'. However, the molecular bases of priming still need to be clarified and the identification of molecular indicators of seed vigor is nowadays a relevant goal for the basic and applied research in seed biology. It is generally acknowledged that enhanced seed vigor and successful priming depend on DNA repair mechanisms, activated during imbibition. The complexity of the networks of DNA damage control/repair functions has been only partially elucidated in plants and the specific literature that address seeds remains scanty. The DNA repair pathways hereby described (Nucleotide and Base Excision Repair, Non-Homologous End Joining, Homologous Recombination) play specific roles, all of them being critical to ensure genome stability. This review also focuses on some novel regulatory mechanisms of DNA repair (chromatin remodeling and small RNAs) while the possible use of telomere sequences as markers of aging in seed banks is discussed. The significant contribution provided by Electron Paramagnetic Resonance in elucidating the kinetics of seed aging, in terms of free radical profiles and membrane integrity is reported.

Bioinformatic identification of novel elements potentially involved in messenger RNA fate control during spermatogenesis

In eukaryotic cells, 3' untranslated regions (3' UTRs) of mRNA transcripts contain conserved sequence elements (motifs), which, once bound by RNA-binding proteins, can affect mRNA stability and translational efficacy. Despite abundant sequences contained within the 3' UTRs, only a limited number of motifs are known to interact with RNA-binding proteins and have a role in mRNA fate control. Spermatogenesis represents an excellent in vivo model for studying posttranscriptional regulation of gene expression because numerous mRNAs are transcribed in late pachytene spermatocytes and/or round spermatids, but their translation will not occur until many hours or even days later, when they have developed into elongated spermatids, in which transcription has long been shut off because of the increasingly condensed chromatin. Translationally suppressed mRNAs are sequestered and confined to ribonuclear protein particles, and their loading onto the ribosomes marks their translation. By bioinformatic sequence analyses of the 3' UTRs of translationally suppressed mRNAs during spermatogenesis, we identified numerous novel sequence elements overrepresented in the transcripts subject to posttranscriptional regulation than in the unregulated transcripts. These include AU(U/A)(U/A)UGAGU and (A/U)AUUA(U/C/G) for genes translationally upregulated in early spermiogenesis, and (G/A)GUACG(U/C/A)(A/U)(A/U) and UGUAGC for genes translationally upregulated in late spermiogenesis. The bioinformatic approach reported in this study can be adapted for rapid discovery of novel regulatory elements involved in mRNA fate control in a wide range of tissues or organs.

Smaug assembles an ATP-dependent stable complex repressing nanos mRNA translation at multiple levels

The nanos (nos) mRNA encodes the posterior determinant of the Drosophila embryo. Translation of the RNA is repressed throughout most of the embryo by the protein Smaug binding to Smaug recognition elements (SREs) in the 3' UTR. Translation is locally activated at the posterior pole by Oskar. This paper reports that the SREs govern the time- and ATP-dependent assembly of an exceedingly stable repressed ribonucleoprotein particle (RNP) in embryo extract. Repression can be virtually complete. Smaug and its co-repressor Cup as well as Trailer hitch and the DEAD box protein Me31B are part of the repressed RNP. The initiation factor eIF4G is specifically displaced, and 48S pre-initiation complex formation is inhibited. However, later steps in translation initiation are also sensitive to SRE-dependent inhibition. These data confirm several previously untested predictions of a current model for Cup-dependent repression but also suggest that the Cup model by itself is insufficient to explain translational repression of the nos RNA. In the embryo extract, recombinant Oskar relieves translational repression and deadenylation by preventing Smaug's binding to the SREs.

A novel RNA-binding protein associated with cell plate formation

Building a cell plate during cytokinesis in plant cells requires the participation of a number of proteins in a multistep process. We previously identified phragmoplastin as a cell plate-specific protein involved in creating a tubulovesicular network at the cell plate. We report here the identification and characterization of a phragmoplastin-interacting protein, PHIP1, in Arabidopsis (Arabidopsis thaliana). It contains multiple functional motifs, including a lysine-rich domain, two RNA recognition motifs, and three CCHC-type zinc fingers. Polypeptides with similar motif structures were found only in plant protein databases, but not in the sequenced prokaryotic, fungal, and animal genomes, suggesting that PHIP1 represents a plant-specific RNA-binding protein. In addition to phragmoplastin, two Arabidopsis small GTP-binding proteins, Rop1 and Ran2, are also found to interact with PHIP1. The zinc fingers of PHIP1 were not required for its interaction with Rop1 and phragmoplastin, but they may participate in its binding with the Ran2 mRNA. Immunofluorescence, in situ RNA hybridization, and green fluorescent protein tagging experiments showed the association of PHIP1 with the forming cell plate during cytokinesis. Taken together, our data suggest that PHIP1 is a novel RNA-binding protein and may play a unique role in the polarized mRNA transport to the vicinity of the cell plate.

Mice deficient for RNA-binding protein brunol1 show reduction of spermatogenesis but are fertile

RNA-binding proteins are involved in post-transcriptional processes like mRNA stabilization, alternative splicing, and transport. Brunol1 is a novel mouse gene related to elav/Bruno family of genes encoding for RNA-binding proteins. We report here the expression and functional analysis of murine Brunol1. Expression analysis of Brunol1 during embryogenesis by RT-PCR showed that Brunol1 expression starts at 9.5 dpc and continues to the later stages of embryonic development. In adult mice, the Brunol1 expression is restricted to brain and testis. We also analyzed the Brunol1 expression in testes of different mutants with spermatogenesis defects: W/W(V), Tfm/y, Leyl(-/-), olt/olt, and qk/qk. Brunol1 transcript was detectable in Leyl(-/-), olt/olt, and qk/qk mutant but not in W/W(V) and Tfm/y mutants. We also showed by transfection of a fusion protein of green fluorescent protein and Brunol1 protein into NIH3T3 cells, that Brunol1 is localized in cytoplasm and nucleus. In order to elucidate the function of the Brunol1 protein in spermatogenesis, we disrupted the Brunol1 locus in mouse by homologous recombination, which resulted in a complete loss of the Brunol1 transcript. Male and female Brunol1(+/-) and Brunol1(-/-) mice from genetic backgrounds C57BL/6J x 129/Sv hybrid and 129X1/SvJ when inbred exhibited normal phenotype and are fertile, although the number and motility of sperms are significantly reduced. An intensive phenotypic analysis showed no gross abnormalities in testis morphology. Collectively our results demonstrate that Brunol1 might be nonessential protein for mouse embryonic development and spermatogenesis.

Xp54 and related (DDX6-like) RNA helicases: roles in messenger RNP assembly, translation regulation and RNA degradation

The DEAD-box RNA helicase Xp54 is an integral component of the messenger ribonucleoprotein (mRNP) particles of Xenopus oocytes. In oocytes, several abundant proteins bind pre-mRNA transcripts to modulate nuclear export, RNA stability and translational fate. Of these, Xp54, the mRNA-masking protein FRGY2 and its activating protein kinase CK2α, bind to nascent transcripts on chromosome loops, whereas an Xp54-associated factor, RapA/B, binds to the mRNP complex in the cytoplasm. Over-expression, mutation and knockdown experiments indicate that Xp54 functions to change the conformation of mRNP complexes, displacing one subset of proteins to accommodate another. The sequence of Xp54 is highly conserved in a wide spectrum of organisms. Like Xp54, Drosophila Me31B and Caenorhabditis CGH-1 are required for proper meiotic development, apparently by regulating the translational activation of stored mRNPs and also for sorting certain mRNPs into germplasm-containing structures. Studies on yeast Dhh1 and mammalian rck/p54 have revealed a key role for these helicases in mRNA degradation and in earlier remodelling of mRNP for entry into translation, storage or decay pathways. The versatility of Xp54 and related helicases in modulating the metabolism of mRNAs at all stages of their lifetimes marks them out as key regulators of post-transcriptional gene expression.

Bruno acts as a dual repressor of oskar translation, promoting mRNA oligomerization and formation of silencing particles

Prior to reaching the posterior pole of the Drosophila oocyte, oskar mRNA is translationally silenced by Bruno binding to BREs in the 3' untranslated region. The eIF4E binding protein Cup interacts with Bruno and inhibits oskar translation. Validating current models, we directly demonstrate the mechanism proposed for Cup-mediated repression: inhibition of small ribosomal subunit recruitment to oskar mRNA. However, 43S complex recruitment remains inhibited in the absence of functional Cup, uncovering a second Bruno-dependent silencing mechanism. This mechanism involves mRNA oligomerization and formation of large (50S-80S) silencing particles that cannot be accessed by ribosomes. Bruno-dependent mRNA oligomerization into silencing particles emerges as a mode of translational control that may be particularly suited to coupling with mRNA transport.

Unshackling the links between reovirus oncolysis, Ras signaling, translational control and cancer

Reovirus has an inherent preference for replicating in cells with dysregulated growth factor signaling cascades that comprise Ras activation. Precisely how reovirus exploits the host cell Ras pathway is unclear, but there is evidence suggesting that activated Ras signaling is important for efficient viral protein synthesis. Defining the molecular mechanism of reovirus oncolysis will shed light on reovirus replication and important aspects of cellular transformation, Ras signaling cascades and regulation of protein translation.

Reciprocal function of movement proteins and complementation of long-distance movement of Cymbidium mosaic virus RNA by Odontoglossum ringspot virus coat protein

Complementation of movement and coat proteins of the orchid-infecting potexvirus Cymbidium mosaic virus (CymMV) and tobamovirus Odontoglossum ringspot virus (ORSV) was investigated. Nicotiana benthamiana, which is susceptible to both CymMV and ORSV, was used as a model system. Four transgenic lines, each harbouring one of the movement protein (MP) or coat protein (CP) genes of CymMV or ORSV, were constructed. The MP of CymMV consists of three overlapping open reading frames, together called the triple-gene block (TGB). CymMV and ORSV mutants, each carrying an inactivated MP or CP, were generated from the respective biologically active full-length cDNA clones. Complementation was studied by infecting transgenic plants with in vitro transcripts generated from these mutants. The cell-to-cell movement of a movement-deficient CymMV was restored in transgenic plants carrying the ORSV MP transgene. Similarly, CymMV TGB1 transgenic plants were able to rescue the cell-to-cell movement of a movement-deficient ORSV mutant. ORSV CP transgenic plants supported systemic movement of a CymMV CP-deficient mutant. However, in these plants, neither encapsidation of CymMV RNA with ORSV CP nor CymMV CP expression was detected. Long-distance movement of an ORSV CP-deficient mutant was not supported by CymMV CP. The complementation of MPs and CPs of CymMV and ORSV facilitates movement of these viruses in plants, except for long-distance movement of ORSV RNA by CymMV CP.

Transition from embryonic to adult transcription pattern of serotonin N-acetyltransferase gene in avian pineal gland

The study reports the change of transcription pattern of serotonin N-acetyltransferase gene and melatonin receptor genes during ontogenesis of the avian pineal gland. The RT-PCR technique was used to investigate the expression of the arylalkylamine N-acetyltransferase (AA-NAT) and melatonin receptor genes during development of the pineal glands isolated from Japanese quail (Coturnix coturnix japonica) embryos incubated from 3 days on until hatching (17 days), and in some organs (pineal, brain hemisphere, eye, leg, heart) of the 3-day-old quail embryo. It was shown that two phases of AA-NAT expression are observed during pineal gland development. The first, embryonic-type phase, lasts from the beginning until 7-10 days of incubation, and is marked by the presence of two RT-PCR products for AA-NAT: the shorter mature form without intron (238 bp), and the longer form (323 bp) containing an unprocessed intron of 85 bp. The second, adult-type phase is characterized by the presence of a single mature transcript, containing no intron; it starts from 7 to 10 days of incubation and lasts until hatching and in the adult pineal. The duration of this transition time from the embryonic to the adult transcription pattern in the quail pineal gland from 7 to 10 days of incubation we attribute to asynchronic embryo development, because quail chicks usually hatch between the 16th and 19th day of incubation. Analysis of the AA-NAT protein sequences for chick and quail (GeneBank accession no. U 46 502 and AF 007 068, respectively) revealed their perfect homology with the part of protein read from the sequence present in the adult-type phase of the pineal gland (the RT-PCR product of 238 bp). The presence of the intron (in the 323 bp RT-PCR product, accession no. AY 197 460) in the embryonic-phase of the pineal gland changes the reading frame of the mRNA sequence and the hypothetical resulting protein loses its homology with the chick and quail AA-NAT enzyme starting with 105th amino acid of the complete chick AA-NAT protein comprising 205 amino acids (accession no. U 46 502). In the whole embryos at stages 1-8 (according to the Hamburger-Hamilton classification) both RT-PCR products with and without intron were consistently found, and individual tissues from 3-day-old embryos also produced two AA-NAT products, i.e., the expression was of the embryonic-type. At the time of transition from the embryonic to the adult AA-NAT transcription pattern, in 7-11-day-old embryos, all three melatonin receptor transcripts (mel-1a, mel-1b, and mel-1c) were observed in the pineals, without consistent modifications of the band intensity. In the adult pineal, a single mature AA-NAT transcript was present as well as all three melatonin receptor transcripts, usually with preferential expression of the mel-1a band. The transition time from the embryonic to adult AA-NAT expression pattern coincides well with the acquisition of functional activity and the appearance of melatonin synthesis in the embryonic pineal reported for chicken, as related to quail. We suggest that the change in transcription pattern of the AA-NAT gene may reflect another, still unknown mechanism of regulating AA-NAT activity during ontogenesis, at the level of mRNA processing, whose specificity (or not) for embryonic development we wish to establish in the future.

Accumulation of zinc, copper, and metallothionein mRNA in lizard ovary proceeds without a concomitant increase in metallothionein content

The possible role of metallothionein (MT) in metal homeostasis has been investigated in growing oocytes and eggs of the lizard Podarcis sicula. Chromatographic analysis does not reveal the presence of MT in both ovary and eggs, the only metal-binding proteins detected being represented by high molecular mass components. De novo synthesis of MT could be observed in the ovary of cadmium-treated lizards. A cDNA encoding MT was obtained from the liver of P. sicula by RT-PCR followed by a RACE strategy, using primers designed on consensus motifs of vertebrate MT. In spite of the lack of MT in the ovary of untreated animals, Northern blot analysis demonstrates that the maternal untranslated MT transcript is expressed constitutively in the ovary in all the periods of the ovarian cycle. MT mRNA content increases during the oocyte growth, reaching the highest level in ovulated eggs, concomitantly with the accumulation of zinc and copper. Our findings suggest that maternal MT mRNA accumulates in the egg and is translated sometime during development to cope with the future needs of the growing embryo. The appearance of MT after cadmium treatment suggests that the block that makes the oocytarian MT mRNA untranslatable is removed by the metal.

Alternative patterns of transcription and translation of the ribosomal protein L32 mRNA in somatic and spermatogenic cells in mice

The patterns of transcription and translation of the ribosomal protein L32 (Rpl32) mRNA differ greatly in adult testis and somatic tissues. Northern blots reveal that the levels of Rpl32 mRNA are four- to five-fold higher in prepubertal and adult testes, and purified pachytene spermatocytes and round spermatids than in a variety of nongrowing adult somatic tissues. 5' RACE demonstrates that transcription in 8-day prepubertal testis, which lacks meiotic and haploid cells, strongly prefers the same start site in the 5' terminal oligopyrimidine tract (5' TOP) that is used is somatic cells. The 5' TOP is a cis element that inhibits translation of many mRNAs in nongrowing somatic cells. Although the sizes of deadenylated Rpl32 mRNAs are indistinguishable in somatic and spermatogenic cells, transcription initiates at 11 sites over a 31-nt segment in adult testis and approximately 62% of Rpl32 mRNAs lack a 5' TOP. In agreement with previous studies, low levels of cycloheximide increase the proportions and sizes of polysomes in absorbance profiles, and increase the proportions and sizes of polysomes translating four 5' TOP mRNA species including the Rpl32 mRNA in 8-day seminiferous tubules. In contrast, cycloheximide has little or no effect on the absorbance profiles and distribution of Rpl32 mRNA and 5' TOP mRNAs in adult seminiferous tubules. The failure of cycloheximide to increase the size of polysomes in adult seminiferous tubules implies a block in the pathway by which ribosomes are recruited onto translationally active mRNAs.

Presence and developmental regulation of serotonin N-acetyltransferase transcripts in oocytes and early quail embryos (Coturnix coturnix japonica)

By RT-PCR two transcripts for arylalkylamine N-acetyltransferase (AA-NAT; serotonin N-acetyltransferase; EC 2.3.1.87), the key enzyme in melatonin synthesis, were found, for the first time, in the oocytes and blastoderms from freshly laid eggs (323- and 238-bp RT-PCR products), and one (238-bp product) in the pineal gland of Japanese quail (Coturnix coturnix japonica). The two products differed by an intron of 85-bp present in the 323-bp band and absent from the 238-bp band. The identity of the products was confirmed by restriction analysis and sequencing. The ratio of the 323:238-bp bands changed during oogenesis from approximately 17:1 in small 3-mm oocytes to approximately 4:1 in immature vitellogenic oocytes and approximately 1:1 in mature, preovulatory oocytes; it was reversed to approximately 0.2:1 in blastoderms from fertile freshly laid eggs, corresponding to embryo of approximately 40,000 cells. It is proposed that the longer 323-bp product, containing an intron, represents a translationally inactive form of the transcript, stored in maternal RNA. The shorter 238-bp product lacking an intron may represent the mature active AA-NAT mRNA found in the pineal gland and in early embryos, and-to a lower proportion-in older oocytes. These data constitute the first direct proof of an intron sequence in maternal RNA of avian oocyte. It is possible that differential processing of the immature mRNA is part of a transcriptional regulation mechanism of AA-NAT activity. A possible role of extrapineal melatonin in early avian development is discussed.

Splicing in murine CABYR and its genomic structure

Human calcium-binding tyrosine-phosphorylation regulated protein (CABYR) is a polymorphic, testis-specific, calcium binding protein that undergoes tyrosine phosphorylation during in vitro capacitation. A protein kinase A (PKA) regulatory subunit type II alpha (RII-alpha) homologous domain in the N-terminus, phosphorylation dependent Ca(++) binding isoforms, and localization to the principal piece of the human sperm tail suggest that CABYR may be involved in sperm motility. In this paper, four mouse orthologous cDNAs and the genomic DNA of CABYR were cloned, nucleotide and protein sequences of mouse and humans were compared, and the genomic organization of the mCABYR gene was analyzed. Human and mouse CABYR conserve potential functional motifs including a domain homologous to the dimerization interface of cyclic adenosine monophosphate dependent PKA RII-alpha, 14 PXXP motifs, and regions of homology with extensins and src homology-3-binding protein 1. mCABYR is arranged into six exons spanning about 14 kb of DNA. Mouse CABYR showed several similarities with human CABYR: (1) the protein was localized to the principal piece of mouse epididymal spermatozoa; (2) mouse CABYR has two coding regions (CR-A and CR-B), with 66 and 82% identity, respectively to human; and (3) mCABYR showed the presence of two testis-specific transcripts of approximately 1.4 and approximately 2.4 kb. Three murine splice variants were identified, two of which spliced into CR-B. Exon 4, present in all human and mouse variants and comprising 85% of CR-A appears suitable for targeted deletion. The overall 81% nucleotide identity between mouse and human CABYR, the common genomic organization, presence of similar testis-specific transcripts, localization in the principal piece of tail and occurrence of homologous splice variants indicate an authentic murine orthologue of CABYR has been identified.

Poly(A)-binding protein positively affects YB-1 mRNA translation through specific interaction with YB-1 mRNA

The major protein of cytoplasmic mRNPs from rabbit reticulocytes, YB-1, is a member of an ancient family of proteins containing a common structural feature, cold-shock domain. In eukaryotes, this family is represented by multifunctional mRNA/Y-box DNA-binding proteins that control gene expression at different stages. To address possible post-transcriptional regulation of YB-1 gene expression, we examined effects of exogenous 5'- and 3'-untranslatable region-containing fragments of YB-1 mRNA on its translation and stability in a cell-free system. The addition of the 3' mRNA fragment as well as its subfragment I shut off protein synthesis at the initiation stage without affecting mRNA stability. UV cross-linking revealed four proteins (69, 50, 46, and 44 kDa) that specifically interacted with the 3' mRNA fragment; the inhibitory subfragment I bound two of them, 69- and 50-kDa proteins. We have identified these proteins as PABP (poly(A)-binding protein) (69 kDa) and YB-1 (50 kDa) and demonstrated that titrating out of PABP by poly(A) strongly and specifically inhibits YB-1 mRNA cap(+)poly(A)(-) translation in a cell-free system. Thus, PABP is capable of positively affecting YB-1 mRNA translation in a poly(A) tail-independent manner.

Embryotrophic factor-3 from human oviductal cells affects the messenger RNA expression of mouse blastocyst

Our previous results showed that embryotrophic factor-3 (ETF-3) from human oviductal cells increased the size and hatching rate of mouse blastocysts in vitro. The present study investigated the production of ETF-3 by an immortalized human oviductal cell line (OE-E6/E7) and the effects of ETF-3 on the mRNA expression of mouse embryos. The ETF-3 was purified from primary oviductal cell conditioned media using sequential liquid chromatographic systems, and antiserum against ETF-3 was raised. The ETF-3-supplemented Chatot-Ziomek-Bavister medium was used to culture Day 1 MF1 x BALB/c mouse embryos for 4 days. The ETF-3 treatment significantly enhanced the mouse embryo blastulation and hatching rate. The antiserum, at concentrations of 0.03-3%, abolished the embryotrophic effect of ETF-3. Positive ETF-3 immunoreactivity was detected in the primary oviductal cells, OE-E6/E7, and blastocysts derived from ETF-3 treatment. Vero cells (African Green Monkey kidney cell line), fibroblasts, and embryos cultured in control medium did not possess ETF-3 immunoreactivity. The mRNA expression patterns of the treated embryos were studied at the blastocyst stage by mRNA differential display reverse transcription-polymerase chain reaction (DDRT-PCR). The DDRT-PCR showed that some of the mRNAs were differentially expressed after ETF-3 treatment. Twelve of the differentially expressed mRNAs that had high homology with cDNA sequences in the GenBank were selected for further characterization. The differential expression of seven of these mRNAs (ezrin, heat shock 70-kDa protein, cytochrome c oxidase subunit VIIa-L precursor, proteinase-activated receptor 2, eukaryotic translation initiation factor 2beta, cullin 1, and proliferating cell nuclear antigen) was confirmed by semiquantitative RT-PCR. In conclusion, immortalized oviductal cells produce ETF-3, which influences mRNA expression of mouse blastocyst.

Stage-specific gene expression during sexual development in Phytophthora infestans

Eight genes that are upregulated during sexual development in the heterothallic oomycete, Phytophthora infestans, were identified by suppression subtractive hybridization. Two genes showed very low but detectable expression in vegetative hyphae and became induced about 40- to >100-fold early in mating, before gametangial initials appeared. The remaining six loci were not induced until later in mating, coincident with the formation of gametangia and oospores, with induction levels ranging from 60- to >100-fold. Five genes were single copy, and three were members of families. Sequence analysis revealed that the predicted products of three of the genes had similarity to proteins that influence RNA stability, namely a ribonuclease activator, the pumilio family of RNA-binding proteins and RNase H. The products of two other mating-induced genes resembled two types of Phytophthora proteins previously shown to elicit plant defence responses. Each mating-induced gene was also expressed in a self-fertile strain, which was shown to be a heterokaryon. However, quantitative and qualitative differences existed in their expression in normal matings and in the self-fertile heterokaryon. Besides the mating-induced genes, two extrachromosomal RNA elements were identified.

The major messenger ribonucleoprotein particle protein p50 (YB-1) promotes nucleic acid strand annealing

p50, a member of the Y-box binding transcription factor family, is tightly associated with eukaryotic mRNAs and is responsible for general translational regulation. Here we show that p50, in addition to its previously described ability to melt mRNA secondary structure, is capable of promoting rapid annealing of complementary nucleic acid strands. p50 accelerates annealing of RNA and DNA duplexes up to 1500-fold within a wide range of salt concentrations and temperatures. Phosphorylation of p50 selectively inhibits DNA annealing. Moreover, p50 catalyzes strand exchange between double-stranded and single-stranded RNAs yielding a product bearing a more extended double-stranded structure. Strikingly, p50 displays both RNA-melting and -annealing activities in a dose-dependent manner; a relatively low amount of p50 promotes formation of RNA duplexes, whereas an excess of p50 causes unwinding of double-stranded forms. Our results suggest that the alteration of nucleic acid conformation is a basic mechanism of the p50-dependent regulation of gene expression.

Regulation of mRNA stability in mammalian cells

The regulation of mRNA decay is a major control point in gene expression. The stability of a particular mRNA is controlled by specific interactions between its structural elements and RNA-binding proteins that can be general or mRNA-specific. Regulated mRNA stability is achieved through fluctuations in half-lives in response to developmental or environmental stimuli like nutrient levels, cytokines, hormones and temperature shifts as well as environmental stresses like hypoxia, hypocalcemia, viral infection, and tissue injury. Furthermore, in specific disorders like some forms of neoplasia, thalassemia and Alzheimer's disease, deregulated mRNA stability can lead to the aberrant accumulation of mRNAs and the proteins they encode. This review presents a discussion of some recently identified examples of regulated and deregulated mRNA stability in order to illustrate the diversity of genes regulated by alterations in the degradation rates of their mRNAs.

Molecular components related to egg viability in the gilthead sea bream, Sparus aurata

In pelagic egg spawners, the production of large numbers of sinking eggs, unable to develop into embryos, represents one of the major limiting factors in controlled reproduction. The aim of this study is to elucidate the molecular differences between floating and nonfloating eggs at cytoplasmic and nuclear level. Comparison of analyses between floating and nonfloating sea bream Sparus aurata eggs evidenced differences in vitelline envelope protein components, such differences being probably related with the hydration process but not with fertilization as supported by the assessment of DNA that doubled after in vitro insemination. These data clearly indicated that the absence of embryo development in nonfloating eggs is not due to lack of fertilization. The cytoplasmic composition was also different, the number of protein components being higher in floating eggs, and these extra components may generate the appropriate osmotic pressure at the base of the hydration process. Some lysosomal enzymes, such as cathepsin D and L both involved in yolk proteolysis, in virgin nonfloating eggs were significantly higher with respect to floating ones; the levels of these two enzymes significantly increased in the latter after fertilization. On the contrary, in nonfloating eggs cathepsin L significantly decreased after fertilization. These changes may be related with a series of metabolic processes vital for the production of viable offspring. The capacity of egg transcription and the protein synthesis in these two types of eggs, indicated by the RNA/DNA and RNA/protein ratios, evidenced that the status of cell transcription rate and protein synthesis capacity is significantly higher in floating eggs. This, in turn, suggested that the lack of embryo development may be due to low levels of proteins involved in cell cycle regulation.

mRNA stability and polysome loss in hibernating Arctic ground squirrels (Spermophilus parryii)

All small mammalian hibernators periodically rewarm from torpor to high, euthermic body temperatures for brief intervals throughout the hibernating season. The functional significance of these arousal episodes is unknown, but one suggestion is that rewarming may be related to replacement of gene products lost during torpor due to degradation of mRNA. To assess the stability of mRNA as a function of the hibernation state, we examined the poly(A) tail lengths of liver mRNA from arctic ground squirrels sacrificed during four hibernation states (early and late during a torpor bout and early and late following arousal from torpor) and from active ground squirrels sacrificed in the summer. Poly(A) tail lengths were not altered during torpor, suggesting either that mRNA is stabilized or that transcription continues during torpor. In mRNA isolated from torpid ground squirrels, we observed a pattern of 12 poly(A) residues at greater densities approximately every 27 nucleotides along the poly(A) tail, which is a pattern consistent with binding of poly(A)-binding protein. The intensity of this pattern was significantly reduced following arousal from torpor and undetectable in mRNA obtained from summer ground squirrels. Analyses of polysome profiles revealed a significant reduction in polyribosomes in torpid animals, indicating that translation is depressed during torpor.

RNA-binding proteins in mouse oocytes and embryos: expression of genes encoding Y box, DEAD box RNA helicase, and polyA binding proteins

Growth and differentiation of early embryos depends almost entirely on information which is maternally inherited in the form of macromolecules accumulated by the female gamete during its growth phase. Most of the maternal mRNAs synthesized by growing oocytes are not immediately recruited onto polysomes but are stored as translationally dormant messenger ribonucleoprotein (mRNP) particles. mRNA binding proteins which have been associated with masked mRNP complexes in Xenopus oocytes fall into two main categories, those having affinity for a variety of RNA sequences (members of the Y box and DEAD box RNA helicase families) and those which interact more specifically with 3' polyA tails (the polyA binding proteins or PABPs). The objective of this study was to determine whether mouse oocytes and embryos express sequences encoding a Y box protein, (MSY1); on RNA helicase, (RCK/p54); and a universally expressed PABP and testis specific isoform (PABP1 and PABPt, respectively). RNAs were amplified by RT/PCR and the identities of targeted cDNAs were confirmed by restriction analysis and/or direct sequencing. Relative steady state levels and time courses of accumulation/decay were compared by Northern hybridization. All of the sequences are transcribed as maternal mRNAs. MSY1 transcripts accumulated during the growth phase appear to be degraded in parallel with the bulk of maternal mRNAs by the mid-late two-cell stage. RCK/p54 mRNAs are most abundant in growing oocytes; steady state levels decline in primary and secondary oocytes, and degradation appears to be complete by the mid-late two-cell stage. Zygotic transcription of MSY1 and RCK/p54 is evident in four-cell stage embryos. Most of the PABP1 message accumulated by growing oocytes decays during meiotic maturation with transcription resuming in two-cell embryos. PABPt is expressed at very low levels in oocytes and embryos. Based on the temporal patterns of expression and the reported activities of homologous sequences in other systems, we suggest that these RNA binding proteins may participate in the post-transcriptional regulation of gene expression during the period of maternal control of development in the mouse.

Reconstitution of TCP80/NF90 translation inhibition activity in insect cells

Acid beta-glucosidase (GCase) is the enzyme deficient in Gaucher disease, an inherited metabolic prototype for enzyme and gene therapy. An 80-kDa mammalian cytoplasmic protein (TCP80/NF90) was discovered to interact with the GCase mRNA coding region and inhibit its translation in vitro and ex vivo. Human TCP80/NF90 is identical to NF90, an IL-2 enhancer protein, and MPP4, an M-phase phosphoprotein. The interaction of recombinant TCP80/NF90 with GCase mRNA was evaluated using the baculovirus/Sf9 insect cell system since these cells lack this protein. Purified recombinant and isolated mammalian cytoplasmic TCP80/NF90 had identical functions including binding of coding regions of selected RNAs and inhibition of their in vitro translation. Individual baculoviruses containing the human TCP80/NF90 cDNA (vSf9/TCP80) and GCase cDNA (vSf9/GCase) were used to co-infect Sf9 cells. The presence of preformed TCP80/NF90 significantly (>87%) inhibited wild-type GCase mRNA translation in these cells, but baculovirus containing a mutant GCase did not. Sf9 cells co-infected with vSf9/TCP80 showed a major reduction of GCase RNA polysome association. These results show that the multifunctional protein, TCP80/NF90, can function in vivo as a translation inhibitory protein and include alterations of mRNA binding to polysomes as a component of its mechanism of action.

Interaction of the universal mRNA-binding protein, p50, with actin: a possible link between mRNA and microfilaments

We have shown previously that p50 is the most abundant protein associated with a variety of eukaryotic mRNAs and exhibits about 98% amino acid sequence identity to mammalian Y-box binding transcription factors. The dual function of p50 in the cell as a regulator of both transcription and translation has been suggested. To gain insight into the role of p50 in these processes, we performed the yeast two-hybrid screen to identify p50 molecular partners. Here we report the identification of actin as a p50-interacting protein. Coimmunoprecipitation of p50 and actin from HeLa extracts as well as in vitro binding studies indicate specificity and a high affinity for the interaction between p50 and actin. Interestingly, p50 binding to actin is affected by mRNA; binding was observed at a low p50/mRNA ratio and was greatly reduced at higher ratios. Since the p50/mRNA ratio appears to be important for mRNA translatability, we speculate that p50 can regulate the attachment of mRNA to the actin network depending on its translational activity. Using immunofluorescence, we show that p50 binds to actin filaments in permeabilized cells and causes actin fibers to bundle in vitro. Together, these findings support the view that p50 may play an important role in mRNA transport, anchoring, and localization on actin filaments in the cell.

Sequence-independent assembly of spermatid mRNAs into messenger ribonucleoprotein particles

During mammalian spermatogenesis, meiosis is followed by a brief period of high transcriptional activity. At this time a large amount of mRNA is stored as messenger ribonucleoprotein (mRNP) particles. All subsequent processes of sperm maturation occur in the complete absence of transcription, primarily using proteins which are newly synthesized from these stored mRNAs. By expressing transgene mRNAs in the early haploid spermatids of mice, we have investigated the sequence requirements for determining whether specific mRNAs in these cells will be stored as mRNP particles or be assembled into polysomes. The results suggest that mRNAs which are transcribed in spermatids are assembled into mRNP particles by a mechanism that acts independently of mRNA sequence. Our findings reveal a fundamental similarity between the mechanisms of translational control used in spermatogenesis and oogenesis.

Subcellular RNA compartmentalization

The phenomenon of mRNA sorting to defined subcellular domains is observed in diverse organisms such as yeast and man. It is now becoming increasingly clear that specific transport of mRNAs to extrasomal locations in nerve cells of the central and peripheral nervous system may play an important role in nerve cell development and synaptic plasticity. Although the majority of mRNAs that are expressed in a given neuron are confined to the cell somata, some transcript species are specifically delivered to dendrites and/or, albeit less frequently, to the axonal domain. The physiological role and the molecular mechanisms of mRNA compartmentalization is now being investigated extensively. Even though most of the fundamental aspects await to be fully characterized, a few interesting data are emerging. In particular, there are a number of different subcellular distribution patterns of different RNA species in a given neuronal cell type and RNA compartmentalization may differ depending on the electrical activity of nerve cells. Furthermore, RNA transport is different in neurons of different developmental stages. Considerable evidence is now accumulating that mRNA sorting, at least to dendrites and the initial axonal segment, enables local synthesis of key proteins that are detrimental for synaptic function, nerve cell development and the establishment and maintenance of nerve cell polarity. The molecular determinants specifying mRNA compartmentalization to defined microdomains of nerve cells are just beginning to be unravelled. Targeting appears to be determined by sequence elements residing in the mRNA molecule to which proteins bind in a manner to direct these transcripts along cytoskeletal components to their site of function where they may be anchored to await transcriptional activation upon demand.

Translational regulation by Y-box transcription factor: involvement of the major mRNA-associated protein, p50

p50, the major core protein of messenger ribonucleoprotein particles (mRNPs), is a universal protein found exclusively in association with different mRNA species in the cytoplasm of somatic mammalian cells. Furthermore, p50 is the most abundant and tightly bound protein within both inactive mRNPs and active mRNPs derived from polysomes, although the latter contain a lower level of p50. Recent experiments have revealed that, depending on the p50 to mRNA ratio, p50 may either act as a repressor or an activator of protein synthesis. On the other hand, p50 exhibits about 98% amino acid sequence identity to mammalian transcription factors that bind specifically to Y-box containing DNA. Thus, it is a counterpart of the Y-box binding proteins which are found in bacteria, plants and animals, exhibiting multiple biological activities ranging from transcriptional regulation of a wide variety of genes to 'masking' mRNA activity in germinal cells. This review summarizes our current knowledge of p50 structure and function. It also discusses the biological roles of p50 and related proteins in gene expression and describes the likely mechanisms of their action.

Transient expression of a translation initiation factor is conservatively associated with embryonic gene activation in murine and bovine embryos

In the present study the abundance of mRNAs for eukaryotic translation initiation factors eIF-1A (formerly known as eIF-4C), -2alpha, -4A, -4E, and -5 was examined in in vivo-derived mouse embryos throughout preimplantation development using a semiquantitative reverse transcription-polymerase chain reaction assay. Although the mRNA profile for each gene is unique, only mRNA for eIF-1A transiently increases during embryonic gene activation (EGA) at the 2-cell stage, and this was confirmed by an independent hybridization-based assay. In in vitro-developed bovine embryos, mRNA for eIF-1A was transiently detected at the 8-cell stage, when the major activation of the genome occurs in this species. As in the mouse, detection in 8-cell bovine embryos was sensitive to the transcriptional inhibitor alpha-amanitin. It was also observed at the same time relative to cleavage in embryos cultured in defined medium under a reduced oxygen environment, and in medium supplemented with serum and somatic cells in 5% CO2 in air. Neither the chronology of early cleavage divisions nor the yield of bovine blastocysts differed in these culture media. Our results suggest that transient expression of eIF-1A in the mouse and cow is a conserved pattern of gene expression associated with EGA in mammals.

The 5′ Untranslated Region, Signal Peptide, and the Coding Sequence of the Carboxyl Terminus of IL-15 Participate in Its Multifaceted Translational Control

We previously reported that the AUG-burdened 5′ untranslated region (UTR) of IL-15 mRNA impedes its translation. Here we demonstrate that the nucleotide or protein sequences of the IL-15 signal peptide and carboxyl terminus also contribute to the poor translation of IL-15 transcripts. In particular, the exchange of the IL-15 signal peptide coding sequence with that of IL-2 increased cellular and secreted levels of IL-15 protein 15- to 20-fold in COS cells, while IL-2 transcripts with the IL-15 signal peptide generated 30- to 50-fold less IL-2 protein than wild-type IL-2. Furthermore, the addition of an artificial epitope tag to the 3′ coding sequence of IL-15 increased its protein production 5- to 10-fold. Combining these two IL-15 message modifications, in addition to removing the 5′ UTR, increased IL-15 synthesis 250-fold compared with a wild-type construct with an intact 5′ UTR. These data suggest that IL-15 mRNA, unlike IL-2 mRNA, may exist in translationally inactive pools. By storing translationally quiescent IL-15 mRNA, cells might respond to intracellular infections or other stimuli by rapidly transforming IL-15 message into one that can be efficiently translated.

The major core protein of messenger ribonucleoprotein particles (p50) promotes initiation of protein biosynthesis in vitro

The major core protein of cytoplasmic messenger ribonucleoprotein particles (p50) has been shown previously to inhibit protein synthesis in vitro and in vivo. Furthermore, p50 is highly homologous to the Y-box-binding transcription factor family of proteins, binds DNA containing the Y-box motif, and thus may have a dual function in cells as a regulator of both transcription and translation. Here we show that binding or removal of p50 from rabbit reticulocyte lysate by monospecific antibodies to p50 strongly inhibits translation of endogenous and exogenous globin mRNAs as well as prokaryotic beta-galactosidase mRNA in a rabbit reticulocyte cell-free system. Thus, depending on the conditions, p50 not only may act as a translational repressor, but may also be required for protein synthesis. Translation inhibition with anti-p50 antibodies is not a result of mRNA degradation or its functional inactivation. The inhibition does not change the ribosome transit time, and therefore, it does not affect elongation/termination of polypeptide chains. The inhibition with anti-p50 antibodies is followed by a decay of polysomes and accumulation of the 48 S preinitiation complex. These results suggest that p50 participates in initiation of protein biosynthesis. Although uninvolved in the formation of the 48 S preinitiation complex, p50 is necessary either for attachment of the 60 S ribosomal subunit or for previous 5'-untranslated region scanning by the 43 S preinitiation complex.

Structural basis for sequence-nonspecific recognition of 5'-capped mRNA by a cap-modifying enzyme

Sequence-nonspecific binding of RNA, recognition of a 7-methylguanosine 5' mRNA cap, and methylation of a nucleic acid backbone are three crucial and ubiquitous events in eukaryotic nucleic acid processing and function. These three events occur concurrently in the modification of vaccinia transcripts by the methyltransferase VP39. We report the crystal structure of a ternary complex comprising VP39, coenzyme product S-adenosylhomocysteine, and a 5' m7 G-capped, single-stranded RNA hexamer. This structure reveals a novel and general mechanism for sequence-non-specific recognition of the mRNA transcript in which the protein interacts solely with the sugar-phosphate backbone of a short, single-stranded RNA helix. This report represents the first direct and detailed view of a protein complexed with single-stranded RNA or 5'-capped mRNA.

Isolation and characterization of goldfish Y box protein, a germ-cell-specific RNA-binding protein

Cyclin B is a regulatory subunit of maturation-promoting factor. In goldfish (Carassius auratus) oocytes, cyclin B is synthesized de novo after stimulation by 17alpha,20beta-dihydroxy-4-pregnen-3-one (maturation-inducing hormone). In this study, we examined goldfish oocyte proteins bound to cyclin B mRNA. Using oligo(dT)-cellulose affinity chromatography and northwestern blotting analysis, we identified a 54-kDa cyclin B mRNA-binding protein (p54). Southwestern blotting analysis showed the binding of p54 to the Y box DNA element (CTGATTGGCCAA), suggesting that p54 is a Y box protein in goldfish. We isolated two cDNA clones, GFYP1 and GFYP2, the latter of which encodes a germ-cell-specific Y box protein. An antibody against a GFYP2 protein recognized p54, suggesting that p54 is identical or highly similar to GFYP2 protein. This is also supported by the finding that a recombinant GFYP2 expressed in bacteria bound to both the Y box DNA element and the goldfish cyclin B mRNA synthesized in vitro. These results suggest that p54 is a germ-cell-specific Y box protein and is a potential masking protein of cyclin B mRNA in goldfish oocytes.

Acquisition of meiotic competence in growing mouse oocytes is controlled at both translational and posttranslational levels

Full-grown mouse oocytes spontaneously resume meiosis in vitro when released from their follicular environment. By contrast, growing oocytes are not competent to resume meiosis; the molecular basis of meiotic competence is not known. Entry into M phase of the eukaryotic cell cycle is controlled by MPF, a catalytically active complex comprising p34cdc2 kinase and cyclin B. Incompetent oocytes contain levels of cyclin B comparable to those in competent oocytes, while their level of p34cdc2 is markedly lower; p34cdc2 accumulates abruptly at the end of oocyte growth, at the time of meiotic competence acquisition. We show here that this change in p34cdc2 concentration is not secondary to a corresponding change in the concentration of the cognate mRNA, indicating that translational control may be involved. Microinjection of translatable p34cdc2 mRNA into incompetent oocytes yielded high levels of the protein, but it did not lead to resumption of meiosis. Similarly, microinjection of cyclin B1 mRNA resulted in accumulation of the protein, but not in the acquisition of meiotic competence. By contrast, the microinjection of both p34cdc2 and cyclin B1 mRNAs in incompetent oocytes induced histone H1 and MAP kinase activation, germinal vesicle breakdown, and entry into M-phase including the translational activation of a dormant mRNA. Thus, endogenous cyclin B1 in incompetent oocytes is not available for interaction with p34cdc2, suggesting that a posttranslational event must occur to achieve meiotic competence. Microinjection of either p34cdc2 or cyclin B1 mRNAs accelerated meiotic reinitiation of okadaic acid-treated incompetent oocytes. Taken together, these results suggest that acquisition of meiotic competence by mouse oocytes is regulated at both translational and posttranslational levels.

The RNA-binding protein, TB-RBP, is the mouse homologue of translin, a recombination protein associated with chromosomal translocations

The mouse RNA-binding protein, TB-RBP, suppresses translation in vitro and attaches mRNAs to microtubules by binding to conserved elements in the 3' untranslated regions of specific mRNAs. We have now purified TB-RBP from testicular and brain cytoplasmic extracts and cloned its cDNA. We find that the mouse TB-RBP cDNAs contain an open reading frame of 228 amino acids with a leucine zipper domain within its C terminus, a transmembrane helix, and a group of putative phosphorylation sites. TB-RBP shows 99% identity to the human protein, translin, a recombination hotspot-binding protein associated with chromosomal translocations [Aoki, K., Suzuki, K., Sugano, T., Tasaka, T., Nakahara, K., Kuge, O., Omori, A. & Kasai, M. (1995) Nat. Genet. 10, 167-174]. As shown for translin, TB-RBP also binds to single-stranded DNAs containing a broad range of consensus sequences, many of which are similar to the Y and H RNA-binding sequences. Recombinant TB-RBP was synthesized and an antiserum was prepared against the recombinant protein. The identity between translin and TB-RBP was confirmed by demonstrating that immunoprecipitation of TB-RBP from testicular extracts abolished formation of the RNA-TB-RBP complex. Based upon its DNA binding to target sequences in clustered breakpoint regions, we propose that TB-RBP may be involved in DNA recombination or DNA repair in male germ cells.

Regulated unmasking of in vivo synthesized maternal mRNA at oocyte maturation. A role for the chaperone nucleoplasmin

We examine the translational regulation of histone H4 mRNA when Xenopus laevis oocytes are induced to mature with progesterone. Histone H4 mRNA synthesized from plasmid templates microinjected into oocyte nuclei is translationally silenced (masked). This masked mRNA becomes translationally active only after oocyte maturation. In contrast, histone H4 mRNA injected into the oocyte cytoplasm is translationally active both before and after oocyte maturation. Thus, transcription in vivo is required to mask histone H4 mRNA and to allow subsequent translational regulation. Protein association with histone H4 mRNA synthesized in vivo was determined before and after oocyte maturation. UV cross-linking of radiolabeled RNA to protein and immunoprecipitation of cross-linked proteins reveals an increased association of the chaperone nucleoplasmin with ribonucleoprotein particles dependent on the oocyte maturation process. The Y-box protein FRGY2 inhibits translation of histone H4 mRNA in vitro. Nucleoplasmin is able to partially relieve this repression. We discuss the potential role of nuleoplasmin in the remodeling of repressive ribonucleoprotein particles containing maternal mRNA to facilitate translational activation.

In vivo antisense oligodeoxynucleotide mapping reveals masked regulatory elements in an mRNA dormant in mouse oocytes

In mouse oocytes, tissue-type plasminogen activator (tPA) mRNA is under translational control. The newly transcribed mRNA undergoes deadenylation and translational silencing in growing oocytes, while readenylation and translation occur during meiotic maturation. To localize regulatory elements controlling tPA mRNA expression, we identified regions of the endogenous transcript protected from hybridization with injected antisense oligodeoxynucleotides. Most of the targeted sequences in either the 5' untranslated region (5'UTR), coding region, or 3'UTR were accessible to hybridization, as revealed by inhibition of tPA synthesis and by RNase protection. Two protected regions were identified in the 3'UTR of tPA mRNA in primary oocytes: the adenylation control element (ACE) and the AAUAAA polyadenylation signal. These sequences were previously shown to be involved in the translational control of injected reporter transcripts. During the first hour of meiotic maturation, part of the ACE and the AAUAAA hexanucleotide became accessible to hybridization, suggesting a partial unmasking of the 3'UTR of this mRNA before it becomes translationally competent. Our results demonstrate that in vivo antisense oligodeoxynucleotide mapping can reveal the dynamics of regulatory features of a native mRNA in the context of the intact cell. They suggest that specific regions in the 3'UTR of tPA mRNA function as cis-acting masking determinants involved in the silencing of tPA mRNA in primary oocytes.

Xp54, the Xenopus homologue of human RNA helicase p54, is an integral component of stored mRNP particles in oocytes

In investigating the composition of stored (maternal) mRNP particles in Xenopus oocytes, attention has focussed primarily on the phosphoproteins pp60/56, which are Y-box proteins involved in a general packaging of mRNA. We now identify a third, abundant, integral component of stored mRNP particles, Xp54, which belongs to the family of DEAD-box RNA helicases. Xp54 was first detected by its ability to photocrosslink ATP. Subsequent sequence analysis identifies Xp54 as a member of a helicase subfamily which includes: human p54, encoded at a chromosomal breakpoint in the B-cell lymphoma cell line, RC-K8; Drosophila ME31B, encoded by a maternally-expressed gene, and Saccharomyces pombe Ste13, cloned by complementation of the sterility mutant ste13. Expression studies reveal that the gene encoding Xp54 is transcribed maximally at early oogenesis: no transcripts are detected in adult tissues, other than ovary. Using a monospecific antibody raised against native Xp54, its presence in mRNP particles is confirmed by immunoblotting fractions bound to oligo(dT)-cellulose and separated by rate sedimentation and buoyant density. On isolating Xp54 from mRNP particles, it is shown to possess an ATP-dependent RNA helicase activity. Possible functions of Xp54 are discussed in relation to the assembly and utilization of mRNP particles.

The transcriptional and translational control of diazepam binding inhibitor expression in rat male germ-line cells

The diazepam binding inhibitor [DBI, also known as acyl-CoA-binding protein, (ACBP), or endozepine] is a 10-kD protein that has been suggested to be involved in the regulation of several biological processes such as acyl-CoA metabolism, steroidogenesis, insulin secretion, and gamma-aminobutyric acid type A (GABA(A))/benzodiazepine receptor modulation. DBI has been cloned from vertebrates, insects, plants, and yeasts. In mammals, DBI is expressed in almost all the tissues studied. Nevertheless, DBI expression is restricted to specific cell types. Here we have studied DBI gene expression in the germ-line cells of rat testis. The DBI gene was intensively transcribed in postmeiotic round spermatids from stages VI to VIII of the seminiferous epithelial cycle. A prominent, spermatid-specific upstream transcription initiation site was identified in addition to the multiple common transcriptional initiation sites found in the somatic tissues. However, no DBI protein was detected in round spermatids, suggesting that the DBI transcripts were translationally arrested. The DBI protein was detected in the late spermatogenic stages starting from elongating spermatids from step 18 (stage VI) onward. The DBI protein was also detected in mature spermatozoa and in ejaculated human sperms. The majority of DBI was located at the middle piece of the spermatozoons tail enriched with mitochondria. On the basis of this observation and the well-established role of DBI in acyl-CoA metabolism, we propose that DBI expression in spermatozoa reflects the usage of fatty acids as a primary energy source by spermatozoa. The biological function of DBI in spermatozoa could thus be related to the motility function of sperm.

Transcription regulatory signals in the 5' and 3' regions of Oreochromis aureus ER gene

The Oreochromis aureus estrogen receptor (OaER) 5' region is 62.2% AT-rich and does not display well-positioned consensus TATA and CAAT boxes. A functional imperfect 13 bp ERE, TGTTAtggTGACC, deviating at 2 bases is located in the leader exon. Transient transfection assays indicate that this ERE confers a 5-fold increase in SEAP reporter gene activity at 20 h post E2-induction. The continued elevation of transcription after the initial peak could be responsible for the "memory' effect of E2-induced vitellogenesis. The 4.4 kb OaER 3' region is AT-rich and has a high representation of 1/2 EREs and GREs. There are 10 copies of the destabilizing pentamer ATTTA which, in transient transfection experiments strongly suppressed SEAP activity. The 3' EREs are functional, and on their own, induce a bimodal increase in SEAP activity of 9-fold at 6 h and 11-fold at 18 h post-E2 induction. A recombinant construct of SEAP gene flanked by the 5' upstream and 3' flanking regions of the OaER gene allowed these regulatory signals to "cross-talk' to achieve a 5- and 25-fold increase for the first and second peaks, respectively. This bimodal response to E2-induction is attributed to both transcriptional and translational controls over the SEAP gene. This work represents a novel illustration of a synergistic interaction between the 5' and 3' regulatory elements of a steroid receptor in autoregulation.

Translational repression dependent on the interaction of the Xenopus Y-box protein FRGY2 with mRNA. Role of the cold shock domain, tail domain, and selective RNA sequence recognition

We have examined the determinants of the translational repression of mRNA by the Xenopus oocyte-specific Y-box protein FRGY2 using in vitro and in vivo assays. In vitro reconstitution of messenger ribonucleoprotein (mRNP) complexes demonstrates that the sequence-specific RNA-binding cold shock domain is not required for translational repression, whereas the RNA-binding C-terminal tail domain is essential. However, microinjection of reconstituted mRNPs into Xenopus oocytes demonstrates that although translational repression occurs in the absence of consensus RNA binding sequences for FRGY2, the presence of FRGY2 recognition elements within mRNA potentiates translational repression. Analysis of the in vivo assembly of mRNP shows that the cold shock domain alone is not stably incorporated into mRNP, whereas the C-terminal tail domain is sufficient for stable incorporation. We suggest that translational repression of mRNA by FRGY2 is favored by sequence-selective recognition of RNA sequences by the cold shock domain. However, translational repression in vitro and the assembly of mRNP in vivo requires the relatively nonspecific interaction of the C-terminal tail domain with mRNA. Thus two distinct domains of FRGY2 are likely to contribute to translational control.

Characteristics of poly(A)-degrading factor present in the avian oocytes and early embryos

The presence of poly(A)-degrading activity was studied in vitro in the quail and mouse oocytes and early embryos using 3H-poly(A) as a substrate. The activity was measured by adsorption of the undegraded substrate to DE-81 filter paper discs, by chromatographic separation on Sephadex G-50 column and by agarose gel electrophoresis followed by transfer onto a Zeta-probe membrane (BioRad, Richmond, CA) and autoradiography. High poly(A)-degrading activity was found in the quail previtellogenic and vitellogenic oocytes and lower activity in the early embryos from cleavage stage to gastrulation. This activity is localized predominantly in the nucleus and, to a lesser degree, in the cytoplasm and in the vitellus of vitellogenic oocytes. The length of the poly(A) degradation product was estimated to be of about (A)10. Optimum activity was at pH 8.7 and at Mn2+ concentration of 0.5 mM. This makes the deadenylating enzyme from the quail oocytes similar to endoribonuclease IV from the chick and quail oviducts (Müller [1976] Eur. J. Biochem., 70:241-248; Müller [1976], Eur. J. Biochem., 70:249-258). We suggest that the poly(A)-degrading enzyme, similar to endoribonuclease IV found in the quail oocytes, might be the "deadenylating factor" reported in Xenopus oocytes (Varnum et al. [1992] Dev. Biol., 153:283-290). Such poly(A)-degrading activity is undetectable in unfertilized mouse eggs; however, a slight, statistically insignificant tendency for poly(A) degradation was seen in two-cell embryos.

RNase A activity in Japanese quail oocytes

The presence of RNase A activity was studied in vitro in homogenates of quail oocytes and early embryos using [3H]poly(U) as a substrate. The activity was measured by adsorption of the undegraded substrate onto DE-81 filter paper discs and by chromatographic separation on a Sephadex G-50 column. RNase A activity examined by these methods was almost undetectable in quail previtellogenic, vitellogenic and ovulated oocytes as well as in the embryos from laid eggs. It is estimated to be about 1.1 x 10(-5) Kunitz units per ovulated oocyte. Higher activity starts to appear in gastrulating embryos. These findings are discussed in relation to other studies demonstrating the high stability of maternal RNA during early development, especially in growing oocytes.

In postmeiotic male germ cells poly (A) shortening accompanies translation of mRNA encoding gamma enteric actin but not cytoplasmic beta and gamma actin mRNAs

In the mammalian testis the cytoplasmic beta and gamma actins are expressed in all stages of germ-cell differentiation, whereas gamma enteric actin is expressed in germ cells solely in postmeiotic stages. Northern blot analysis of mouse testicular RNAs reveals actin mRNAs of about 2.1, 1.5, and 1.4 kB. The 2.1-kB mRNAs encode the cytoplasmic beta and gamma actins, whereas the two faster-migrating actin mRNAs encode gamma enteric actin. When postmitochondrial mouse testis extracts are fractionated by sucrose gradient centrifugation, the 1.5-kB gamma enteric actin mRNA is primarily found in the nonpolysomal fraction, whereas the 1.4-kB gamma enteric actin is polysomal. When the poly (A) tails are removed, the nonpolysomal and polysomal gamma enteric actin mRNAs both migrate at 1.3 kB, indicating that the difference in electrophoretic mobilities of the two gamma enteric actin mRNAs is caused by poly (A) length differences. The nonpolysomal and polysomal forms of the cytoplasmic beta and gamma actins show similar electrophoretic mobilities before and after deadenylation. Sequence comparison of the 3' untranslated region of the mouse gamma enteric actin to the 3' untranslated regions of other testicular mRNAs that undergo partial deadenylation reveals three highly-conserved sequence elements. These data demonstrate that the poly (A) shortening of polysomal mRNAs previously seen only with testis-specific mRNAs that are stored as mRNPs also occurs with mRNAs of widely-expressed genes that are expressed in postmeiotic male germ cells. The mRNAs all contain specific conserved sequence elements in their 3' untranslated regions.