Local and spatially coordinated movements in Dictyostelium discoideum amoebae during chemotaxis

We have studied chemotaxis by individual Dictyostelium discoideum amoebae using strong, local gradients of the chemoattractant cyclic AMP. Gradients were provided by diffusion of cyclic AMP from a microneedle, which could be positioned at various points around the cell. Responses to changes in the gradient indicate how the cell is structurally organized for chemotactic movement. There is a polarity in the responsiveness of the surface to stimulation by cyclic AMP along the length of the amoeba. Furthermore, two aspects of chemotactic movement can be distinguished. The first response to cyclic AMP is a locally generated extension of a hyaline pseudopod from the region of the surface nearest the stimulus. The second response, the flow of cytoplasm in the direction of the stimulus, is coordinated and separate from the first response. The coordination appears to depend on the nucleus or on the microtubule-organizing center.     

Surface solar ultraviolet radiation for paleoatmospheric levels of oxygen and ozone

Many investigators have concluded that the level of solar ultraviolet radiation (200–300 nm) reaching the surface was a key parameter in the origin and evolution of life on Earth. The level of solar ultraviolet radiation between 200 and 300 nm is controlled primarily by molecular absorption by ozone, whose presence is trongly coupled to the level of molecular oxygen. In this paper, we present a series of calculations of the solar ultraviolet radiation reaching the surface for oxygen levels ranging from 10^–4 present atmospheric level to the present level. The solar spectrum between 200 and 300 mn has been divided into 34 spectral intervals. For each spectral interval, we have calculated the solar ultraviolet radiation reaching the Earth's surface by considering the attenuation of the incoming beam due to ozone and oxygen absorption. A one-dimensional photochemical model of the atmosphere was used for these calculations.     

Problems in estimating growth rates of marine phytoplankton from short-term14C assays

Growth rate estimates () of phytoplankton populations that were sampled from nitrogen-limited continuous cultures and then incubated for short durations in batch culture with added¹⁴C-HCO₃ ^– were significantly different than steady-state growth rates () for 3 of 5 marine phytoplankton species. Two diatoms,Thalassiosira weissflogii andChaetoceros simplex, displayed virtually identical growth rates (=) over a wide range of, whereas for a third diatom,Phaeodactylum tricornutum, was overestimated by an average of 40% compared to. In contrast, was underestimated by the¹⁴C technique for the two remaining species: up to 40% at a steady-state of 1.0 day^–1 for the chlorophyteDunaliella tertiolecta and up to 100% at of 1.4 day^–1 for the haptophytePavlova lutheri. For the latter two species the divergence between and appeared to increase with increasing steady-state. A simple model of labeled and total carbon flow between the aqueous phase and cellular biomass was constructed to demonstrate that respiration was negligible when=, but was significant when>. In the cases in which<, a rapid physiological alteration presumably took place once the steady state was disturbed and cells were placed in the incubation chambers, which perhaps was related to the nutritional state of the cultures at the time of sampling. Questions thus are raised regarding our ability to measure accurately primary productivity from shipboard experiments with confined samples of phytoplankton from nutrient-impoverished waters that probably are less hardy than the laboratory cultures used in these studies.     

Valine-sensitive acetohydroxy acid synthases in Escherichia coli K-12: unique regulation modulated by multiple genetic sites.

Summary Spontaneous mutants (146) of Escherichia coli K-12 were selected that were resistant to inhibition of growth by 1.2 mM L-valine (Val^r). The Val^r isolates, containing acetohydroxy acid synthase resistant to feedback inhibition by L-valine (AHAS^r), were classed according to cotransduction of the mutation with leu. Several mutations resulting in an AHAS^r phenotype were found to be cotransducible with glyA. However, no mutations causing a Val^r phenotype were linked to ilv. AHAS activity was more closely examined in representatives of three classes of mutants with Val^r linked to leu, labeled ilv-660, ilv-661, and ilv-662. The ilvE503 allele in E. coli K-12, known to cause a two- to three-fold derepression of AHAS, was found to affect regulation of synthesis of both valine-sensitive AHAS (AHAS^s) and AHAS^r in the mutants containing ilv-660 and ilv-661, whereas it affected repression of AHAS^s, only, in the mutant containing ilv-662. Further, both AHAS^s and AHAS^r in the ilv-661 mutant were repressed by valine, whereas valine did not repress AHAS^r synthesis in the strain carrying ilv-660 and only partially repressed AHAS^r in the strain carrying ilv-662. Unexpectedly, AHAS^r synthesis in strains carrying ilv-660 or ilv-662 was repressible by leucine. The ilv-660 locus appears to be similar in position to ilvH and encodes a product that confers valine-sensitivity upon AHAS activity in the wild-type E. coli K-12. The ilv-660 and ilv-662 loci may normally encode products that influence both the feedback sensitivity of AHAS and control of AHAS biosynthesis.     

Dissociation of cellular K+ accumulation from net Na+ transport by toad urinary bladder

Summary A number of published data suggest a variable stoichiometry between the rates of cellular potassium uptake and net sodium transport (J _Na) across the urinary bladder of the toad. This problem was examined by simultaneously studying the intracellular chemical activity of potassium (a _K) with open-tip K⁺-selective microelectrodes and micropipets, and monitoringJ _Na by measuring the short-circuit current (SCC). When bathed in the short-circuited state with solutions containing ana _K of 2.7mm, the mean ±sem values for intracellulara _K were 43±0.6mm.Ouabain, at a concentration of 10^–2 m, reduced intracellulara _K by 56–67% and SCC by 96–100%. At 5×10^–4 m, ouabain reversibly reduced intracellulara _K by 40–55%, and SCC by 63–68%; the inhibition of SCC was only partly reversible during the period of observation.Removal of external potassium reduced intracellulara _K by 69–80% and SCC by 51–76%. Restoration of external potassium entirely returned intracellulara _K to its control value, but only partially reversed the inhibition of SCC during the period of study. Furthermore, recovery ofa _K began 19–43 min before that of SCC; recovery ofa _K was 90–97% complete before any increase in SCC could be measured. Although other interpretations are possible, the simplest interpretation of the data is that the processes responsible for potassium accumulation and transepithelial sodium transport are not identical. We propose the existence of a separate transfer mechanism at the basolateral cell membrane, responsible for accumulating intracellular potassium, and not directly coupled to active sodium transport.     

At least six different actins are expressed in a higher mammal: An analysis based on the amino acid sequence of the amino-terminal tryptic peptide

The protein chemical characterization of the amino-terminal tryptic peptide of actin from different bovine tissues shows that at least six different actin structural genes are expressed in this mammal.Unique amirio acid sequences are found for actin from skeletal muscle, for actin from heart muscle, for two different actin species from smooth muscle, and for two different actin species typical of non-muscle tissues such as brain and thymus. The presence of more than one actin species in the same tissue (e.g. nonmuscle tissues and smooth muscles) is demonstrated by different amino-terminal peptides which, however, are closely related. The actins from the sarcomeric muscles (e.g. skeletal muscle and heart muscle) show unique but extremely similar amino-terminal peptides. A limited comparison of bovine and avian actins involving smooth and skeletal muscles emphasizes that among higher vertebrates actin divergence involves tissue rather than species specificity.For the lower eukaryotic organism Physarum polycephalum a single actin amino-terminal peptide is found, indicating that only one actin species is present during the plasmodial stage. The amino acid sequence of this peptide although unique reveals a high degree of homology with the corresponding mammalian cytoplasmic actin peptides.Different actin extraction and purification procedures have been compared by the relative yields of the different amino-terminal peptides. The results indicate that the various actin species obtained by the current purification procedures are a true reflection of the actual actins present in the tissue. In addition we compare the resolution provided by either isoelectric focusing analysis of different actins or by the protein chemical characterization of the amino-terminal peptides of different actins. We show that the latter procedure is more suitable for recording changes in actin expression during evolution and differentiation.     

Mechanism of action of choleragen

Choleragen exerts its effect on cells through activation of adenylate cyclase. Choleragen initially interacts with cells through binding of the B subunit of the toxin to the ganglioside G_M1 on the cell surface. Subsequent events are less clear. Patching or capping of toxin on the cell surface may be an obligatory step in choleragen action. Studies in cell-free systems have demonstrated that activation of adenylate cyclase by choleragen requires NAD. In addition to NAD, requirements have been observed for ATP, GTP, and calcium-dependent regulatory protein. GTP also is required for the expression of choleragen-activated adenylate cyclase. In preparations from turkey erythrocytes, choleragen appears to inhibit an isoproterenol-stimulated GTPase. It has been postulated that by decreasing the activity of a specific GTPase, choleragen would stabilize a GTP-adenylate cyclase complex and maintain the cyclase in an activated state. Although the holotoxin is most effective in intact cells, with the A subunit having 1/20th of its activity and the B subunit (choleragenoid) being inactive, in cell-free systems the A subunit, specifically the A₁ fragment, is required for adenylate cyclase activation. The B protomer is inactive. Choleragen, the A subunit, or A₁ fragment under suitable conditions hydrolyzes NAD to ADP-ribose and nicotinamide (NAD glycohydrolase activity) and catalyzes the transfer of the ADP-ribose moiety of NAD to the guandino group of arginine (ADP-ribosyltransferase activity). The NAD glycohydrolase activity is similar to that exhibited by other NAD-dependent bacterial toxins (diphtheria toxin, Pseudomonas exotoxin A), which act by catalyzing the ADP-ribosylation of a specific acceptor protein. If the ADP-ribosylation of arginine is a model for the reaction catalyzed by choleragen in vivo, then arginine is presumably an analog of the amino acid which is ADP-ribosylated in the acceptor protein. It is postulated that choleragen exerts its effects on cells through the NAD-dependent ADP-ribosylation of an arginine or similar amino acid in either the cyclase itself or a regulatory protein of the cyclase system.     

Fate of thymocytes: studies with 125I-iododeoxyuridine and 3H-thymidine in mice.

Cortical thymocytes of young adult mice were labeled in situ with radioactive DNA precursors. As a result of cell emigration and cell death, total thymic radioactivity decreased within 8 days to 10% or less of that present on day 1. Accumulation of thymic migrants in peripheral lymphoid organs was estimated by computing the net thymus-derived radioactivity in these tissues. Thymic cell death was assessed by comparing values obtained with 125I-UdR to those acquired with 3H-TdR; The results indicate that cortical thymocytes migrate to the spleen, mesenteric lymph node, femurs and intestine; nevertheless, only a small fraction of the activity originally present in the thymus was recovered in these organs; the vast majority of newly formed cortical thymocytes apparently die after a relatively short life span. Exclusive of the fraction which dies in situ, evidence for thymocyte death is seen in bone marrow; however, most migrants appear to terminate in the intestine.     

Free Ammonia Inhibition of Algal Photosynthesis in Intensive Cultures

The effect of free NH₃ inhibition on short-term photosynthesis was investigated in three microalgal species: the freshwater chlorophyte Scenedesmus obliquus, the marine diatom Phaeodactylum tricornutum and the marine chlorophyte Dunaliella tertiolecta. By performing a series of assays at various concentrations of added NH₄Cl and culture pH, we demonstrated that the inhibitory compound was free NH₃ and that pH played no role in determining the magnitude of inhibition, other than in establishing the degree of dissociation of nontoxic NH₄⁺ to toxic NH₃. When corrections were made for pH, all three species displayed the same sigmoidal response curve to free NH₃ concentration; 1.2 mM NH₃ led to 50% reduction in photoassimilation of ¹⁴C. Based on literature values, some marine phytoplankton appear to be significantly more sensitive to free NH₃ than were the test species, which are noted for their excellent growth characteristics. However, the combination of low algal biomass and strong pH buffering commonly found in most marine and many freshwater environments probably limits the possibilities for NH₃ toxicity to low alkalinity freshwaters and intensive algal cultures in which NH₄⁺ is the main source of N. Such conditions occur commonly in algal wastewater treatment systems.