Polyribosomes of Escherichia coli. I. Isolation of polysomes from a complex of DNA and membrane

Summary The distribution of pulse labeled RNA, pulse labeled protein, soluble enzyme (-galactosidase) and polyribosomes between low speed (10 min 20000xg) supernatant and pellet of E. coli lysates was examined. This distribution was greatly changed by addition of deoxyribonuclease to the lysing medium. Large amounts of polysomes sedimented with DNA at low centrifugal forces. A complex of membrane, DNA, polyribosomes and RNA polymerase could be separated from unlysed cells by surcrose gradient centrifugation. The polysomes present in this complex (Polysomes II) were separated from the polysomes which were found in the cytoplasma (Polysomes I). Polysomes II contain very few ribosomal subunits and 70S ribosomes.     

Separate ribosomal pools in sea urchin embryos: ammonia activates a movement between pools

Monoribosomes from unfertilized eggs of Strongylocentrotus purpuratus were shown to translate mRNA less efficiently than ribosomes derived from polyribosomes of embryos, as measured by globin synthesis in a ribosome-dependent rabbit reticulocyte lysate [Danilchik, M. V., & Hille, M. B. (1981) Dev. Biol. 84, 291-298]. Data presented in this paper show that monoribosomes from 16-cell and blastula embryos resemble monoribosomes from unfertilized eggs in translational capacity and are less active than the ribosomes associated with polyribosomes. Thus, we find two distinct populations of ribosomes in embryos. We define the less active monoribosome population as "naive" ribosomes and the more active, functioning polysome-derived ribosomes as "experienced" ribosomes. Naive and experienced ribosomes have the same elongation rates. The relationship between ionic triggers and the conversion of monoribosomes to experienced ribosomes was studied with the Ca2+ ionophore A23187, which releases intracellular Ca2+ stores, and NH4Cl, which alkalinizes the cytoplasm. We found that ribosomes in the monoribosome populations from A23187-activated eggs or from NH4Cl-activated eggs resembled naive monoribosomes from unfertilized eggs in their translational activity. In contrast, ribosomes derived from the polysomes of NH4Cl-treated eggs were as active as the experienced polysome-derived ribosomes from normal embryos. Eggs activated with A23187 did not produce polyribosomes. The presence of significant amounts of experienced ribosomes in NH4Cl-treated eggs implicates alkalinization of the cytoplasm as a stimulus for ribosome activation, which occurs slowly during initial development.     

Quantitative changes in protein synthesis during oogenesis in Xenopus laevis

Protein synthesis rates in Xenopus laevis oocytes from stage 1 through stage 6 were measured. In addition, the translational efficiencies, total RNA contents, and percentages of ribosomes in polysomes in growing oocytes at several stages were determined. Stage 1 oocytes synthesize protein at a mean rate of 0.18 ng hr⁻¹ while stage 6 oocytes make protein at a rate of 22.8 ng hr⁻¹. Polysomes from growing and full-grown oocytes sedimented in a sucrose gradient with a peak value of 300 S, corresponding to a weight-average packing density of 10 ribosomes per mRNA. Ribosome transit times of endogenous mRNAs were essentially unchanged at all stages examined. While the oocyte's total ribosomal RNA content was observed to increase about 115-fold during oogenesis, the percentage of ribosomes in polysomes remained constant at approximately 2%. Taken together, the data suggest that the 127-fold increase in protein synthesis which occurs during Xenopus oogenesis involves the progressive recruitment onto polysomes of mRNA from the maternal stockpile.     

Association of maternal and newly synthesized ribosomes with membranous noncytoskeletal structures in Xenopus laevis embryonic cells

In Xenopus laevis embryos a high concentration of both KCl and 0.5% DOC (sodium deoxycholate) is needed for maximal extraction of ribosomes and polysomes. We studied the nature of the structures that keep ribosomes and polysomes immobilized within the cytoplasm of embryonic cells at cleavage through tailbud stages, using various combinations of a low-salt buffer (20 mM KCl), a high-salt buffer (500 mM KCl), 0.5% DOC, and 0.5% Triton X-100. With a low-salt buffer and 0.5% DOC, but not Triton X-100, 80S ribosomal monomers and polysomes were liberated from the cytoplasmic rapidly sedimenting structures (RSS) to the soluble fraction. With a high-salt buffer (500 mM KCl), ribosomes were solubilized as 60S and 40S subunits together with about one-half of the total polysomes. When cells were homogenized in a low-salt buffer with added inhibitors of the cytoskeleton (cytochalasin B or colchicine), the majority of polysomes but not ribosomes were solubilized. These results provide evidence for the following conclusions. 1) Polysomes are bound to cytoskeletal structures in Xenopus embryos, but ribosomes, both maternal and newly synthesized, are associated with membranous noncytoskeletal structures. 2) The membranous structures consist of two compartments, one high-salt sensitive and the other high-salt resistant. 3) Ribosomes of the high-salt resistant group increase in amount with developmental stage and appear to be the precursor to the ribosomes of the high-salt sensitive group.     

Zum Vorkommen helixförmig angeordneter Ribosomen bei Rhodopseudomonas palustris

Zusammenfassung Es wird das Vorkommen von Polysomensäulen bei Rhodopseudomonas palustris beschrieben. An Hand der Daten ergab sich, daß diese Polysomen aus zwei helixförmig angeordneten Ribosomensträngen aufgebaut sind, die eine linksdrehende Schraube bilden. Der Querschnitt der Schraube zeigt hexagonale Anordnung der Ribosomen. Der Steigungswinkel der Helices beträgt 20⁰, der Gesamtdurchmesser der hexagonalen Polysomensäule beträgt 375 Å. Diese Polysomenart kommt lediglich in der begeißelten Polregion vor und ist dicht unter der Cytoplasmamembran lokalisiert. Die Polysomen stehen mit der Cytoplasmamembran in Kontakt. Ihre Längsachse läuft parallel zur Plasmamembran.

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The occurrence of helical arranged ribosomes of rhodopseudomonas palustris

Summary Helical arranged ribosomes were detected in the polar region of Rhodopseudomonas palustris cells. They are attached to the cytoplasmic membrane. The columnar polysomes are shaped of two helical strings of ribosomes, which are arranged around the axis of the screw. The vertical projection of the single ribosomes in one turn of the screw is a hexagon. The helices are left handed. The angle between the horizontal line and the ribosome chain in the helix is =20⁰. The diameter of the polyribosome column amounts to 375 Å.

     

Untersuchungen zur Entwicklung des Phycomyceten Allomyces arbuscula

Polysomes were isolated both from growing gametophytes of Allomyces arbuscula and from gametangia prepared from mycelia at different periods during gametogenesis. Analysis of polysomes by sucrose gradients showed that ribosomes present in the gametangia monosome pool were shifted into polysomes. This shift was found to be correlated with gametangia differentiation. The ribosome distribution remained virtually unchanged during the early stage of gamete formation. In mature gametes and swarming zygotes a low level of polysomes was detected. Labeling of rRNA by ³²PO₄ demonstrated a de novo synthesis of monosomes throughout the period of gametangia differentiation. No incorporation of ³²PO₄ was found to be present in ribosomes prepared from gametangia after onset of gamete formation. On the basis of these labeling experiments it is concluded that radioactivity in polysomes extracted from mature gametes and swarming zygotes can be attributed in part to conserved mRNA.Synchronous formation of gametangia was induced by transferring the vegetative mycelia from growth medium into a low salt buffer. Under these conditions the incorporation of either ³²PO₄ or ³H-uridine into RNA, particularly into rRNA, was found to be markedly decreased. This obviously indicates a shutdown of RNA synthesis. rRNA from induced mycelia examined by polyacrylamide gel electrophoresis was found to be severely degraded. In contrast to this, rRNA isolated from ribosomes of developing gametangia and from gametes exhibited no degradation products. It is suggested that endonucleases cause rRNA hydrolysis in the hyphal cytoplasm during gametangia differentiation. Ribosomes compartmentalized in gametangia seem to be inaccessible to nucleases during the later process of gametogenesis.

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Abkürzungen MAK Methyl-Albumin-Kieselgur - PAA Polyacrylamid - stains all 4,5,4,5-Dibenzo-3,3-diäthyl-9-methylthiacarbocyaninbromid (Serva, Heidelberg)     

Synthesis and turnover of polysomal mRNAs in sea urchin embryos

The synthesis and turnover kinetics of polysomal mRNA have been measured in sea urchin embryos. Polysomes were isolated from stages ranging between mesenchyme blastula and late gastrula Strongylocentrotus purpuratus embryos which had been exposed to exogenous ³H-guanosine. The amount of radioactivity incorporated into messenger and ribosomal RNAs was determined separately as a function of time, and the precursor pool specific activity was measured in the same embryos. Synthesis and decay rate constants were extracted from the data by a leastsquares procedure. Per embryo, the rate of mRNA synthesis was calculated to be about 0.13 pg min^?1, while the rate of rRNA synthesis is about 0.022 pg min^?1. The newly synthesized mRNA turns over with a half-time of 5.7 hr. The data support only a single decay rate for the mRNA, but small fractions of mRNA decaying at different rates cannot be excluded. Previous studies have shown that a minor fraction of the mRNA includes the least abundant, most highly diverse set of messages (“complex class” mRNAs). To determine whether mRNAs of the complex class are synthesized and degraded at similar rates, labeled mRNA was measured in hybrids formed in mRNA excess reactions with single copy DNA. These experiments showed that complex class mRNAs represent an approximately proportional amount of the new mRNA synthesis, and turn over at the same average rate as does the bulk of the mRNA. Most of the mRNAs in the embryo polysomes are newly synthesized, rather than maternal. This statement refers both to complex class mRNAs and to prevalent mRNAs. Considering the sequence homology between embryo and oocyte mRNAs shown earlier, these results indicate that many of the same structural genes active during oogenesis are being transcribed in embryos at these stages.     

Cytogenetic analysis of oocytes and early preimplantation embryos from XO mice.

The cytoplasm of early sea urchin embryos contains nonribosomal, high molecular weight RNA both associated with ribosomes in polysomes and free of ribosomes in particles termed free RNP. In a 1-hr labeling period, 50% of the newly synthesized RNA enters the pool of ribosome-free RNP particles during the cleavage stages, and this percentage decreases until less than 20% of the new RNA in the mesenchyme blastula stage is found in the free RNP. mRNA from both polysomes and free RNP contain poly(A)(+) and poly(A)(?) species. During the cleavage stages only 8–10% of the RNA from each fraction is polyadenylated; however, in the blastula, 40–50% of the nonhistone polysomal RNA is polyadenylated while only 22–30% of the free RNP RNA is polyadenylated. At any developmental stage, the poly(A)(+)RNA from the free RNA and polysomes have identical sedimentation profiles; this is also the case for the poly(A)(?)RNA except for the absence of the 9 S histone mRNA from the free RNP. Changes in poly(A)(+)RNA content and sedimentation profiles during development occur simultaneously in the free RNP and the polysomes. Kinetic studies of these two RNP populations as well as nuclear RNP show that the bulk of the free RNP are not unusually stable cytoplasmic components. The free RNP decay with a half-life of about 40 min while nuclear RNA and polysomal RNA display half-lives of about 12 and 65 min, respectively. Further, the rate of synthesis of the free RNP is not consistent with their being the only precursors for polysomes. Our estimates of the rates of synthesis for nuclear RNA, polysomes, and free RNP are, respectively, 1.1 × 10^?15, 2.2 × 10^?16, and 5.0 × 15^?17 g/min/nucleus. The data on free RNP is discussed in terms of translational regulation of protein synthesis in the developing sea urchin.     

Elevation of Cyclic AMP-Dependent Protein Kinase Activity during Migration of Primary Mesenchyme Cell in Sand Dollar Blastulae

Concetration of intracellular cyclic AMP (cAMP), and activities of adenylate cyclase and cAMP-dependent protein kinase were examined in swimming and mesenchyme blastulae and primary mesenchyme cells (PMCs) of the sand dollar, Clypeaster japonicus , respectively. In mesenchyme blastulae, the concentration of cAMP increased 45% from that in swimming blastulae. PMCs contained a concentration of cAMP 40% higher than that in whole embryos at the mesenchyme blastula stage. The activity of adenylate cyclase in mesenchyme blastulae was 100% higher than that in swimming blastulae. The activites of cAMP-dependent protein kinase in whole embryos at the above two developmental stages, on the other hand, were quite similar to each other. However, in PMCs the activity of the enzyme was conspicuously higher than that in these embryos, and it reached 190% higher than that in these embryos. Inhibition of cAMP-dependent protein kinase activity by a synthetic inhibitor, H8, caused severe inhibition of PMC migration but it did not exert any effect on PMC ingression. These results suggest that the cAMP-dependent protein kinase activity is involved in PMC migration, but not in PMC ingression.     

RIBOSOME CRYSTALLIZATION IN CHICKEN EMBRYOS : II. Conditions for the Formation of Ribosome Tetramers In Vivo

Slow cooling of fertilized chicken eggs permits the elongation and termination of nascen^t polypeptides in the polysomes but prevents the initiation of new protein chains. This leads to polysome disaggregation during the first 30 min of cooling, and to the formation, of a pool of inactive ribosomes prone to crystallization. After 2 hr these ribosomes began to form tetramers, which do not contain any labeled proteins synthesized during cooling. If protein synthesis is inhibited by cycloheximide, added to eggs before cooling, tetramer formation in the embryos is prevented. Puromycin, on the other hand, leads to polysome disassembly and does not prevent tetramer formation. Rapid cooling of explanted embryos after short incubation at 37°C, with or without cycloheximide, largely prevents polysome disaggregation and the formation of tetramers. On the other hand, the addition of puromycin to explanted embryos promotes tetramer formation after rapid cooling. When cooled eggs are rewarmed, tetramers are disassembled into monomers, even if protein synthesis is inhibited. When those embryos were rapidly recooled tetramers reformed spontaneously from tetramer-derived monomers, even in the presence of cycloheximide. We conclude that the formation of tetramers at low temperature is an inherent property of the normal ribosomes.     

Isolation and properties of Acheta accessory gland polysomes

Polysomes have been prepared from Acheta male accessory gland. The following factors are important in ensuring maximum yield: (1) homogenization at a large buffer to tissue excess (40 : 1), (2) use of 200 mM NH₄Cl as a nuclease inhibitor, (3) addition of either sodium deoxycholate or Triton X−100 to the buffer before homogenization and (4) use of the Dounce homogenizer. Inclusion of NH₄⁺ gives no deleterious effects with respect to dissociation or activity of the ribosomes.The occurrence of cold-induced ‘run-off’ of polysomes has been confirmed by direct measurement. Polysomes reassemble within 15 to 30 min after return of the animals to higher temperatures, with restoration of endogenous protein synthetic capacity to control levels; the ribosomes appear to retain an elevated capacity for poly(U)-directed protein synthesis.     

Mammalian sperm-egg interaction: Identification of a glycoprotein in mouse egg zonae pellucidae possessing receptor activity for sperm

Sperm-egg interaction in mammals is initiated by binding of sperm to the zona pellucida, an acellular coat completely surrounding the plasma membrane of unfertilized eggs. Zonae pellucidae of mouse eggs are composed of three different glycoproteins, designated ZP1, ZP2 and ZP3, having apparent molecular weights of 200,000, 120,000 and 83,000, respectively Bleil and Wassarman, 1978, Bleil and Wassarman, 1980a, Bleil and Wassarman, 1980b. In this investigation, ZP1, ZP2 and ZP3 were purified from zonae pellucidae isolated individually from unfertilized mouse eggs and 2-cell embryos. Each of the glycoproteins was then tested for its ability to interfere with the binding of sperm to eggs in vitro. Solubilized zonae pellucidae isolated from unfertilized eggs, but not from 2-cell embryos, reduced binding of sperm to as little as 10% of control values. Similarly, ZP3 purified from zonae pellucidae of unfertilized eggs reduced the binding of sperm to eggs in vitro to an extent comparable to that observed with solubilized zonae pellucidae. On the other hand, ZP3 purified from zonae pellucidae of 2-cell embryos had no significant effect on the extent of sperm binding, consistent with the inability of solubilized zonae pellucidae from 2-cell embryos to affect sperm binding. In no case did purified ZP1 and ZP2 interfere significantly with the binding of sperm to eggs in vitro. These results suggest that ZP3 possesses the receptor activity responsible for the binding of sperm to zonae pellucidae of unfertilized mouse eggs. Fertilization apparently results in modification of ZP3 such that it can no longer serve as a receptor for sperm.     

Quantitative changes in total RNA, total poly(A), and ribosomes in early mouse embryos

To obtain information on the amounts and major classes of RNA stored in the mouse egg and accumulated during cleavage, we determined the contents of total RNA, total poly(A), and ribosomes from the 1-cell stage to blastocyst. Using purified RNA for assay, we obtained an RNA content of 0.35 ng in the unfertilized egg, 0.24 ng in 2-cell, 0.69 ng in 8- to 16-cell, and 1.47 ng in early bastocyst (32 cells). As derived from EM morphometry, the number of ribosomes accounts for 60–70% of the total RNA content at all these stages; the marked increase in ribosomal number during cleavage is attributable entirely to new synthesis. Hybridization with [³H]poly(U) in solution yielded a poly(A) content of 0.7 pg for the unfertilized egg and 0.83 pg for the 1-cell embryo. The poly(A) content dropped sharply, to 0.26 pg per embryo, by the late 2-cell stage and increased to 0.44 pg in 8- to 16-cell embryos and 1.42 pg in early blastocysts. Hybridization in situ gave a similar pattern and also revealed a heavy labeling of embryo nuclei from the 2-cell onward but very little, if any, labeling of the pronuclei of 1-cell embryos, suggesting an absence, or low level, of poly(A)+ RNA synthesis at the 1-cell but an active synthesis at the 2-cell and later stages. These findings and other available evidence(e.g., R. Bachvarova and V. De Leon, 1980, Develop. Biol.74, 1–8) suggest that the mouse embryo inherits a large supply of maternal mRNA but that the bulk of this RNA is eliminated in the 2-cell embryo. In situ hybridization was used to study the relative concentration of poly(A) in ovarian oocytes. In growing oocytes, the cytoplasmic concentration of poly(A) remains about the same, suggesting that the accumulation of poly(A)+ RNA is proportional to oocyte growth. The poly(A) content declines about twofold between the time of completion of oocyte growth and fertilization. The germinal vesicle continues to be labeled up to the time of ovulation, raising the possibility that poly(A)+ RNA synthesis (and presumably turnover) occurs in fully grown oocytes.     

Translational control in sea urchin eggs and embryos: Initiation is rate limiting in blastula stage embryos

To determine whether initiation is rate-limiting in protein synthesis during the embryogenesis of sea urchins, polyribosome profiles of unfertilized eggs and cleavage, blastula and prism stage embryos were examined after incubation of the eggs and embryos in the presence and absence of low amounts of emetine, an inhibitor of polypeptide elongation. The ribosomes were radioactively labeled with [³H]uridine by injection of the adults during oogenesis so that we could monitor emetine-dependent shifts of monoribosomes to polyribosomes. Although initiation is not rate limiting in unfertilized eggs or 2- to 16-cell embryos of Strongylocentrotus purpuratus, it is rate limiting in blastula and prism embryos. We suggest that initiation becomes rate limiting to allow the selective translation of certain classes of mRNA during later development.     

The toxicity of mercuric chloride and methylmercuric chloride to Fundulus heteroclitus embryos in relation to exposure conditions

Synopsis Embryos in specific stage of the estuarine teleost, Fundulus heteroclitus, were exposed to mercuric chloride (MC) and methylmercuric chloride (MMC) under several distinct treatment conditions. Four-eight cell stage eggs (0-day old) were exposed for 4 days (continuous), 2 days and one day to each mercury compound. One-day old (mid-blastula), 2-day old (mid-neurula) and 5-day old (beating heart) embryos were exposed 4 days to MC and MMC. Mortality for the four days immediately following the initiation of exposure was the embryonic response measured. Under most exposure conditions to the 4–8 cell eggs, progressive and significant reductions in survival were observed at all concentrations above 40 and 30 gHg⁺⁺l^–1 as MC and MMC, respectively. Reducing the duration of exposure to 1 day most significantly increased the survival potential of the 4–8 cell eggs. For all exposure treatments to the 4–8 cell eggs, significant differences in survival, between eggs exposed to MC and MMC, were determined at 40, 60 and 80 gHg⁺⁺l^–1, indicating the presence of compound-dependent response differences. In all cases demonstrating response differences between MC and MMC exposed embryos, survival was significantly lower following exposure to MMC. Survival of embryos was progressively increased when the initiation of continuous exposure (4 days) was delayed 1, 2 and 5 days after fertilization. As a result, compound-dependent response differences were progressively shifted to higher He⁺⁺ concentrations. For both MC and MMC, survival of 1-day old embryos exposed for 4 days was greater than that of 0-day old eggs exposed for 1 day. Of the embryonic stages examined, it appears that the earlier cleavage stages are the most sensitive to mercury intoxication.     

The cytoplasmic distribution and characterization of poly(A)+RNA in oocytes and embryos of Drosophilia.

Using the presence of poly(A) tracts as a marker for mRNA, we have examined the distribution of this class of RNA between polysomes and free RNP particles. This has been done in mature oocytes and in embryos aged for various times from fertilization through to hatching of a larva. The proportion of ribosomes that are in polysomes to those that are not has been calculated. In mature oocytes, 58% of the poly(A)⁺ RNA and 72% of the ribosomes are not in polysomes. By 1 hr, this drops to 51% of the poly(A)⁺ RNA and 48% of the ribosomes. By 7 hr, a plateau is reached: 30% of each are not in polysomes. The poly(A)⁺ RNA in the cytoplasm of oocytes and 1-hr embryos is found in particles with an average size of 50S and a range of 30–70S. The poly(A)⁺ RNA ranges in size from 7 to 40S, with an average size of 22S. The polyA from this RNA is 50–200 nucleotides long with an average of 115 nucleotides. These data have allowed us to calculate that 1–2% of the total RNA is poly(A)⁺ RNA.     

Oral/aboral ectoderm differentiation of the sea urchin embryo depends on a planar or secretory signal from the vegetal hemisphere

A monoclonal antibody that recognizes oral ectoderm and esophagus of sea urchin larvae was newly produced. Distribution of the antigen, named Hpoe, was examined by indirect immunofluorescence microscopy. Hpoe did not exist in eggs and appeared during the cleavage stage. In hatched blastulae, Hpoe was detected on the apical surface of all cells. As embryogenesis progressed, Hpoe disappeared from the primary mesenchyme, archenteron and aboral ectoderm. Hpoe reappeared in foregut at the prism stage and was restricted to the oral ectoderm and esophagus at the pluteus stage. Using this antigen as a molecular marker of oral/aboral ectoderm differentiation, the role of the vegetal hemisphere in ectoderm differentiation was examined. All animal hemispheres isolated from 16-cell stage embryos, mesenchyme blastulae, early gastrulae and mid gastrulae developed into epithelial balls and every cell expressed Hpoe. These epithelial balls failed in oral/aboral ectoderm differentiation. Twenty millimolar LiCI-treated whole embryos developed into exo-gastrulae but Hpoe restriction in ectoderm occurred in these exo-gastrulae. These results show that oral/aboral ectoderm differentiation requires an inductive interaction from the vegetal hemisphere and indicate that the inductive interaction depends on a planar or secretory signal, rather than the contact of the esophagus and ectoderm.     

Functional heterogeneity of the 30 S ribosomal subunit of Escherichia coli. 3. Requirement of protein S1 for translation

Poly(U)-dependent polyphenylalanine synthesis is completely dependent on the presence of ribosomal protein S1. Polysomes generated under the direction of poly(U) contain approximately one molecule of S1 per ribosome. Isolation of 30 S ribosomes from poly(U)-generated polysomes by a procedure requiring a low concentration of Mg²⁺ (0·25 mM) results in loss of S1. S1 is probably also required for the phage RNA-dependent binding of formylmethionyl-tRNA. The data are discussed in relation to current concepts of the functional aspects of ribosome heterogeneity.     

Polysome Turnover During Amino Acid Starvation in Escherichia coli

The experiments presented in this paper support earlier evidence that ribosomes are released from polysomes when they encounter a codon for which no charged transfer ribonucleic acid is available. However, it is further shown that these ribosomes then reinitiate and resume translation. The size and the level of polysomes during deprival of an amino acid is a function of the frequency with which that particular amino acid appears in cellular proteins. Polysomes from starved cells are more stable than those from growing cells, and, moreover, polysomes from starved relaxed strains are more stable than those from starved stringent strains.     

Uptake and release of63Ni2+ byXenopus embryos during early cleavage stages

Summary Uptake and release of⁶³Ni was studied in dejelliedXenopus laevis embryos exposed to⁶³Ni²⁺ (0.3–30 mol/1) for 0.5-h intervals during the period 1–4.5 h post-fertilization (i.e. from first cleavage to early blastula stage). At first cleavage, the mean uptake of⁶³Ni by embryos was 12-17 times that by non-fertilized eggs, suggesting that conversion of the vitelline envelope to the fertilization envelope enhanced integumental permeability to⁶³Ni²⁺. 63 Ni uptake by embryos at the 1-2-cell stage averaged 1.8–2.5 times that at the early blastula stage. An average of 5% of total⁶³Ni in washed embryos was recovered in isolated fertilization envelopes, indicating that⁶³Ni²⁺ passed through the envelope into internal compartments. Progressive increases of⁶³Ni uptake were seen with increasing exposure levels; after exposure during 1–1.5 h post-fertilization to the highest concentration of⁶³Ni²⁺ (30 mol/1),⁶³Ni uptake averaged 11.4 (SD±5.1) pmol/embryo. Rapid efflux of⁶³Ni was noted after⁶³Ni²⁺-exposed embryos were transferred to nickel-free medium; mean⁶³Ni contents at 0.25 h and 2 h post-exposure diminished to 50% and 15% of the initial values, regardless of the exposure level. The finding thatXenopus embryos are permeable to⁶³Ni²⁺ during early cleavage stages provides a convenient experimental system to investigate the embryotoxicity and teratogenicity of nickel.