The effect of 5-fluorouracil on cells and regenerating protoplasts ofSaccharomyces cerevisiae

The regeneration of the yeast cell-wall was studied using 5-fluorouracil and yeast protoplasts. Protein synthesis in yeast cells (Saccharomyces cerevisiae) was kept reduced in the presence of this inhibitor at a rate corresponding to that before inhibition and was independent on the concentration of the inhibitor (10 or 100 μg/ml). The inhibition of the RNA synthesis was incomplete and dependent on the concentration of the inhibitor. Synthesis of thymidine and DNA was not inhibited as indicated by the growth tests. On the basis of the obtained data it may be concluded that fluorouracil inhibits only thede novo and the induced protein synthesis while permitting protein synthesis that has already been started before inhibition. Fluorouracil was then applied during the regeneration of yeast protoplasts. The results obtained have shown that fluorouracil does not inhibit the synthesis of the yeast cell wall but that the normal course of cell division is impaired by fluorouracil. The low efficiency of the fluorouracil inhibition of the cell wall synthesis indicates that processes leading to the regeneration of the cell wall are in fact only a continuation of those taking place under normal growth conditions.     

Effects of cytochalasin B on division and calcium carbonate extrusion in a calcareous alga.

Cytochalasin B (CB) (100 μg/ml) reversibly blocked cell division and cuased the formation of abnormal cytoplasmic bodies in the alga Cricosphaera carterae. Concentrations of 20 μg/ml and 40 μg/ml CB were without effects. In the presence of CB, calcified bodies (coccoliths) which form in Golgi vesicles and are normally extruded through the plasma membrane were not extruded and accumulated within the cell. CB appeared to alter the membranes of Golgi vesicles containing coccoliths. DMSO (10% $\text{v}\text{v}$), the solvent for CB, was without effect on cell division and coccolith extrusion. A concentration of 20% $\text{v}\text{v}$ DMSO inhibited cell division irreversibly.     

Total carbohydrate determination in dimethyl sulfoxide and guanidine hydrochloride as solvents

An automated total carbohydrate determination method based on phenol-sulfuric acid is presented for use with dimethyl sulfoxide or 4 M guanidine hydrochloride as solvents. The analysis system may be adjusted to yield linear optical density versus concentration plots up to concentrations of 100 μg carbohydrate/ml at concentration gradients of 10 μg/ml between samples. Sampling rates of 20 samples/hr and 30 samples/hr are used for 4 M guanidine hydrochloride and dimethyl sulfoxide, respectively.     

Lepidopteran anti-juvenile hormones: Effects on development of Apanteles congregatus in Manduca sexta

Parasitism by the braconid wasp Apanteles congregatus decreases the effectiveness of the anti-juvenile hormone agents ETB (ethyl 4-[2-{ittert-butyl carbonyloxy}bytoxy]benzoate) and fluoromevalonolactone (FMev) in inducing precocious metamorphosis of Manduca sexta larvae. Topical application of 1–200 μg ETB to parasitized third-instar larvae had no effect on either host or parasite development, whereas doses of 50μg or more ETB applied to unparasitized third-instar larvae caused formation of larval-pupal intermediates after the fourth instar. Parasitism also decreased the effectiveness of 100–200 μg FMev in causing metamorphosis at the moult following its application. In contrast to ETB, FMev disrupted development of the parasitoids. No wasps emerged when preterminal stage hosts were treated with FMev and the hosts formed larval-pupal intermediates. After treatment of terminal stage hosts with FMev, the number of emerging parasitoids was reduced by one-third. Precocene II (100 μg per larvae) had no effect on development of either M. sexta or A. congregatus.     

Biological activity in extracts of ascidians (Tunicata, Ascidiacea) from the northeastern Brazilian coast

Ascidians are marine animals with a great ability to synthesize bioactive substances. This study examined the cytotoxic potential of 10 ascidians found in the coastal waters of Northeast Brazil. Samples of the species Eudistoma vannamei Millar, 1977, Eudistoma sp., Didemnum ligulum Monniot F., 1983, Didemnum psammatodes (Sluiter, 1895), Didemnum sp., Polysyncraton sp., Trididemnum sp., Cystodytes dellechiajei (Della Valle, 1877), Euherdmania sp., and an unidentified species belonging to the Holozoidae family were extracted in methanol 5:1 (v/w). The extracts were tested for cytotoxicity using the brine shrimp lethality assay, sea urchin egg development assay, hemolysis assay, and MTT assay using tumor cell lines. The extract of E. vannamei showed the highest toxicity in brine shrimp (LD₅₀=34.7 μg/ml) and in all tumor cell lines tested, with an IC₅₀ of <2 μg/ml for CEM, 11.2 μg/ml for HL-60, 23.8 μg/ml for B16, and 14.3 μg/ml for HCT-8. In sea urchin eggs, it inhibited the cell cycle progression mainly at the blastula stage (IC₅₀=74.8 μg/ml). The extract of Euherdmania sp. also exhibited some toxicity in these assays, but at a lower potency than that of E. vannamei. The extracts of D. psammatodes and Polysyncraton sp. showed a strong inhibition of the sea urchin egg cell cycle during both phases examined, first cleavage and blastula, with a possible action on the cell microfilament apparatus. The extract of D. ligulum showed selective toxicity toward HCT-8 cells (IC₅₀=35.3 μg/ml). The extract from the Holozoidae was the only one that possessed a hemolytic effect, with an IC₅₀ of 175.2 μg/ml. Further studies are necessary for a better characterization of the active principles of these extracts and a possible elucidation of the mechanisms of action.     

Quantitative in vitro assay to evaluate the capability of yeast cell wall fractions from Trichosporon mycotoxinivorans to selectively bind gram negative pathogens

Yeast cell wall fractions have been proposed to bind enteropathogenic bacteria. The aim of this study was to develop a quantitative assay by measuring the optical density as growth parameter of adhering bacteria. The exponential growth phase of adhering bacteria was determined by optical density reading and compared with the colony count (CFU/mL). A linear regression was compiled and the bacterial number bound to the yeast cell wall product could be determined. Further focus was the investigation of a yeast cell wall from strain Trichosporon mycotoxinivorans (MTV) for its ability to bind gram negative Salmonella, E. coli and Campylobacter strains and gram positive probiotic bacteria of the genera lactobacilli and bifidobacteria as well as gram positive Clostridium perfringens quantitatively. The gram negative probiotic strain E. coli Nissle 1917 was also investigated. Seven out of 10 S. Typhimurium and S. Enteritidis strains adhered to the cell wall product with an amount between 10³ and 10⁴ CFU/10 μg. Four out of 7 E. coli strains showed an average binding capability (10² CFU/10 µg) whereas 4 × 10³E. coli F4 cells bound per 10 μg yeast cell wall. E. coli 0149 K91, E. coli 0147 K89, C. jejuni and C. perfringens as well the genera lactobacilli and bifidobacteria did not bind to the yeast cell wall. E. coli Nissle 1917 was bound with 2 × 10² CFU/10 μg. These results demonstrate that cell wall from MTV can be used to differentially bind E. coli spp. and Salmonella spp. up to 8 × 10⁴ CFU/10 μg. Thus certain yeast cell walls may prevent enteric infections caused by selective bacteria. This methodical approach would be an accurate tool in the feed industry for quality control of yeast cell wall products.     

Action of two juvenile hormone antagonists on lepidopteran epidermis

The juvenile hormone antagonist ETB (ethyl-4-2(t-butylcarbonyloxy)-butoxybenzoate) caused formation of precocious larval-pupal intermediates after the 4th (penultimate)-larval instar of the tobacco hornworm, Manduca sexta, when 50 μg were applied to any 3rd stage larvae or to 4th stage larvae within 12 hr after ecdysis. This dose was most effective within 12 hr after ecdysis to the 3rd stage. In the black mutant larval assay for juvenile hormone, ETB had activity, 0.75 μg per larva giving half-maximal score. In vitro ETB acted as a juvenile hormone to prevent the ecdysteroid-induced change in commitment at concentrations above 0.1 μg/ml with an ED₅₀ at 2.8 μg/ml and as a partial juvenile hormone antagonist to 0.1 μg/ml juvenile hormone I at concentrations between 10^?3 and 10^?2 μg/ml. By contrast, EMD (ethyl-E-3-methyl-2-dodecenoate) had little juvenile hormone-like activity in vitro up to its limits of solubility (100 μg/ml) and exhibited sporadic partial juvenile hormone antagonistic activity in vitro at concentrations between 1 and 100 μg/ml. Since these concentrations were 10–1000 times that of juvenile hormone I in the medium, EMD apparently is not an efficient competitor.     

R-Plasmids in Pasteurella multocida.

Multiple drug-resistant strains of Pasteurella multocida were associated with a high incidence of fatal pneumonia in feedlot cattle. A representative strain, CAH160, resistant to tetracycline (Tc), streptomycin (Sm), and sulfonamide (Su) was studied. The minimal inhibitory concentration (MIC) of Tc was 32 μg/ml while Sm had an MIC of 256 μg/ml. Plasmid DNA was isolated from CAH160 by cesium chloride-ethidium bromide centrifugation. Agarose gel electrophoresis showed that at least three distinct species of plasmid DNA were present. DNA isolated from CAH160 was used to transform Escherichia coli K12 strain C600 r_k^?m_k^?. Transformants resistant to Tc; to Sm, Su; and to Tc, Sm, Su were obtained. Contour length measurements of plasmid DNA isolated from transformant cells showed that Tc resistance was associated with a 3-Mdal plasmid (pSR10), while Sm, Su resistance resided on a 2.7-Mdal molecule (pSR11). More than 20% of the transformants were resistant to Tc, Sm, Su and contained both plasmid species. In E. coli the MIC of Tc was 256 μg/ml and that of Sm was 64 μg/ml. The buoyant density of pSR10 was 1.699 g/cm³, while the density of pSR11 was 1.709 g/cm³.     

Ultrastructural Alterations of Candida albicans Induced by a New Imidazole Antimycotic Omoconazole Nitrate

The antifungal effects of an imidazole-antimyeotic omoconazole nitrate (OMZ) on the morphology and ultrastructure of Candida albicans yeast cells were studied using scanning and transmission electron microscopy. The treatment of growing Candida cultures with fungistatic doses (0.4 to 4 μg/ml) of OMZ produced the formation of a chain or cluster of cells. Thickening of the cell wall and accumulation of electron-dense vesicles in the wall were clearly observed. Development of Golgi-like complex membranous structures in the cytoplasm was the most prominent finding. The cytological alteration induced by exposure to a higher concentration (40 μg/ml) of the drug was characterized by disruption of the intracytoplasmic organelles. Our results confirm the strong antifungal activity of OMZ against fungal cells.     

The composition of fresh and stored oyster mushrooms (Pleurotus ostreatus)

Soluble carbohydrate, protein, polysaccharide and cell wall composition were assayed in freshly harvested Pleurotus ostreatus sporophores and those stored for 4 days at 2° or 18°. Mannitol and trehalose were present at 1.8 and 6.5% dry wt respectively in fresh sporophores, and at reduced levels in those stored at 18°. In sporophores stored at 2°, trehalose levels increased by up to 122%. Soluble polysaccharide appeared to be composed of glycogen-like material, which was susceptible to post-harvest breakdown, and components containing mannose and other sugars. The total protein content was 42% dry wt; no protein degradation was seen in sporophores stored at 2°, but about 25% of the protein disappeared during storage at 18°. Cell wall polysaccharide was utilised during storage. Respiration rate was about 8–10 ml CO₂/g dry wt/hr at harvest and declined to about 5 ml/g dry wt/hr after 40 hr storage at 18°.     

Stability of rapidly labelled messenger ribonucleic acid in Bacillus amyloliquefaciens during the phases of minimum and maximum extracellular enzyme formation.

The stability of rapidly labelled hybridizable messenger RNA in both exponential and post-exponential phase cells of Bacillus amyloliquefaciens was measured in terms of the rate of loss of its radioactivity. In the exponential phase, where 96% of the mRNA was specific for cell proteins and only 4% was exoprotein mRNA, the label was lost exponentially from the rapidly labelled hybridizable mRNA fraction with a half-life of six minutes at 30 °C. The antibiotic rifampicin, at a concentration of 10 μg/ml, had no effect on the characteristics of decay of this exponential-phase mRNA. In the post-exponential phase, where there were equal amounts of cell protein and exoprotein-specific mRNA, rapidly labelled hybridizable mRNA decayed exponentially in the presence of rifampicin (10 μg/ml), with a half-life of six minutes at 30 °C. In the absence of rifampicin the characteristics of decay were more complex. The evidence available suggested that this was due to the superimposition of a component attributable to reincorporation of degradation products of radioactive RNA on the characteristic exponential decay pattern of the post-exponential mRNA.Measurement of the stability of active mRNA, by studying the loss of ability to incorporate l-[¹⁴C]leucine into protein in the presence of rifampicin (10 μg/ml), gave half-lives of 4.5 minutes and six minutes, respectively, for exponential and post-exponential material.     

Lamotrigine analysis in plasma by gas chromatography–mass spectrometry after conversion to a tert.-butyldimethylsilyl derivative

Lamotrigine (lamictal) is a new anticonvulsant drug recently approved by the FDA for clinical use. Therapeutic monitoring of lamotrigine is useful for patient management (therapeutic range 1–4 μg/ml). Here we describe a gas chromatography–mass spectrometric identification and quantitation of lamotrigine after extraction from human serum and derivatization. Lamotrigine was extracted from alkaline serum with chloroform and derivatized with N-methyl-N-(tert.- butyldimethysilyl) trifluoroacetamide containing 2% tert.-butyldimethylchlorosilane. Oxazepam-d₅ was used as an internal standard. The tert.-butyldimethylsilyl derivative of lamotrigine showed distinct molecular ions at m/z 483 and 485 as well as other peaks at m/z 426, 370 and 334 for unambiguous identification. The base peak was observed at m/z 199. Similarly, the tert.-butyldimethysilyl derivative of oxazepam-d₅ showed molecular ions at m/z 519 and 521 along with other characteristic peaks at m/z 462, 376 and 318. For the analysis of lamotrigine, the mass spectrometer was operated in the selective ion monitoring mode. The within-run and between-run precisions were 4.3% (mean=3.01, S.D.=0.13 μg/ml) and 5.1% (mean=2.93, S.D.=0.15 μg/ml), respectively at a serum lamotrigine concentration of 3.0 μg/ml. The within-run and between-run precisions were 8.2% (mean=0.49, S.D.=0.04 μg/ml) and 10.6% (mean=0.47, S.D.=0.05 μg/ml), respectively at a serum lamotrigine concentration of 0.5 μg/ml. The assay was linear for serum lamotrigine concentrations of 0.5–20 μg/ml. The detection limit was 0.25 μg/ml. The assay was free from interferences from common tricyclic antidepressants, benzodiazepines, other common anticonvulsants, salicylate and acetaminophen.     

Chemical profiling of Streptomyces sp. Al-Dhabi-2 recovered from an extreme environment in Saudi Arabia as a novel drug source for medical and industrial applications

Filamentous bacterial belonged to Streptomyces species were novel drug source for medical and industrial applications. However, the detailed identification of Streptomyces species from Saudi Arabian extreme environment for the identification novel drug source for medical and industrial applications were rarely studied. The Streptomyces strain Al-Dhabi-2 obtained from the thermophilic region kingdom of Saudi Arabia, exhibited antimicrobial potentials against the pathogenic microorganism were characterized. Biochemical and phylogenetic analysis confirmed that the strain was closely associated to the Streptomyces species. The chromatogram of GC-MS analysis of this ethyl acetate extract (EA) had diverse of chemical compounds namely benzene acetic acid (7.81%), acetic acid, methoxy-, 2-phenylethyl ester (6.01%) were the major compounds. EA of Al-Dhabi-2 showed inhibition zone ranged from 14 to 25 mm at 5 mg/well concentration against the tested microbial pathogens. Results revealed that the significant MIC values were observed against B. cereus, and E. faecalis by (less than 39 μg/ml) and against S. agalactiae with (78 μg/ml). Minimum inhibitory concentrations (MIC) for fungi: were also reported against Cryptococcus neoformans and Trichophyton mentagrophytes by (156 μg/ml), whilst Candida albicans and Aspergillus niger by (312 μg/ml). Results of this study showed that thermophilic actinobacteria could be promise source in the context of searching for unique antimicrobial agents with novel properties.     

Effect of melittin-induced membrane alterations on rat heart adenylate cyclase activity

Melittin, a surface-active, 26-amino acid polypeptide from bee venom, has been reported to alter a variety of membrane properties including stability, permeability, and fluidity, the latter having been shown to be altered in a biphasic manner. Melittin induced a biphasic alteration of rat heart microsomal adenylate cyclase activity, stimulating it at low concentration (<30 μg/ml) and inhibiting it at higher concentrations (100 μg/ml or higher). Melittin potentiated sodium fluoride and 5′-guanylylimidodiphosphate activation of adenylate cyclase below 40 μg/ml but it inhibited at high concentrations, except in the presence of high concentrations of 5′-guanylylimidodiphosphate (10^?4m). Basal and fluoride-activated adenylate cyclase exhibited no significant change in the K_m for ATP in the presence of melittin at <40 μg/ml, but the V was elevated. Potentiation by melittin of adenylate cyclase was observed at all fluoride, 5′-guanylylimidodiphosphate, and magnesium concentrations tested. The observed effects of melittin on rat heart adenylate cyclase are consistent with it acting by altering the properties of membrane lipids with which the enzyme is associated.     

Inhibition by heparin and dextran sulfate of stimulated rat pancreatic adenylate cyclase

Heparin inhibited the adenylate cyclase activity of semipurified rat pancreatic plasma membranes stimulated by hormones and by Gpp(NH)p but not by fluoride or when in the persistently active state. When observed, the inhibition was rapid and sustained. It was of a noncompetitive type and never exceeded 20% for secretin. The inhibition of Gpp(NH)p-stimulated activity was more pronounced (48% inhibition at a heparin concentration of 50 μg/ml). For the C-terminal octapeptide of pancreozymin (CCK-8)-stimulated adenylate cyclase, the inhibition amounted to 93% at 50 μg/ml. This inhibition was competitive at low heparin concentration and of a mixed type above 10 μg/ml. Besides, heparin inhibited (I₅₀ = 6 μg/ml) the binding of peptides of the CCK family to their specific receptors without affecting the apparent K_d value of binding. Taken together, these relatively specific effects of heparin gave evidence in favor of the existence of CCK spare receptors. Dextran sulfate was more potent than heparin as an inhibitor of adenylate cyclase activation while chondroitin-4-sulfate and chondroitin-6-sulfate were ineffective. Dansylated pancreatic plasma membranes exhibited characteristics of adenylate cyclase activation by CCK-8 which were similar to those found for untreated membranes exposed to heparin.     

Determination of trovafloxacin,a new quinolone antibiotic,in biological samples by reversed-phase high-performance liquid chromatography

A simple, accurate and precise high-performance liquid chromatographic method was developed and validated for the determination of trovafloxacin, a new quinolone antibiotic, in serum and urine. Following solid-phase extraction, chromatographic separation was accomplished using a C₁₈ column with a mobile phase consisting of 0.04 M H₃PO₄-acetonitrile-tetrabutylammonium hydroxide-0.005 M dibutyl amine phosphate (D-4) reagent (83:16.85:0.05:0.1, v/v), pH 3. Trovafloxacin and the internal standard (a methyl derivative of trovafloxacin) were detected by ultraviolet absorbance at 275 nm. The lower limit of quantification for trovafloxacin was 0.1 μg/ml and the calibration curves were linear over a concentration range of 0.1 to 20..0 μg/ml (r² = 0.9997). The average recoveries were greater than 70% for both trovafloxacin and internal standard. The intra-day and inter-day coefficients of variation were generally less than 5% in urine and serum over the concentration range of 0.1 to 20.0 μg/ml. Human serum samples could be stored for up to 12 months at −20°C and urine samples could be stored up to 18 months at −80°C.     

Ability of intestinal lactic bacteria to bind or/and metabolise phenol and p-cresol

Intestinal microflora can contribute to colon cancer by the production of substances playing a role in carcinogenesis. Metabolites of protein fermentation in the colon, such as ammonia, H₂S, indole, phenol, skatole are toxic. Lactic bacteria existing in the colon may exert an anti-carcinogenic action, but the mechanism is poorly understood. In the present study the ability of intestin|al lactobacilli to bind or metabolise phenol and p-cresolin vitro was determined.Lactobacillus strains were cultivated in MRS and in a modified MRS broth with reduced concentrations of carbon source. Phenol and p-cresol content in the media were from 2 to 10 μg/ml. In MRS medium lactobacilli could decrease the concentration of phenol and p-cresol and it was 0.2-5.8 μg/ml for phenol and 0.2-1.4 μg/ml for p-cresol. After cultivation in a modified MRS broth, the decrease was 0.5-2.0 μg/ml for phenol and 0.5-2.4 μg/ml for p-cresol. The binding capacity of bacterial cells was rather low. After incubation of non-growing bacteria the decrease of phenol concentration was 0.1-0.5 μg/ml and p-cresol 0.1-2.8 μg/ml. But the ability of growing lactobacilli to metabolise the compounds cannot be excluded. After interaction of lactobacilli with 10 μg/ml of phenol they displayed a lower genotoxicity, as evaluated by the alkaline comet assay. The phenomenon not always depended on the decrease of phenol concentration, but on the medium, the strain of bacteria and for phenol it ranged from 32 to 48%.Lactobacillus strains tested did not lower the genotoxicity of p-cresol.     

Streptomycin resistance in Rhizobium japonicum

Mutants resistant to varying concentrations of streptomycin were recovered from two streptomycin-sensitive, effective nitrogen-fixing strains of Rhizobium japonicum. To determine if there were an upper limit of resistible antibiotic concentration, 3 mutants which were resistant to 10000 μg/ml were challenged by higher concentrations of streptomycin. Only one grew well at 25000 μg/ml, and none grew at 50000 μg/ml. All mutants maintained a smooth colonial morphology, and none exhibited streptomycin-dependence. Streptomycin-resistant mutants of both strains were examined for properties of infectivity and effectiveness. All mutants tested retained the symbiotic properties of the parental strains. The retention of these parental properties by the streptomycin-resistant mutants of R. japonicum is different from the properties described for phenotypically similar mutants in certain other rhizobial species.     

Anticoccidial drugs: Effects on infectivity and survival intracellularly of Eimeria tenella sporozoites

The effects of various anticoccidial drugs on extracellular and intracellular sporozoites were studied in cell culture and in chickens. Treatment of freshly excysted, extracellular sporozoites of Eimeria tenella for 18 hr with monensin, decoquinate, or robenidine at 100 ppm had no effect on oocyst production 7–10 days after the sporozoites were rinsed free of drugs and fed to chickens. Treatment of cultures of E. tenella in chick kidney cell monolayers with monensin (0.001 μg/ml), decoquinate (0.01 μg/ml), zoalene (20.0 μg/ml), or robenidine (0.01 μg/ml) had no effect on intracellular sporozoites at 4 hr following introduction of sporozoites and drugs into the culture. A significant reduction of intracellular parasites occurred at 24 hr in the cultures treated with monensin or zoalene. Remaining intracellular sporozoites in monensin-treated cultures were morphologically abnormal or degenerate, while sporozoites in other cultures appeared normal. The number and condition of sporozoites in the nontreated cultures were unchanged at 24 hr postinoculation. These results indicate that sporozoites undergo changes subsequent to penetration of host cells that render them susceptible to drug action.     

The relationship of protein synthesis to cell division and oral development in synchronized Tetrahymena pyriformis GL-C: An analysis employing cycloheximide

The effects of cycloheximide on synchronized Tetrahymena pyriformis strain GL-C were investigated at concentrations ranging from 0.01 to 10 μg/ml. The initial inhibition of protein synthesis was nearly total (>85%) at 1 μg/ml and above, partial (50–80%) at 0.2 to 0.05 μg/ml, and slight (<30%) at 0.02 μg/ml. Eventual recovery of protein synthesis to a rate approaching that of the controls took place at concentrations of 1 μg/ml and less. When the drug was added before a “transition point” at 55 minutes after the end of the synchronizing treatment (EST), cell division was blocked by 10 μg/ml, and delayed at concentrations of 1 μg/ml or less. The duration of delay was related to the degree of initial inhibition, and to the time required for recovery of protein synthesis; it also depended on the time after EST at which the drug was added. At a given concentration, maximum division delay was observed just prior to the “transition point;” this maximum delay was correlated with resorption of differentiating oral primordia, followed by the appearance of new primordia. The lesser delays observed at earlier times were correlated with temporary blockage of development of primordia in the “stomatogenic field” stage. Resumption of oral primordium development was, in both cases, temporally correlated with a substantial recovery of protein synthesis. After the “transition point,” cell division, and completion of oral development, was delayed slightly at the lower concentrations, and more substantially at 1 and 10 μg/ml, with some division-arrest at the latter concentration. Except for the recovery phenomenon, the developmental responses elicited by cycloheximide were similar to those observed earlier with puromycin. The bearing of these findings on the mechanism of synchronization in Tetrahymena is considered in the Discussion.