Somatic Cell Cycle Research Articles

The maternal-effect gene futile cycle is essential for pronuclear congression and mitotic spindle assembly in the zebrafish zygote.

Embryos have been successfully used for the general study of the cell cycle. Although there are significant differences between the early embryonic and the somatic cell cycle in vertebrates, the existence of specialised factors that play a role during the early cell cycles has remained elusive. We analysed a lethal recessive maternal-effect mutant, futile cycle (fue), isolated in a maternal-effect screen for nuclear division defects in the zebrafish (Danio rerio). The pronuclei fail to congress in zygotes derived from homozygous fue mothers. In addition, a defect in the formation of chromosomal microtubules prevents mitotic spindle assembly and thus chromosome segregation in fue zygotes. However, centrosomal functions do not appear to be affected in fue embryos, suggesting this mutant blocks a subset of microtubule functions. Cleavage occurs normally for several divisions resulting in many anucleate cells, thus showing that nuclear- and cell division can be uncoupled genetically. Therefore, we propose that in mitotic spindle assembly chromosome-dependent microtubule nucleation is essential for the coupling of nuclear and cell division.

Identification and characterization of a Xenopus homolog of Dbf4, a regulatory subunit of the Cdc7 protein kinase required for the initiation of DNA replication.

Dbf4 is a regulatory subunit for the Cdc7 protein kinase that is required for the initiation of eukaryotic DNA replication, but the precise roles of Dbf4-Cdc7 remain to be determined. Here we identified a Xenopus homolog of Dbf4 (XDbf4) and characterized XDbf4 and Xenopus Cdc7 (XCdc7) in Xenopus egg extracts. XDbf4 formed a complex with XCdc7 in egg extracts and activated XCdc7 kinase activity in vitro. In contrast with Dbf4 in yeast and mammalian cultured cells, the XDbf4 levels in egg extracts did not change during the cell cycle progression. XDbf4 was a phosphoprotein in interphase extracts, and was apparently hyperphosphorylated in cytostatic factor (CSF)-mediated, metaphase-arrested extracts and in mitotic extracts. However, the hyperphosphorylation of XDbf4 did not seem to affect the level of kinase activation, or chromatin binding of the XDbf4-XCdc7 complex. Upon release from CSF-arrest, XDbf4 was partially dephosphorylated and bound to chromatin. Interestingly, XDbf4 was loaded onto chromatin before XCdc7 during DNA replication in egg extracts. These results suggest that the function of XDbf4-XCdc7 during the early embryonic cell cycle is regulated in a manner distinct from that during the somatic cell cycle.

Dominant negative E2F inhibits progression of the cell cycle after the midblastula transition in Xenopus.

The cleavage cycle, which is initiated by fertilization, consists of only S and M phases, and the gap phases (G1 and G2) appear after the midblastula transition (MBT) in the African clawed frog, Xenopus laevis. During early development in Xenopus, we examined the E2F activity, which controls transition from the G1 to S phase in the somatic cell cycle. Gel retardation and transactivation assays revealed that, although the E2F protein was constantly present throughout early development, the E2F transactivation activity was induced in a stage-specific manner, that is, low before MBT and rapidly increased after MBT. Introduction of the recombinant dominant negative E2F (dnE2F), but not the control, protein into the 2-cell stage embryos specifically suppressed E2F activation after MBT. Cells in dnE2F-injected embryos appeared normal before MBT, but ceased to proliferate and eventually died at the gastrula. These cells contained decreased cdk activity with enhanced inhibitory phosphorylation of Cdc2 at Tyr15. Thus, E2F activity is required for cell cycle progression and cell viability after MBT, but not essential for MBT transition and developmental progression during the cleavage stage.

In vivo analysis of the cyclin D1 promoter during early embryogenesis in Xenopus.

Cyclin D1 plays an important role in the regulation of G1 progression of the somatic cell cycle by functioning as a regulatory subunit of cdk4 and 6. Since both cyclin D1-mRNA and -protein turn over very rapidly, the expression of cyclin D1 is mostly regulated at the transcription level. Detection of the cyclin D1-specific mRNA during early embryogenesis in Xenopus laevis revealed that expression was induced soon after the midblastula stage but not during the cleavage stage, and was mainly found in the neural plate and eye vesicles. The recently-developed transgenic frog technique enabled us to analyze the frog cyclin D1 promoter activity in vivo by using green fluorescent protein (GFP) as a marker. During early embryogenesis, a GFP signal was detected at the neural plate in the neurula of the transgenic embryos. Transgenic analysis using sequential-deletion and point mutants of the cyclin D1 promoter revealed that a single, putative TCF/LEF binding site, located approximately 3.5 kb upstream of the transcriptional initiation site, was required for neural plate-specific expression. Our results suggest that the TCF/LEF signaling pathway participates in the regulation of cyclin D1 induction during the generation of the dorsal nervous system in early frog embryogenesis.

Cell cycle-dependent localization of macroH2A in chromatin of the inactive X chromosome.

One of several features acquired by chromatin of the inactive X chromosome (Xi) is enrichment for the core histone H2A variant macroH2A within a distinct nuclear structure referred to as a macrochromatin body (MCB). In addition to localizing to the MCB, macroH2A accumulates at a perinuclear structure centered at the centrosome. To better understand the association of macroH2A1 with the centrosome and the formation of an MCB, we investigated the distribution of macroH2A1 throughout the somatic cell cycle. Unlike Xi-specific RNA, which associates with the Xi throughout interphase, the appearance of an MCB is predominantly a feature of S phase. Although the MCB dissipates during late S phase and G2 before reforming in late G1, macroH2A1 remains associated during mitosis with specific regions of the Xi, including at the X inactivation center. This association yields a distinct macroH2A banding pattern that overlaps with the site of histone H3 lysine-4 methylation centered at the DXZ4 locus in Xq24. The centrosomal pool of macroH2A1 accumulates in the presence of an inhibitor of the 20S proteasome. Therefore, targeting of macroH2A1 to the centrosome is likely part of a degradation pathway, a mechanism common to a variety of other chromatin proteins.

Control of chromosomal DNA replication in the early Xenopus embryo.

In order for the genome to be faithfully maintained, chromosomal DNA must be precisely replicated and segregated in each cell cycle. Over the last decade an enormous amount has been learned about how this is achieved. Much of the progress has come from genetic analysis in yeast. However, biochemical analysis of cell cycle events using extracts prepared from eggs of the South African clawed toad Xenopus laevis has also played an important role. Although there are differences in the detailed regulation of the yeast and frog cell cycles, the basic cell cycle machinery appears to have been conserved throughout evolution. The results obtained in these model organisms, therefore, seem likely to be generally applicable to the cell cycles of all eukaryotes. ### Xenopus eggs and egg extracts The fertilized Xenopus egg undergoes 12 synchronous rounds of cell division in ∼8 h. These cell divisions take place in the absence of growth, subdividing the large (∼1 mm diameter) single‐celled egg into ∼4000 smaller cells. Transcription does not occur during these early embryonic divisions, although translation of pre‐existing maternal mRNA continues. Most of the proteins required for cell cycle progression are pre‐formed in the egg, and the continuing translation of a single protein (cyclin B) can support passage through the whole cell cycle (Murray and Kirschner, 1989). The stockpile of cell cycle proteins present in the Xenopus egg became exploitable by biochemical means following the development of cell‐free extracts that support all the nuclear events of the early embryonic cell division cycle (Lohka and Masui, 1983). Gentle lysis associated with minimal dilution of the cytoplasm yields a ‘low speed supernatant’ that maintains all the cell cycle activities present in the intact egg. These ‘low speed supernatants’ support precise rounds of DNA replication on exogenously added DNA templates, and like the intact egg, only re‐replicate DNA after passage through …

Cell cycle regulation in early mouse embryos.

For a long time it has been thought that the cell cycles of the early mouse embryo do not differ from the somatic cell cycle. They are long and are composed of classical G1, S, G2 and M phases and have functional checkpoint controls. However, a few characteristics observed during the earliest mitotic cleavage divisions suggest that the embryonic cell cycle could differ significantly from the somatic ones. Understanding these differences could have an important impact on our understanding of both general cell cycle mechanisms as well as the developmental programme of the early mouse embryo. Over the last few years our laboratories have undertaken a project focused on describing the differences in the first two cell cycles of the mouse embryo. We discuss here the results concerning (1) the way mouse oocytes switch from the meiotic to the mitotic cell cycle upon activation of development (inactivation of the cytostatic factor, CSF); (2) how the entry into the first and the second mitotic M phase is regulated (nucleus-independent activation of M phase-promoting factor, MPF); and (3) how the duration of the early embryonic mitoses is regulated. These data show that developmentally regulated phenomena are superimposed on and highly coordinated with the cell cycle machinery.

Cell Cycle in the Fucus Zygote Parallels a Somatic Cell Cycle but Displays a Unique Translational Regulation of Cyclin-Dependent Kinases

Cell Cycle in the Fucus Zygote Parallels a Somatic Cell Cycle but Displays a Unique Translational Regulation of Cyclin-Dependent Kinases

Requirement of a centrosomal activity for cell cycle progression through G1 into S phase.

Centrosomes were microsurgically removed from BSC-1 African green monkey kidney cells before the completion of S phase. Karyoplasts (acentrosomal cells) entered and completed mitosis. However, postmitotic karyoplasts arrested before S phase, whereas adjacent control cells divided repeatedly. Postmitotic karyoplasts assembled a microtubule-organizing center containing gamma-tubulin and pericentrin, but did not regenerate centrioles. These observations reveal the existence of an activity associated with core centrosomal structures-distinct from elements of the microtubule-organizing center-that is required for the somatic cell cycle to progress through G1 into S phase. Once the cell is in S phase, these core structures are not needed for the G2-M phase transition.

Dissection of the XChk1 signaling pathway in Xenopus laevis embryos.

Checkpoint pathways inhibit cyclin-dependent kinases (Cdks) to arrest cell cycles when DNA is damaged or unreplicated. Early embryonic cell cycles of Xenopus laevis lack these checkpoints. Completion of 12 divisions marks the midblastula transition (MBT), when the cell cycle lengthens, acquiring gap phases and checkpoints of a somatic cell cycle. Although Xenopus embryos lack checkpoints prior to the MBT, checkpoints are observed in cell-free egg extracts supplemented with sperm nuclei. These checkpoints depend upon the Xenopus Chk1 (XChk1)-signaling pathway. To understand why Xenopus embryos lack checkpoints, xchk1 was cloned, and its expression was examined and manipulated in Xenopus embryos. Although XChk1 mRNA is degraded at the MBT, XChk1 protein persists throughout development, including pre-MBT cell cycles that lack checkpoints. However, when DNA replication is blocked, XChk1 is activated only after stage 7, two cell cycles prior to the MBT. Likewise, DNA damage activates XChk1 only after the MBT. Furthermore, overexpression of XChk1 in Xenopus embryos creates a checkpoint in which cell division arrests, and both Cdc2 and Cdk2 are phosphorylated on tyrosine 15 and inhibited in catalytic activity. These data indicate that XChk1 signaling is intact but blocked upstream of XChk1 until the MBT.

Mis3 with a conserved RNA binding motif is essential for ribosome biogenesis and implicated in the start of cell growth and S phase checkpoint

In normal somatic cell cycle, growth and cell cycle are properly coupled. Although CDK (cyclin-dependent kinase) activity is known to be essential for cell cycle control, the mechanism to ensure the coupling has been little understood. We here show that fission yeast Mis3, a novel evolutionarily highly conserved protein with the RNA-interacting KH motif, is essential for ribosome RNA processing, and implicated in initiating the cell growth. Growth arrest of mis3-224, a temperature sensitive mutant at the restrictive temperature, coincides with the early G2 block in the complete medium or the G1/S block in the release from nitrogen starvation, reflecting coupling of cell growth and division. Genetic interactions indicated that Mis3 shares functions with cell cycle regulators and RNA processing proteins, and is under the control of Dsk1 kinase and PP1 phosphatase. Mis3 is needed for the formation of 18S ribosome RNA, and may hence direct the level of proteins required for the coupling. One such candidate is Mik1 kinase. mis3-224 is sensitive to hydroxyurea, and the level of Mik1 protein increases during replication checkpoint in a manner dependent upon the presence of Mis3 and Cds1. Mis3 is essential for ribosome biogenesis, supports S phase checkpoint, and is needed for the coupling between growth and cell cycle. Whether Mis3 interacts solely with ribosomal precursor RNA remains to be determined.

The NIMA-related kinase X-Nek2B is required for efficient assembly of the zygotic centrosome in Xenopus laevis.

Nek2 is a mammalian cell cycle-regulated serine/threonine kinase that belongs to the family of proteins related to NIMA of Aspergillus nidulans. Functional studies in diverse species have implicated NIMA-related kinases in G(2)/M progression, chromatin condensation and centrosome regulation. To directly address the requirements for vertebrate Nek2 kinases in these cell cycle processes, we have turned to the biochemically-tractable system provided by Xenopus laevis egg extracts. Following isolation of a Xenopus homologue of Nek2, called X-Nek2B, we found that X-Nek2B abundance and activity remained constant through the first mitotic cycle implying a fundamental difference in Nek2 regulation between embryonic and somatic cell cycles. Removal of X-Nek2B from extracts did not disturb either entry into mitosis or the accompanying condensation of chromosomes providing no support for a requirement for Nek2 in these processes at least in embryonic cells. In contrast, X-Nek2B localized to centrosomes of adult Xenopus cells and was rapidly recruited to the basal body of Xenopus sperm following incubation in egg extracts. Recruitment led to phosphorylation of the X-Nek2B kinase. Most importantly, depletion of X-Nek2B from extracts significantly delayed both the assembly of microtubule asters and the recruitment of gamma-tubulin to the basal body. Hence, these studies demonstrate that X-Nek2B is required for efficient assembly of a functional zygotic centrosome and highlight the possibility of multiple roles for vertebrate Nek2 kinases in the centrosome cycle.

Ste20-like kinase (SLK), a regulatory kinase for polo-like kinase (Plk) during the G2/M transition in somatic cells.

Activation of the cyclin-dependent kinase cdc2-cyclin B1 at the G2/M transition of the cell cycle requires dephosphorylation of threonine-14 and tyrosine-15 in cdc2, which in higher eukaryotes is brought about by the Cdc25C phosphatase. In Xenopus, there is evidence that a kinase cascade comprised of xPlkk1 and Plx1, the Xenopus polo-like kinase 1, plays a key role in the activation of Cdc25C during oocyte maturation. In the mammalian somatic cell cycle, a polo-like kinase homologue (Plk1) also functions during mitosis, but a kinase upstream of Plk is still unknown. We show here that human Ste20-like kinase (SLK), which is a ubiquitously expressed mammalian protein related to xPlkk1, can phosphorylate and activate murine Plk1. During progression through the G2 phase of the mammalian cell cycle, the activity of endogenous SLK is increased. The amount of SLK protein is decreased in quiescent and differentiating cells. Treatment with okadaic acid induces a phosphorylation-dependent enhancement of SLK activity. We propose that SLK has a role in the regulation of Plk1 activity in actively dividing cells during the somatic cell cycle. SLK itself is suggested to be regulated by phosphorylation.

Specific regulation of CENP-E and kinetochores during meiosis I/meiosis II transition in pig oocytes

To understand the mechanisms which regulate meiosis-specific cell cycle and chromosome distribution in mammalian oocytes, the level and the localization of CENP-E and the kinetochore number and direction on a half bivalent were examined during pig oocyte maturation. CENP-E is a kinetochore motor protein whose intracellular level and localization are strictly regulated in the somatic cell cycle. The localizations of CENP-E on meiotic chromosomes from diakinesis stage to anaphase I and at the spindle midzone at telophase I were shown by immunofluorescent confocal microscopy to be similar to those in somatic cells of pig and other species. Further, ultrastructural analysis revealed the presence of CENP-E on fibrous corona and outer plate of kinetochores of the meiotic chromosomes. However, unlike mitosis, CENP-E staining was continuously detected either at the spindle midzone or on the kinetochores of segregated chromosomes during the first polar body emission. Consistent with this, immunoblot analysis revealed that CENP-E level remained high during meiosis I/meiosis II (MI/MII) transition and that some of CENP-E survived through the transition even in cycloheximide-treated oocytes in which cyclin B1 was completely degraded. Furthermore, examinations of CENP-E signals in confocal microscopy and kinetochores in electron microscopy in MI and MII oocytes provide the cytological evidence in mammalian oocytes which suggests that each sister chromatid in a pair has its own kinetochore which localizes side-by-side so that two sister chromatids on a half bivalent are oriented toward and connected to the same pole in MI. Mol. Reprod. Dev. 56:51–62, 2000. © 2000 Wiley-Liss, Inc.

Specific regulation of CENP-E and kinetochores during meiosis I/meiosis II transition in pig oocytes.

To understand the mechanisms which regulate meiosis-specific cell cycle and chromosome distribution in mammalian oocytes, the level and the localization of CENP-E and the kinetochore number and direction on a half bivalent were examined during pig oocyte maturation. CENP-E is a kinetochore motor protein whose intracellular level and localization are strictly regulated in the somatic cell cycle. The localizations of CENP-E on meiotic chromosomes from diakinesis stage to anaphase I and at the spindle midzone at telophase I were shown by immunofluorescent confocal microscopy to be similar to those in somatic cells of pig and other species. Further, ultrastructural analysis revealed the presence of CENP-E on fibrous corona and outer plate of kinetochores of the meiotic chromosomes. However, unlike mitosis, CENP-E staining was continuously detected either at the spindle midzone or on the kinetochores of segregated chromosomes during the first polar body emission. Consistent with this, immunoblot analysis revealed that CENP-E level remained high during meiosis I/meiosis II (MI/MII) transition and that some of CENP-E survived through the transition even in cycloheximide-treated oocytes in which cyclin B1 was completely degraded. Furthermore, examinations of CENP-E signals in confocal microscopy and kinetochores in electron microscopy in MI and MII oocytes provide the cytological evidence in mammalian oocytes which suggests that each sister chromatid in a pair has its own kinetochore which localizes side-by-side so that two sister chromatids on a half bivalent are oriented toward and connected to the same pole in MI.

CDNA and promoter sequences for MCM3 homologues from maize, and protein localization in cycling cells

Abstract Key words: Cell cycle, DNA replication, licensing factor,MCM proteins, plant.A partial cDNA from maize, ROA, encoding a proteinhomologous to the MCM3 family of essential factorsfor the initiation of DNA replication, has been isolated Introduction previously. In the present work, a longer version of theoriginal ROA cDNA, encoding a full-length protein, was The eukaryotic genome is generally replicated exactlyisolated and termed ZmROA1. In addition, three other once, during the S phase of the somatic cell cycle, andclosely related cDNAs, ZmROA2, ZmROA3 and this is essential to maintain the DNA content throughZmROA4, were also isolated. ZmROA2 end ZmROA3 numerous generations of cells. The once-per-cell cycleappear to encode full-length proteins, whereas control over DNA replication ensures that DNA synthesisZmROA4 a partial polypeptide. Two clusters of basic is initiated only once, and that re-replication is inhibitedamino acids comprising putative nuclear localization during S phase (reviewed by Coverly and Laskey, 1994).signals were identified in the N-terminal domain This involves a tight control over replication origins,of these proteins, together with a potential leucine which must be timed to activate upon S phase entry,zipper. Immunofluorescence studies on cycling meris- but then rendered unable to support further rounds oftematic root-tip cells revealed that these proteins are DNA synthesis until cells complete mitosis and enter thelocalized in the nucleus throughout interphase with a next interphase. A complex array of interactions betweenpattern overlapping that of chromatin. However, as initiation factors and chromosomal origins is essential tochromatin condenses at prophase, ZmROA proteins make origins competent for DNA replication, to fire thembecome increasingly distinct from chromatin and during S phase, and to prevent them from firing again inappear totally dissociated from the segregating chro- the same cell cycle (reviewed by Kearsey

Characterization of a novel nucleolar protein that transiently associates with the condensed chromosomes in mitotic cells

We report the isolation and characterization of a murine gene encoding a conserved mammalian nucleolar protein. The protein, called Tsg118, has a predicted molecular mass of 59.4 kDa and a high content of basic amino acids. A homologous human gene was localized to chromosome 16p12.3. The Tsg118 protein is predominantly expressed in proliferating somatic cells and in male germ cells. Indirect immunofluorescence microscopy analysis using an affinity-purified anti-Tsg118 serum shows colocalization of Tsg118 and a known nucleolar protein, fibrillarin, to the dense fibrillar component of the nucleolus. The nucleolar localization of the Tsg118 protein appears to be temporally restricted to the interphase stages of the somatic cell cycle and to the meiotic phase of spermatogenesis. We find that the Tsg118 protein localizes to the nucleolus in both proliferating and serum-starved cells. Interestingly, as the nucleolar signal disappears in mitotic cells, the Tsg118 protein instead becomes associated with the surface of the condensed chromosomes.

Cell cycle analysis and synchronization of the Xenopus laevis XL2 cell line: study of the kinesin related protein XlEg5.

Cell free extracts prepared from Xenopus eggs are one of the most powerful in vitro systems to analyze cell cycle-regulated mechanisms such as DNA replication, nuclear assembly, chromosome condensation, or spindle formation. Xenopus embryos can complete several synchronous cell cycles in the absence of transcription, consequently Xenopus extracts are very helpful to study the molecular level of cellular mechanisms. Many key cell cycle regulators like p34cdc2 and cdk2 have been discovered and characterized using those extracts, but their regulation during somatic cell cycles have only been studied in mammalian cultured cells. In this paper, we describe optimized conditions to obtain cell cycle arrested Xenopus XL2 cultured cells. Synchronization of XL2 cells at different stages of the cell cycle was achieved by serum starvation and drug treatments such as aphidicolin, nocodazole, and ALLN. The degree of synchronization was assessed by indirect fluorescence microscopy and FACS analysis. This method was used to study the cell cycle expression of the Xenopus kinesin-related protein, XlEg5, a microtubule-based motor protein involved in movement and cell division in early development. We found that the expression of the protein was maximum in mitosis and minimum in G1, which correlated with the expression of its messenger RNA. XL2 cultured cells were also used to analyze the ultrastructural sub-cellular localization of XlEg5. During mitosis, the protein was found around the centrosome in prophase, on the spindle microtubules in metaphase, and, interestingly, around the minus end of the midbody microtubules in telophase.

Novel ribosomal genes from maize are differentially expressed in the zygotic and somatic cell cycles.

We have isolated cDNAs representing more than 50 different genes from libraries of unfertilised egg cells and zygotes of maize, expression of which is up- or downregulated after in vitro fertilisation (IVF). Among the cDNAs isolated are seven which encode proteins that are probably involved in translation and two encoding proteins probably involved in DNA replication. The latter genes are strongly induced on fertilisation. This indicates that zygotic gene activation (ZGA) - the switch from maternal to embryonic control of development - occurs in the zygote shortly after fertilisation in higher plants - earlier than in animal systems so far investigated. Four novel transcripts for ribosomal proteins (S21A, S21B, L39, P0) involved in ribosome biosynthesis and translation were analysed in more detail. The expression of all four genes correlates with cell division activity and is strongly induced during the G1 phase of the somatic cell cycle. A different mode of regulation operates in the first embryonic cell cycle: relatively large amounts of transcript are stored in the unfertilised egg cells, and by 18 h after IVF, two ribosomal genes are induced while a third is downregulated. These results indicate that using the combination of single-cell culture techniques with novel molecular methods, it is possible to isolate and study numerous genes expressed in female gametes and zygotes of higher plants. The detailed analysis of the four ribosomal protein genes demonstrates that the zygotic and somatic cell cycles are differentially regulated.

A transcript encoding translation initiation factor eIF-5A is stored in unfertilized egg cells of maize.

Differential screening of cDNA libraries of unfertilized egg cells and in vitro zygotes of maize resulted in the isolation of more than 50 different genes whose expression is up- or down-regulated after in vitro fertilization (IVF). Among these genes, we identified a cDNA encoding the eukaryotic translation initiation factor eIF-5A. This highly conserved factor is thought to be necessary for selective mRNA stabilization and translation. It is also the only known protein that contains the unusual amino acid hypusine which is required for biological activity. High transcript amounts are stored in the egg cell, which is, in terms of metabolism, relatively inactive. Upon fertilization transcript amounts decrease, in contrast to metabolically inactive embryos in which the transcript cannot be detected and transcript levels increase upon germination. The expression pattern during the first embryonic cell cycle is also different from that observed during the somatic cell cycle: egg cells in the G0 phase contain high transcript levels, while arrested suspension cells contain few transcripts. In the somatic cell cycle, eif-5A is strongly induced during the G1 phase and transcripts are continuously degraded during the S, G2 and M phases until new induction during the G1 phase of the next cycle. eif-5A, a member of a small gene family in maize, is expressed in most maize tissues investigated. Based on our results, we suggest that the unfertilized egg cell of maize, although relatively inactive regarding its metabolism, is prepared for selective mRNA translation that is quickly triggered after fertilization. We also suggest that the regulation of eif-5A in the first embryonic cell cycle is different from the somatic cell cycle.