Behavior of hobo and P transposons in yellow 2-717 unstable line of Drosophila melanogaster and its derivatives after crossing with a laboratory strain

Using fluorescent in situ hybridization technique (FISH), the frequency of hobo and P mobile elements transpositions on X chromosomes from the y ^2-717 , isolated from the Uman’ population of Drosophila melanogaster, as well as from its phenotypically normal and mutant derivatives, obtained as a result of crosses the males examined with the C(I)DX,ywf/Y females, was evaluated. It was demonstrated that the maximum frequency of hobo transpositions on X chromosomes of the males from derivative strains, subjected to repeated hobo-dysgenic crosses reached a value of 1.2 × 10^?2 per site per X chromosome per generation. The number of hobo copies in male X chromosomes from derivative strains was 3 times higher than in the original initial strain. Furthermore, the “old” hobo sites remained unchanged. In derivative strains, the frequency of hobo insertion was higher than that of excisions. One of the derivative strains, y ^1t-717a1k3-2 , was characterized by high intrastrain instability of hobo element localization. In the y ^2-717a1k3 and y ^1t-717a1k3-2 strains a large inversion, In(1)IB; 13CD, was described. At the absence of the full-sized P element in the strains involved in crosses, maximum frequency of P element transpositions in the derivative strains reached a value of 1.2 × 10^?2 per site per X chromosome per generation.     

Isolation and Estimation of Cytokinins and Cytokinin-Containing Transfer RNAs from Cucumis sativus L. var. Guntur Seedlings

N⁶-(Δ²-Isopentenyl) adenosine antibodies were used for the isolation of free cytokinins and cytokinin-containing tRNAs from parts of Cucumis sativus L. var. Guntur seedlings and for the estimation of cytokinins in them. Immobilized N⁶-(Δ²-isopentenyl) adenosine antibodies retained tRNAs containing N⁶-(Δ²-isopentenyl) adenosine and N⁶-(4-hydroxy-3-methylbut-2-enyl) adenosine with equal efficiencies. There were at least five cytokinins in the free form in cucumber seedlings. N⁶-(4-Hydroxy-3-methylbut-2-enyl) adenosine, N⁶-(Δ²-isopentenyl) adenosine, and N⁶-(Δ²-isopentenyl) adenine were present at least to the extent of 80, 23, and 9 nanograms, respectively, in the cotyledons and 40, 6, and 3 nanograms, respectively, in the decotyledonated seedlings per gram of tissue. Only two cytokinins were found in the tRNAs of cucumber cotyledons, namely N⁶-(Δ²-isopentenyl) adenosine and N⁶-(4-hydroxy-3-methylbut-2-enyl) adenosine in amounts of 12 and 318 nanograms, respectively, per gram of tissue. Immunoaffinity chromatographic analysis of radiolabeled aminoacyl tRNAs from cucumber cotyledons showed that tRNA^Phe and tRNA^Tyr contained cytokinins whereas tRNA^Ala did not.     

The effect of γ-radiation on induction of the hobo element transposition in Drosophila melanogaster

The transposition frequency of the hobo mobile element in four successive generations of Drosophila melanogaster strain y ^2-717 after an acute γ-irradiation with a dose of 30 Gr amounted to 7.5 × 10^?4 per site per genome per generation. Under the same conditions, PCR analysis of the genomic DNA of y ^2-717 flies detected new variants of defective hobo sequence. No changes in the hobo localization and PCR products compared with the control were detected in the case of single irradiation with doses of 3 and 30 Gr. The localizations of hobo element on polytene chromosomes of y ^2-717 strain did not change during 11 generations after five exposures of flies to 30 Gr. Irradiation of a highly unstable D. melanogaster strain y ⁺⁷⁴³ did not increase the number of families with mutant progeny, yet increased the total number of mutant descendants almost twofold, from 5 to 9%.     

Metabolism of cytokinin: ribosylation of cytokinin bases by adenosine phosphorylase from wheat germ

As part of the study of cytokinin metabolic pathways, an enzyme, adenosine phosphorylase (EC 2.4.2.-), which catalyzed the ribosylation of N⁶-(Δ²-isopentenyl)adenine, N⁶-furfuryladenine, and adenine to form the corresponding nucleosides, was partially purified from wheat (Triticum aestivum) germ. The pH optimum for the ribosylation of the cytokinins and adenine was from 6.5 to 7.8; for guanine and hypoxanthine it was from 7.0 to 8.5 At pH 7.2 (63 millimolar N-2-hydroxyethyl piperazine-N′-ethanesulfonic acid) and 37 C the K_m for N⁶-(Δ²-isopentenyl)adenine was 57.1 micromolar; N⁶-furfuryladenine, 46.5 micromolar; adenine, 32.2 micromolar; and the V_max for N⁶-(Δ²-isopentenyl)adenine, N⁶-furfuryladenine, and adenine were 134.7, 137.1, and 193.1 nanomoles per milligram protein per minute, respectively. The equilibrium constants of the phosphorolysis of N⁶-(Δ²-isopentenyl)adenosine and adenosine by this enzyme indicated that the reaction strongly favored nucleoside formation. This enzyme was shown to be distinct from inosine-guanosine phosphorylase based on the differences in the Sephadex G-100 gel filtration behaviors, pH optima, and the product and p-hydroxymercuribenzoate inhibitor studies. These results suggest that adenosine phosphorylase may play a significant role in the regulation of cytokinin metabolism.     

Dynamics and Distribution of Cyanophages and Their Effect on Marine Synechococcus spp.

Cyanophages infecting marine Synechococcus cells were frequently very abundant and were found in every seawater sample along a transect in the western Gulf of Mexico and during a 28-month period in Aransas Pass, Tex. In Aransas Pass their abundance varied seasonally, with the lowest concentrations coincident with cooler water and lower salinity. Along the transect, viruses infecting Synechococcus strains DC2 and SYN48 ranged in concentration from a few hundred per milliliter at 97 m deep and 83 km offshore to ca. 4 × 10⁵ ml^-1 near the surface at stations within 18 km of the coast. The highest concentrations occurred at the surface, where salinity decreased from ca. 35.5 to 34 ppt and Synechococcus concentrations were greatest. Viruses infecting strains SNC1, SNC2, and 838BG were distributed in a similar manner but were much less abundant (<10 to >5 × 10³ ml^-1). When Synechococcus concentrations exceeded ca. 10³ ml^-1, cyanophage concentrations increased markedly (ca. 10² to > 10⁵ ml^-1), suggesting that a minimum host density was required for efficient viral propagation. Data on the decay rate of viral infectivity d (per day), as a function of solar irradiance I (millimoles of quanta per square meter per second), were used to develop a relationship (d = 0.2610I - 0.00718; r² = 0.69) for conservatively estimating the destruction of infectious viruses in the mixed layer of two offshore stations. Assuming that virus production balances losses and that the burst size is 250, ca. 5 to 7% of Synechococcus cells would be infected daily by viruses. Calculations based on contact rates between Synechococcus cells and infectious viruses produce similar results (5 to 14%). Moreover, balancing estimates of viral production with contact rates for the farthest offshore station required that most Synechococcus cells be susceptible to infection, that most contacts result in infection, and that the burst size be about 324 viruses per lytic event. In contrast, in nearshore waters, where ca. 80% of Synechococcus cells would be contacted daily by infectious cyanophages, only ca. 1% of the contacts would have to result in infection to balance the estimated virus removal rates. These results indicate that cyanophages are an abundant and dynamic component of marine planktonic communities and are probably responsible for lysing a small but significant portion of the Synechococcus population on a daily basis.     

Structure of the 3-deoxy-d-manno-octulosonic acid-containing polysaccharide (K6 antigen) from escherichia coli LP 1092

The acidic polysaccharide (K6) antigen from Escherichia coli LP 1092 contains d-ribose and 3-deoxy-d-manno-octulosonic acid in the molar ratio of 2:1, respectively. Spectroscopic data (¹³C- and ¹H-n.m.r.), methylation analyses, and periodate oxidation indicate that the polysaccharide is composed of the foregoing components essentially in the following trisaccharide sequence: →2)-β-d-Ribf-(1→2)-β-d-Ribf-(1→7)-α-d-KDO-(2→The polysaccharide also contains O-acetyl substituents (～0.2–0.3 mol per KDO residue).     

Mechanism for the differential translocation of amiben in plants

The proportion of the total plant radioactivity present in shoots at the end of a 24-hour exposure of the roots to 0.5 milligram per liter ¹⁴C-3-amino-2,5-dichlorobenzoic acid (¹⁴C-amiben) ranged from 1.4 to 74.3% in 13 species. When roots of 10-day-old wheat (Triticum aestivum L. em. Thell., Triumph) and 13-day-old barnyard grass (Echinochloa crusgalli L. Beauv.) plants were treated with 0.5 milligram per liter ¹⁴C-amiben for 12 or 24 hours, barnyard grass shoots contained at least eight times more of the total plant radioactivity than did wheat shoots. In similar experiments with ¹⁴C-2-chloro-4-(ethylamino)-6-(isopropylamine)-s-triazine (¹⁴C-atrazine), there were no differences in translocation between these two species.     

Hydroxylation of N-(Delta-Isopentenyl)adenine to Zeatin: Relative Activities of Organ Systems from Actinidia arguta

By incubating explants from Actinidia arguta seedlings on a nutrient medium supplemented with 20 to 30 micromolar N⁶-(Δ²-isopentenyl)adenine (i⁶Ade) and then measuring zeatin (io⁶Ade) accumulation in tissues, the distribution of i⁶Ade hydroxylase activities in whole plants could be determined. Based on analyses with three entire plants, it is estimated that, as an organ system, roots contain approximately 68% of the plant's hydroxylase, while stems and leaves account for about 26% and 6%, respectively, of the total activity. Depending on the part of the root examined, hydroxylase activities ranged from 20 to 148 nanomoles io⁶Ade accumulated per gram fresh weight per 24 hours of incubation. Stem activities ranged from 17 to 165 nanomoles per gram fresh weight per 24 hours with the lowest activities being found at the tip. Leaf activities were substantially lower (1-10 nanomoles per leaf depending on position) than either root or stem.     

Cytokinin oxidase from wheat: partial purification and general properties

As part of the study of the possible role(s) of CBF-1, a cytokinin-binding protein abundant in wheat embryo, a cytokinin oxidase was found in wheat (Triticum aestivum L.) germ and partially purified by conventional purification techniques and high performance chromatofocusing. This preparation catalyzes conversion of N⁶-(Δ²-isopentenyl)adenosine to adenosine at a V_max of 0.4 nanomol per milligram protein per minute at 30°C and pH 7.5, the K_m being 0.3 micromolar. This high affinity and the apparent molecular weight of 40,000 estimated by high performance gel permeation on a Spherogel TSK-3000 SW column indicate that this enzyme is different from other cytokinin oxidases previously reported. Oxygen is required for the reaction, as for other cytokinin oxidases already described. N⁶-(Δ²-isopentenyl)adenine and zeatin riboside are also degraded, but N⁶-(Δ²-isopentenyl)adenosine-5′-monophosphate is apparently not a substrate. Benzyladenine is degraded, but to a small extent, and it inhibits slightly the degradation of N⁶-(Δ²-isopentenyl)adenosine. The degradation of N⁶-(Δ²-isopentenyl)adenosine is strongly inhibited by diphenylurea and its highly active derivative N-(2-chloro-4-pyridyl)-N′-phenylurea.     

Altitudinal Change in the Photosynthetic Capacity of Tropical Trees: A Case Study from Ecuador and a Pantropical Literature Analysis

In tropical mountains, trees are the dominant life form from sea level to above 4,000-m altitude under highly variable thermal conditions (range of mean annual temperatures: <8 to >28°C). How light-saturated net photosynthesis of tropical trees adapts to variation in temperature, atmospheric CO₂ concentration, and further environmental factors, that change along elevation gradients, is not precisely known. With gas exchange measurements in mature trees, we determined light-saturated net photosynthesis at ambient temperature (T) and [CO₂] (A _sat) of 40 tree species from 21 families in tropical mountain forests at 1000-, 2000-, and 3000-m elevation in southern Ecuador. We tested the hypothesis that stand-level averages of A _sat and leaf dark respiration (R _D) per leaf area remain constant with elevation. Stand-level means of A _sat were 8.8, 11.3, and 7.2?μmol?CO₂?m^?2?s^?1; those of R _D 0.8, 0.6, and 0.7?μmol?CO₂?m^?2?s^?1 at 1000-, 2000-, and 3000-m elevation, respectively, with no significant altitudinal trend. We obtained coefficients of among-species variation in A _sat and R _D of 20–53% (n?=?10–16 tree species per stand). Examining our data in the context of a pan-tropical A _sat data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based A _sat decreases in tropical mountains by, on average, 1.3?μmol?CO₂?m^?2?s^?1?per?km altitude increase (or by 0.2?μmol?CO₂?m^?2?s^?1 per K temperature decrease). The A _sat decrease occurred despite an increase in leaf mass per area with altitude. Local geological and soil fertility conditions and related foliar N and P concentrations considerably influenced the altitudinal A _sat patterns. We conclude that elevation is an important influencing factor of the photosynthetic activity of tropical trees. Lowered A _sat together with a reduced stand leaf area decrease canopy C gain with elevation in tropical mountains.     

Isolation and Purification of Staphylococcal Lipase

An extracellular lipase was isolated from the cell-free supernatant fluid of a 24-hr culture of Staphylococcus aureus grown in Trypticase Soy Broth at 37 C with continuous agitation. The purification was achieved by precipitation with alcohol followed by differential precipitation at pH 8.6 and 4.3. Subsequent purification with Sephadex G 200 and BioGel 300 yielded a preparation which showed a 350- to 450-fold increase in specific activity over the original cell-free supernatant fluid. The purified lipase was homogeneous over a BioGel 300 column and showed a single peak on electrophoresis in a Veronal buffer (pH 8.6, Γ/2 = 0.1). The electrophoretic mobility was -7.78 × 10^-5 cm² per v per sec.     

ATP-dependent calcium transport and its correlation with Ca2+-ATPase activity in basolateral plasma membranes of rat duodenum

Isolated basolateral plasmamembrane vesicles from rat duodenum epithelial cells exhibit ATP-dependent calcium-accumulation and Ca²⁺-dependent ATPase activity. Calcium accumulation stimulated by ATP is prevented by the calcium ionophore A23187, inhibited 80% by 0.1 mM orthovanadate but is not effected by oligomycin. Calcium accumulation is not observed with the substrate β-γ-(CH₂)-ATP, ADP and p-nitrophenyl phosphate. Kinetic studies reveal an apparent K_m of 0.2 μM Ca²⁺ and a V_max of 5.3 nmol Ca²⁺/min per mg protein for the ATP-dependent calcium-uptake system. Calmodulin and phenothiazines have no effect on calcium accumulation in freshly prepared membranes, but small effects are inducable after a wash with a 5 mM EGTA. The kinetic parameters of Ca²⁺-ATPase are: K_m = 0.25 μM Ca²⁺ and V_max = 19.2 nmol P_i/min per mg protein. Three techniques, osmotic shock, treatment with Triton X-100 or the channel-forming peptide alamethacin, reveal that about 40% of the vesicles are resealed. Assuming that half of the resealed vesicles have an inside-out orientation, the V_max of ATP-dependent calcium uptake amounts to 25 nmol Ca²⁺/min per mg protein and of the Ca²⁺-ATPase to 23 nmol P_i/min per mg protein. The close correlation between kinetic parameters of Ca²⁺-ATPase and ATP-dependent calcium-transport strongly suggests that both systems are expressions of a Ca²⁺-pump located in duodenal basolateral plasma membranes.     

Decreased Exopolysaccharide Synthesis by Anaerobic and Symbiotic Cells of Bradyrhizobium japonicum

Experiments were conducted to determine whether symbiotic bacteroids of Bradyrhizobium japonicum produce exopolysaccharide within soybean (Glycine max [L.] Merr. cv `Lee 74') nodules. B. japonicum strains RT2, a derivative of USDA 110 with resistance to streptomycin and rifampicin, and RT176-1, a mutant deficient in exopolysaccharide synthesis, were used. Although aerobically cultured RT2 produced 1550 micrograms of exopolysaccharide per 10¹⁰ cells, root nodules formed by RT2 contained only 55.7 micrograms of polysaccharide per 10¹⁰ bacteroids, indicating that little exopolysaccharide synthesis occurred within the nodules. The polysaccharide level of RT2 nodules was about equal to that of nodules containing the exopolysaccharide mutant RT176-1 (61.0 micrograms per 10¹⁰ bacteroids). Gas chromatographic analysis showed that the sugar composition of polysaccharide from nodules of RT2 or RT176-1 was almost the same as that of polysaccharide from unnodulated root tissue, but differed strikingly from that of rhizobial exopolysaccharide from aerobic cultures. Thus, the host plant and not the bacteroids was probably the source of most or all of the polysaccharide in the nodule extracts. Also, bacteroids from nodules failed to bind soybean lectin, confirming the absence of an exopolysaccharide capsule.     

Metabolic behavior of acetyl glyceryl ether phosphorylcholine on interaction with rabbit platelets

The metabolic fate of 1-O-[³H]alkyl-2-acetyl-sn-glycero-3-phosphorylcholine ([³H]-AGEPC) upon interaction with rabbit platelets was investigated. [³H]AGEPC was converted to a product identified as the long-chain fatty acyl analog. The reaction was unaffected by extracellular calcium. After a lag time of 30 to 60 s the kinetics of the conversion was linear. The rate of the reaction was found to be a function of platelet and AGEPC concentrations. Of the [³H]AGEPC (10^?9m) 85 ± 5% was processed into the-long chain fatty acyl analog within 1 h when incubated at 37 2C with a 1.25 × 10⁹ platelets per milliliter suspension. A maximal number of 1200 to 3600 [³H]AGEPC molecules were converted to the long-chain fatty acyl derivative per minute per platelet in the presence of 2 mm EDTA. Under similar conditions the 1-O-[³H]alkyl-2-(lyso)-sn-glycero-3-phosphorylcholine ([³H]lysoGEPC) also was transformed to a comparable long-chain fatty acyl derivative at a much slower rate and to a lower extent. No significant increase in lysoGEPC was noted in incubation mixtures containing [³H]AGEPC. The possible direct transacylation of AGEPC upon interaction with platelets is discussed as well as the possible involvement of this reaction in directly triggering the platelet response to AGEPC stimuli.     

N-(Delta-Isopentenyl)adenosine: Its Occurrence as a Free Nucleoside in an Autonomous Strain of Tobacco Tissue

Cytokinins from both the free nucleoside pool and the transfer RNA have been isolated and identified in a habituated strain of tobacco pith callus (Nicotiana tabacum [L] var. Wisconsin 38). The transfer RNA of this strain contains both N⁶-(Δ²-isopentenyl) adenosine and N⁶-(4-hydroxy-3-methylbut-2-cis-enyl) adenosine. The trans-hydroxylated derivative is absent from the transfer RNA of this dark-grown tissue. N⁶-(Δ²-Isopentenyl)-adenosine was identified as a component of the free nucleoside pool in concentrations of about 10 micrograms per kilogram of tissue.     

43Ca nuclear magnetic resonance of Ca2+–calmodulin solutions: Effects of trifluoperazine and peptides

By using a conventional FT NMR spectrometer and probe, we first detected ⁴³Ca NMR spectra of the Ca²⁺ (2.9 mM): calmodulin (0.725 mM) (1:1 per binding site) complex in 0.15 M N-2-hydroxyethylpiperazine N′-2-ethanesulfonic acid (HEPES)–K⁺ buffer (pH 7.2). The half-band width of the complex was nearly 160 Hz and the signal of the complex was located at the 2.13 ppm (43 Hz) lower field from that of the free Ca²⁺ ion. By adding trifluoperazine, melittin, substance P or glucagon, the half-band widths of the Ca²⁺–calmodulin complex (1:1 per binding site) were remarkably reduced and the chemical shifts of the complex moved back to the upper field. It is suggested that the Ca²⁺ ion may bind to Ca²⁺ low-affinity sites more tightly in the presence of those effectors than in their absence.     

Ozone Inhibition of Photosynthesis in Chlorella sorokiniana

Exposure of Chlorella sorokiniana (07-11-05) to ozone inhibits photosynthesis. In this study, the effects of ozone on O₂ evolution and fluorescence yields are used to characterize this inhibition. At an ozone dose of about 3 micromoles delivered to 2 × 10⁹ cells, the photosynthetic rate of the cells is inhibited 50%, as indicated by a decrease in bicarbonate-stimulated O₂ evolution (control rate, 1.4 ± 0.3 × 10⁻¹⁵ moles per cell per minute).     

Purification and characterization of myo-inositol hexaphosphate-adenosine diphosphate phosphotransferase from Phaseolus aureus

myo-Inositol hexaphosphate adenosine diphosphate phosphotransferase transfers phosphate from myo-inositol hexaphosphate to adenosine diphosphate to synthesize adenosine triphosphate. This enzyme has been isolated and purified from ungerminated mungbean seeds and found to be different from guanosine diphosphate phosphotransferase. A purification of about 200-fold with 15% recovery has been obtained. The optimal pH of the reaction is 7.0 and is dependent on the presence of a divalent cation, i.e., Mg²⁺ and Mn²⁺. The K_m value for myo-inositol hexaphosphate has been found to be 0.41 × 10^?4m and V is 90.0 nmol of P_i transferred per milligram of protein per 20 min. K_m for ADP is 0.88 × 10^-4m and V is 83.3 nmol of phosphorus transferred to ADP per milligram of protein per 20 min. The ADP phosphotransferase reaction is reversible to the extent of about 50% of the forward reaction. dADP is partly effective as an acceptor but other ribonucleoside mono- and diphosphates cannot substitute for ADP. The products ATP and myo-inositol pentaphosphate have been confirmed by several criteria. It has also been shown that this enzyme transfers phosphate only from a specific phosphoryl group (C-2 position) of myo-inositol hexaphosphate for the synthesis of ATP and 1,3,4,5,6-myo-inositol pentaphosphate or pentakis (dihydrogen phosphate).     

Association and Validation of Yield-Favored Alleles in Chinese Cultivars of Common Wheat (Triticumaestivum L.)

Common wheat is one of the most important crops in China, which is the largest producer in the world. A set of 230 cultivars was used to identify yield-related loci by association mapping. This set was tested for seven yield-related traits, viz. plant height (PH), spike length (SL), spikelet number per spike (SNPS), kernel number per spike (KNPS), thousand-kernel weight (TKW), kernel weight per spike (KWPS), and sterile spikelet number (SSN) per plant in four environments. A total of 106 simple sequence repeat (SSR) markers distributed on all 21 chromosomes were used to screen the set. Twenty-one and 19 of them were associated with KNPS and TKW, respectively. Association mapping detected 73 significant associations across 50 SSRs, and the phenotypic variation explained (R²) by the associations ranged from 1.54 to 23.93%. The associated loci were distributed on all chromosomes except 4A, 7A, and 7D. Significant and potentially new alleles were present on 8 chromosomes, namely1A, 1D, 2A, 2D, 3D, 4B, 5B, and 6B. Further analysis showed that genetic effects of associated loci were greatly influenced by association panels, and the R² of crucial loci were lower in modern cultivars than in the mini core collection, probably caused by strong selection in wheat breeding. In order to confirm the results of association analysis, yield-related favorable alleles Xgwm135-1A₁₃₈, Xgwm337-1D₁₈₆, Xgwm102-2D₁₄₄, and Xgwm132-6B₁₂₈ were evaluated in a double haploid (DH) population derived from Hanxuan10 xLumai14.These favorable alleles that were validated in various populations might be valuable in breeding for high-yield.     

Effect of Urea Concentration on Growth of Ureaplasma urealyticum (T-Strain Mycoplasma)

The effect of urea on growth of Ureaplasma urealyticum type VIII was studied by cultivating the organisms in a dialysate broth, prepared from soy peptone and autoclaved yeast, supplemented with 5% dialyzed horse serum, 100 mM 2-(N-morpholino)ethane sulfonic acid buffer (pH 5.75), and defined amounts of urea. Without urea, growth did not occur. Total growth was directly related to urea concentration. The least amount of urea that supported growth was 0.032 mM, which resulted in 3 × 10⁴ colony-forming units per ml. The maximum yield of organisms, 8.0 × 10⁷ colony-forming units per ml, was observed at 32 mM urea. Growth was limited not only by urea concentration, but also by the buffer capacity of the medium. The maximum amount of 2-(N-morpholino)ethane sulfonic acid buffer that could be employed was 100 mM; at higher concentrations, growth was inhibited. The yield of U. urealyticum was small even in medium with 32 mM urea and 100 mM 2-(N-morpholino)ethane sulfonic acid buffer: 0.63 mg of protein per liter of culture containing 5 × 10¹⁰ total colony-forming units. The molar growth yield was 20 mg of protein per mol of urea. The growth rate was also a function of urea concentration. Generation times ranged from 8 h at 0.032 mM urea to 1.6 h at 3.2 mM urea, where the substrate level was saturating. The K_s value for growth was 2.0 × 10⁻⁴ M urea. Thus, urea is a growth-limiting factor for U. urealyticum, but remarkably large amounts of this substrate are required.