Interactions between anion exchange and other membrane proteins in rabbit kidney medullary collecting duct cells

Summary In separated outer medullary collecting duct (MCD) cells, the time course of binding of the fluorescent stilbene anion exchange inhibitor, DBDS (4,4-dibenzamido-2,2-stilbene disulfonate), to the MCD cell analog of band 3, the red blood cell (rbc) anion exchange protein, can be measured by the stopped-flow method and the reaction time constant, _DBDS, can be used to report on the conformational state of the band 3 analog. In order to validate the method we have now shown that the ID_50,DBDS,MCD (0.5±0.1 m) for the H₂-DIDS (4,4-diisothiocyano-2,2-dihydrostilbene disulfonate) inhibition of _DBDS is in agreement with the ID_50,Cl ^–,_MCD (0.94±0.07 m) for H₂-DIDS inhibition of MCD cell Cl^– flux, thus relating _DBDS directly to anion exchange. The specific cardiac glycoside cation transport inhibitor, ouabain, not only modulates DBDS binding kinetics, but also increases the time constant for Cl^– exchange by a factor of two, from _Cl=0.30±0.02 sec to 0.56±0.06 sec (30mm NaHCO₃). The ID_50,DBDS,MCD for the ouabain effect on DBDS binding kinetics is 0.003±0.001 m, so that binding is about an order of magnitude tighter than that for inhibition of rbc K⁺ flux (K _I,K ⁺,_rbc=0.017 m). These experiments indicate that the Na⁺,K^–-ATPase, required to maintain cation gradients across the MCD cell membrane, is close enough to the band 3 analog that conformational information can be exchanged. Cytochalasin E (CE), which binds to the spectrin/actin complex in rbc and other cells, modulates DBDS binding kinetics with a physiological ID_50,DBDS,MCD (0.076±0.005 m); 2 m CE also more than doubles the Cl^– exchange time constant from 0.20±0.04 sec to 0.50±0.08 sec (30mm NaHCO₃). These experiments indicate that conformational information can also be exchanged between the MCD cell band 3 analog and the MCD cell cytoskeleton.     

Measurement of the diffusion coefficient for salt in salt flat and mangrove soils

Excluded salt accumulated at mangrove roots must be transported away from the root zone by diffusive processes, due to the low permeability of most mangrove soils. The diffusion coefficient for salt in mangrove soils determines the rate of this diffusive transport but has not been determined experimentally before. In this work we used a 12-month long-time series of salt concentration profiles measured in a sediment core over which fresh water was continuously circulated, to determine the diffusion coefficient for salt in the soil. Salt concentrations were measured using an electrical conductivity probe that was developed for use in hypersaline (salt concentration up to and in excess of 120g/l) conditions. A modified formula was experimentally determined to relate electrical conductivity to salt concentration and temperature, applicable up to a salt concentration of 200g/l. This was done because standard formulae relating these variables do not apply in the hypersaline conditions often encountered in salt flat sediments. The salt concentration profiles were used in a simple mathematical model to determine a sediment diffusion coefficient for salt in a salt flat sediment. This value of D=(4.6±0.2) ×10^–5m²/day was approximately half that calculated theoretically.     

Heat-induced reversible changes in Photosystem 1 absorption cross-section of pea chloroplasts and sub-chloroplast preparations

Reversible changes in the room temperature fluorescence quenching at 685 nm and light scattering level at 577 nm, indicating about 15% of granal unstacking, induced by high temperature treatment (40°C, for 5 min) of pea chloroplasts were shown. Analysis of the low temperature excitation fluorescence spectra of the 735 nm Photosystem 1 (PS 1) band (F735), in the 635–725 nm region, has revealed the involvement of light-harvesting (LHC 2, maxima at 650 and 676 nm) and the proximal Photosystem 2 antenna (maxima 668, 687 nm) in heat-induced enhancement of the PS 1 long wavelength antenna absorption cross-section. It was found that the two PS 1 sub-chloroplast preparations, achieved by the digitonin method, possessed different characteristics of this enhancement. For the heavier fraction (100 000 g) the additional absorption cross-section was formed mostly at the expense of PS 2 antennas (apparently spillover), but for the lighter PS 1 fraction (145 000 g) the changes have indicated an -transfer mechanism, i.e., participation of only LHC 2 in the energy transfer towards PS 1. This may indicate the heterogeneous character of the temperature-induced energy redistribution across the PS 1-containing chloroplast membrane compartments. The model of heat-induced changes in the pigment-protein complex arrangement is discussed in terms of domain organisation of the thylakoid membrane.Abbreviations Chl a/b ratio between chlorophyll a and chlorophyll b concentrations - CP43 and CP47 proximal Photosystem 2 antenna complexes - D1/D2 complex Photosystem 2 reaction centre complex - EDTA ethylenediaminetetraacetic acid - F685 and F696 Photosystem 2 low temperature fluorescence bands - F735 Photosystem 1 low temperature fluorescence band - Fp free pigment band in green gel electrophoresis - LHC 2 light-harvesting chlorophyll a/b complex - LHCP I, II and III light-harvesting bands in green gel electrophoresis - Cp1 and Cpa bands in green gel electrophoresis which are associated with Photosystem 1 and 2 reaction centre complexes with internal antennas - P700 Photosystem 1 reaction centre - PPC pigment-protein complex - PS 1 and Photosystem 1 alpha and Photosystem 1 beta - PS 2 and Photosystem 2 alpha and Photosystem 2 beta - RC reaction centre - SDS-PAGE sodiumdodecylsulphate-polyacrylamide gel electrophoresis - St1-St2 state-1-state-2 transitions     

Interrelationships Between Nitrogen Supply and Photosynthetic Parameters in Vicia faba L.

We determined for Vicia faba L the influence of nitrogen uptake and accumulation on the values of photon saturated net photosynthetic rate (P _Nmax), quantum yield efficiency (), intercellular CO₂ concentration (C _i), and carboxylation efficiency (C _e). As leaf nitrogen content (N_L) increased, the converged onto a maximum asymptotic value of 0.0664±0.0049 mol(CO₂) mol(quantum)^–1. Also, as N_L increased the C _i value fell to an asymptotic minimum of 115.80±1.59 mol mol^–1, and C _e converged onto a maximum asymptotic value of 1.645±0.054 mol(CO₂) m^–2 s^–1 Pa^–1 and declined to zero at a N_L-intercept equal to 0.596±0.096 g(N) m^–2. fell to zero for an N_L-intercept of 0.660±0.052 g(N) m^–2. As N_L increased, the value of P _Nmax converged onto a maximum asymptotic value of 33.400±2.563 mol(CO₂) m^–2 s^–1. P _N fell to zero for an N_L-intercept of 0.710±0.035 g(N) m^–2. Under variable daily meteorological conditions the values for N_L, specific leaf area (_L), root mass fraction (R_f), P _Nmax, and remained constant for a given N supply. A monotonic decline in the steady-state value of R_f occurred with increasing N supply. _L increased with increasing N supply or with increasing N_L.     

The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of M I configurations

Whilst reliable estimates of chiasma frequencies can usually not be obtained, the probability (b) of a chromosome arm to be bound by at least one chiasma can often be determined. In the absence of interference this probability equals (1–e ^–2), where 2 is the average chiasma frequency of the chromosome arm and the average crossover frequency or map length. In the presence of interference is shown to retain its genetic meaning as an additive metric that may describe the chromosome arm or other distinctive chromosome segment in terms of genetic recombination. It is a form of potential map length, comparable to, but numerically different from the regular map length. It is termed provisionally crossing-over potential.A chromosome with armsm andn with crossing-over potentials and will form ring bivalents with a frequency (1–e ^–2).(1–e ^–2); open bivalents with a frequency (1–e ^–2).e ^–2+(1–e ^–2).e ^–2; univalent pairs with a frequencye ^–2.e ^–2. Estimates of these frequencies yield equations from which and may be solved. In rye (Secale cereale) their ratio (q) is approximately two and differs from the mitotic arm length ratio of 1.4, indicating localization of chiasmata in the long arms.Graphs are given to show how, with constantq, the relation between the probabilitiesb _m andb _n of the two arms being bound changes with changing averageb.Data are presented on chiasma frequencies in M I, and compared with the frequencies expected in the absence of interference to give an impression of the degree of interference. Apparent fusion of chiasmata simulates interference.     

Memorization and delayed expression of regulatory messages in plants

Plantlets of Bidens pilosus L., considered to be basically symmetrical, can be lateralized (A/B) by being administered a symmetry-breaking signal such as puncturing one of the plant cotyledons. The induced asymmetry remains latent as long as the plants have not been made permissive, i.e. as long as the plant apex is left functioning. When the apex has been removed (plant decapitation), the latent asymmetry is expressed by one of the cotyledonary buds (a/b) statistically beginning to elongate before the other. The interval of time between delivering the symmetry-breaking signal and making the plant permissive is the memorization-time, t. Memorization can be quantified by using a precedence index, q, the values of which range from 0 (no detectable asymmetry with regard to bud growth) to ±1 (bud growth perfectly asymmetric in favour of either bud b or a). Even for memorization times, t, up to 14 d, q-values up to 0.4 (or even larger) are observed. Various experimental characteristics (e.g. light, temperature, presence or absence of the root system) but not the plant age can affect the q-values, at the moment when the treatments are performed, at least in the range of 6 to 25 d. Combining several puncturing treatments either increases or decreases the q-values, depending on the nature of these treatments and the time-intervals, t, between them. Symmetrically removing both cotyledons in the minutes following the puncturing of one of them does not significantly alter the results, which means that the symmetry-breaking message is rapidly transported and memorized within the plant. Non-traumatic asymmetrical treatments (droplets of saline solutions, light-gradients) can also act as symmetry-breaking signals and be memorized. Plants other than Bidens are likely to possess similar memorization ability, although the q-values observed up to now have not been very large.     

Structure and function of the repressor of bacteriophage lambda

Summary By mutagenizing a cIts (cI857) lysogen, a mutant has been isolated with a wild-type phenotype. This mutant phage lysogenizes with low efficiency and produces a low burst. Though the initial rates of repressor synthesis in Escherichia coli after infection with wild-type and mutant are the same, the maximum level of repressor that is synthesized in the latter case is only about 30% of that synthesized in the former. Virulent plates on the lysogen of mutant with slightly less efficiency producing very tiny plaques. Operator-binding studies made in vitro with purified mutant and wild-type repressors show that the binding curve of the former repressor is a rectangular hyperbola while that of the latter is sigmoid. The half-lives of the complexes of mutant and wild-type repressors with right operator are 133 and 27 min, respectively. All these results suggest that the mutant repressor possibly has a higher affinity for the operators. This mutant has been named cIha (ha=high affinity).     

Inheritance and linkage relationships of glutamate oxaloacetate transaminase isoenzymes in apple

Summary Four zones of enzymatic activity for glutamate oxaloacetate transaminase (GOT) were found in apple tissue. A dimeric gene, GOT-1, determining the fastest migrating zone, was identified. Six alleles were found, including a near null allelle which produced detectable heterodimeric bands but not homodimeric bands. A marked deficit or absence of certain geno-types in all backcrosses and in some crosses between unrelated varieties was attributed to the close linkage (r=0.02±0.005) of GOT-1 with the incompatibility S locus. GOT-1 was also closely linked with the isocitrate dehydrogenase locus IDH-1 (0.03±0.01). Proposed incompatibility genotypes for four cultivars, and the linked GOT-1 alleles are Cox: S ₁ b/S ₂ d, Idared: S ₃ a/S ₄ c, Fiesta: S ₃ a/S ₂ d and Kent: S ₃ a/S ₁ b.The results reported in this paper are part of a PhD Thesis by the first author     

Forskolin Modulates Acetylcholine Receptor Gating by Interacting with the Small Extracellular Loop Between the M2 and M3 Transmembrane Domains

1. Forskolin acts as an allosteric modulator of muscle-type nicotinic acetylcholine receptors. Receptors from mouse muscle and Torpedo electroplax demonstrate differential sensitivity to inhibition by forskolin. Previous work from this laboratory suggested that the subunit is responsible for this differential sensitivity.2. We have used a series of mouse/Torpedo species-chimeric subunits to further define the site of forskolin interaction with the subunit. Analysis of the patterns of forskolin inhibition of receptors containing mouse/Torpedo chimeric subunits along with the mouse , , and subunits suggests that forskolin interacts with the small extracellular domain that links the M2 and M3 transmembrane domains (the M2–M3 linker).3. We suggest that the M2–M3 linker domain plays an important role in the transduction of ligand binding to the conformational changes that result in channel opening.     

Interaction of cytochrome c L with methanol dehydrogenase from Methylophaga marina 42:

The reaction of methanol dehydrogenase with cytochrome c _L from Methylophaga marina and the reactions of the non-physiological substrates, Wurster's blue and ascorbic acid, with both proteins were studied as a function of temperature (4–32 °C), pressure (1–2000 bar) and ionic strength using the optical high pressure stopped-flow method. The thermodynamic parameters H, S and V were determined for all reactions where electron transfers are involved. These data allowed the determination of the Maxwell relationships which proved the internal thermodynamic consistency of the system under study. A conformational change on the cytochrome c _L level was deduced from both breaks in the Arrhenius plots and the variation of the V with temperature.Abbreviations MOPS 4-morpholinepropanesulfonic acid - CHES 2-(cyclohexylamino)ethanesulfonic acid - MDH methanol dehydrogenase - EDTA ethylenedinitrilotetraacetic acid disodium salt - BTB bromothymol blue (3,3-dibromothymolsulfoneph-thalein) - PQQ 2,7,9-tricarboxy-lH-pyrrolo-[2,3f]quinoline-4,5-dione - cytochrome c _HH mammalian horse heart cytochrome c     

Adaptive Responses of the Larvae of Cirripede Barnacle Peltogasterella gracilis to Changes in Seawater Temperature and Salinity

The responses of the larvae of the cirripede barnacle Peltogasterella gracilis (Crustacea: Cirripedia: Rhizocephala) that parasitizes the hermit crab Pagurus pectinatus to different combinations of seawater temperature (25, 22, 20, 16, and 12°C) and salinity (from 34 to 8) were studied in a laboratory. The nauplii of P. gracilis completed the entire cycle of development at 22 to 12°C in a narrow range of salinity (from 34 to 28), which agrees well with the environmental conditions of the crab hosts' habitat. At favorable temperatures (22–20°C) and salinity (34–28), the nauplii reached the cypris stage in 88 ± 2 h, while at 12°C and 34–30, the naupliar development took 156 ± 5 h. The cypris larvae appeared more resistant compared with the nauplii, in terms of changes in both the temperature and salinity of seawater. They actively swam at all experimental temperatures and in the salinity range of 34–18. At temperatures (22–16°C) and salinities (34–24) favorable for the cyprids, their longevity in plankton equaled 6–10 days. Thus, the nauplii of P. gracilis is the more vulnerable stage of development in the life cycle of this parasitic barnacle. The tolerance against changes in environmental factors is due to the adaptive capabilities of parasitic larvae and the environmental conditions in the habitats of its host, a typical marine crustacean. The insignificant parasitization rate of the hermit crab by its rhizocephalan parasite may be explained by the death of the nauplii of P. gracilis, which occurs when they enter to the surface water layer.     

Structural and functional properties of proteasome activator PA28

The proteasome activator PA28 or 11S regulator is a protein complex composed of two different but homologous polypeptides, termed PA28 and PA28. The purified activator protein (_200 kDa) is a ring-shaped heteromultimer containing the two polypeptides, possibly with an _{3 3} stoichiometry. The activator, which by itself shows no hydrolytic activity elicits activation of the proteasome's multiple peptidase activities by binding to the terminal rings of the proteinase. In vitro, active PA28 can be reconstituted from isolated and subunits, yielding two different oligomers: with the single subunit, PA28 homomultimers with moderate stimulatory activity toward 20S proteasomes are obtained whereas isolated -subunits are unable to form oligomers and are devoid of stimulatory activity. However, in the presence of both subunits, heteromultimers form, concomitant with restoration of full stimulatory activity. The recent finding that PA28 modulates the proteasome-catalyzed production of antigenic peptides presented to the immune system on MHC class I molecules indicates a cellular function of the activator in antigen processing. Abbreviations: IFN – interferon; LMP – low molecular weight peptide; MHC – major histocompatibility complex.     

Assembly-dependent expression of antigenic epitopes by β2-microglobulinb

In this report we provide evidence for the expression of antigenic epitopes on mouse (₂-microglobulin^b _2m ^b) that result from assembly with cognate H-2 class I heavy chains. For the cell line 69.9.15 (₂m^a × ₂m^b), which expresses a mutant cytosolic form of H-2K^b and wild-type H-2D^b, flow cytometry with rabbit antiserum against mouse ₂m displayed ₂m expression by cells grown in the presence or absence of fetal calf serum. By contrast, the epitopes identified by the ₂m^b-specific monoclonal antibody (mAb) S19.8 and clone 23 were not expressed by 69.9.15 cells grown in serum-containing conditions, and although S19.8 reactivity was weakly recovered by culture in the absence of serum, no such reacitivity was observed with clone 23. Strong expression of these epitopes was achieved following transfection of 69.9.15 cells with the wild-type H-2K ^b gene, indicating that the ₂m^b epitopes defined by mAb S19.8 and clone 23 were expressed when ₂m^b was assembled with an appropriate heavy chain. In support of this conclusion, we observed the recovery of the S19.8 and clone 23 epitopes by in vitro assembly of H-2K^b heavy chains with ₂m^b in the presence of the VSV N protein p52–59; however, such epitopes were expressed neither by ₂m^b prior to heterodimer assembly nor by non-conformed ₂m^b present in tissue culture supernatants recovered from H-2 class I surface positive cells. Taken together, these data indicate that in addition to the property of ₂m to modify the antigenicity of the MHC class I heavy chains, ₂m epitopes are induced in a reciprocal manner by assembly with MHC class I heavy chain molecules. Correspondence to: R. A. Zeff.     

Conformational Change of the Rieske [2Fe–2S] Protein inCytochrome bc 1 Complex

Structures of mitochondrial bc ₁ complex have been reported based on four different crystalforms by three different groups. In these structures, the extrinsic domain of the Rieske [2Fe–2S]protein, surprisingly, appeared at three different positions: the c ₁ position, where the [2Fe–2S]cluster exists in close proximity to the heme c ₁; the b position, where the [2Fe–2S] clusterexist in close proximity to the cytochrome b; and the intermediate position where the[2Fe–2S] cluster exists in between c ₁ and b positions. The conformational changes betweenthese three positions can be explained by a combination of two rotations; (1) a rotation of theentire extrinsic domain and (2) a relative rotation between the cluster-binding fold and thebase fold within the extrinsic domain. The hydroquinone oxidation and the electron bifurcationmechanism at the Q_P binding pocket of the bc ₁ complex is well explained using theseconformational changes of the Rieske [2Fe–2S] protein.     

Flight of the honey bee

Summary Using manometric and gas analytical methods oxygen consumption , carbon dioxide production , respiratory quotientRQ, (Fig. 1A-C) and thorax surface temperature difference T _ts (Fig. 3) were determined in single bees. The animals were either sitting in respiratory chambers or were suspended by the scutum, in which case they were resting, walking (turning a small polystyrene ball) or flying in a closed miniature wind tunnel.During resting (sitting in Warburg vessels) at an ambient temperatureT _a=10°C,RQ was 1.01±0.2 (n=905) with variations due to method (Fig. 1D, E).RQ values during walking were determined in single cases. In no case were they significantly different from 1.00. After the first 10 min of flight meanRQ was 1.00±0.04. It was significantly smaller than 1.00 (RQ=0.97) only during the last 5% of long-time flights (mean flight duration 58.8±28.8 min). With the exception of near-exhaustion conditions no signs of fuels other than carbohydrates were found.Metabolic rateP _m was 19.71±21.38 mW g^–1 during resting at 20°CT _a30°C indicating that many resting bees actively thermoregulate at higherT _a. After excluding bees which were actively thermoregulating, by an approximationP _m was 5.65±2.44 mW g^–1 at 20°CT _a30°C. True resting metabolic rate for sitting bees atT _a=10°C was 1.31±0.53 mW g^–1 (Fig. 2A, B).A significant negative correlation was found between relative (specific) oxygen consumption rel and body massM _b at 85 mgM _b150 mg.At 0°CT _ts16.5°C a significant (-0.01) positive correlation was found between and T _ts in single resting bees: T _Ts+0.099, or betweenP _m and T _ts:P _m=1.343 T _ts+0.581 (Fig. 3D) in ml h^–1,P _m in mW,T in °C).During walking (duration 13.15±5.71 min,n=13) at 12.5°CT _a21°C a stable T _ts of 11.41±3.37°C, corresponding to 167 mW g^–1, was reached for 80 to 90% of the walking time (Fig. 4B).During wind tunnel flights of tethered animals the minimal metabolic power measured in exhaustion experiments was 240 mW g^–1. Calculation of factors of increase inP _m is of limited value in poikilotherms, in which true resting conditions are not exactly defined.     

Energy requirements for growth and maintenance of Scenedesmus protuberans Fritsch in light-limited continuous cultures

Scenedesmus protuberans Fritsch was grown in light-limited continuous cultures with a light-dark cycle, at temperatures of 20° and 28° C. At 20° irradiances of 12 and 38 W m^–2 were used, at 28° 38 W m^–2.The relationships between growth rate and light uptake rate were of diphasic linear character. With the lower growth rates the relationships were defined with the parameters _e , i.e. the specific maintenance rate constant, and c, the true efficiency of light energy conversion into biomass. The _e -value was dependent on temperature, the c on irradiance.In cultures, incubated in prolonged darkness, decrease rates of biomass were comparable to the derived _e -values.Both diphasic linear relationships between growth rate and light uptake rate and the same order of magnitude of _e -values could be derived from literature data on other green algae.     

Efficient and precise measurement of H(alpha)-C(alpha), C(alpha)-C', C(alpha)-C(beta) and H(N)-N residual dipolar couplings from 2D H(N)-N correlation spectra

A suite of experiments are presented for the measurement of H–C, C–C, C–C and H^N–N couplings from uniformly ¹⁵N, ¹³C labeled proteins. Couplings are obtained from a series of intensity modulated two-dimensional H^N–N spectra equivalent to the common ¹H–¹⁵N–HSQC spectra, alleviating many overlap and assignment issues associated with other techniques. To illustrate the efficiency of this method, H–C, C–C, and H^N–N isotropic scalar couplings were determined for ubiquitin from data collected in less than 4.5 h, C–C data collection required 10 h. The resulting couplings were measured with an average error of ±0.06, ±0.05, ±0.04 and ±0.10 Hz, respectively. This study also shows H–C and C–C couplings, valuable because they provide orientation of bond vectors outside the peptide plane, can be measured in a uniform and precise way. Superior accuracy and precision to existing 3D measurements for C–C couplings and increased precision compared to IPAP measurements for H^N–N couplings are demonstrated. Minor modifications allow for acquisition of modulated H^N–C 2D spectra, which can yield additional well resolved peaks and significantly increase the number of measured RDCs for proteins with crowded ¹H–¹⁵N resonances.     

DNA-dependent RNA polymerases from Schneider 2-L cells of Drosophila. I. Preliminary characterization

The DNA-dependent RNA polymerases of Schneider 2-L cells of Drosophila melanogaster are described. These cells contain five readily detectable forms of this enzyme, polymerases I_a, I_b, III_a, II, and III_b, which elute from DEAE-Sephadex at 0.08, 0.12, 0.15, 0.20, and 0.22 m ammonium sulfate, respectively. RNA polymerases III_a and III_b, which each constitute about 5–10% of the total RNA polymerase activity in Drosophila embryos, are found to constitute 30 and 10%, respectively, of the total polymerase activity in cultured cells. The two form III polymerases are further characterized by in vitro response to divalent cations and ionic strength, template utilization, and sensitivity to -amanitin. Verification of the class III designation of these two polymerases is provided by their sensitivity to only very high levels of -amanitin (50% inhibition at approximately 800 µg/ml), their 10-fold greater activity on poly[d(A–T)], and their elution from DEAE-cellulose at lower ionic strengths than from DEAE-Sephadex.This work was supported by the Natural Sciences and Engineering Research Council.     

A DNA sequence of Drosophila melanogaster with a differential telomeric distribution

A DNA sequence (8–19T) of 2.3 kilobase pairs (kb) of Drosophila melanogaster was localized by in situ hybridization to the extreme ends of polytene chromosomes and to the chromocenter. The relative abundance of this sequence at the ends of polytene chromosomes X2L2R3L3R is 13.41.902.7. This differential distribution is probably due to different copy numbers at the individual telomeric regions. Restriction enzyme analysis of genomic DNA shows that 8–19T sequences are interspersed with other sequences. The clone 8–19T, which contains most of this interspersed repetitive sequence, is itself not internally repetitive but has a complex sequence composition. Some of these sequences are transcribed into poly(A)⁺RNA. We suggest that the ends of Drosophila chromosomes are of a complex arrangement with some sequences common to all ends.     

Dendritic properties of uropod motoneurons and premotor nonspiking interneurons in the crayfish Procambarus clarkii Girard

Dendritic properties of uropod motoneurons and premotor nonspiking interneurons of crayfish have been studied using intradendritic recording and current injection. The input resistance of phasic motoneurons (5.20 ± 0.5 M; mean ± standard error) measured by injecting constant hyperpolarizing current was significantly lower than that of tonic motoneurons (10.3 ± 2.6 M; 0.02 < P < 0.05). The membrane time constant of phasic motoneurons (7.3 ± 0.9 ms) was also significantly shorter than that of tonic motoneurons (24.3 ± 2.5 ms; P < 0.001). Both types of motoneurons behaved linearly during hyperpolarization and sub-threshold depolarization. Nonspiking interneurons showed outward rectification upon depolarization. During hyperpolarization, their membrane behaved linearly and showed significantly higher input resistance (19.5 ± 2.5 M) than phasic and tonic motoneurons (P < 0.001). Their membrane time constant (38.0 ± 5.7 ms) was significantly longer than that of phasic motoneurons (P < 0.001) but not than that of tonic motoneurons (P > 0.05). In response to intracellular injection of sinusoidally oscillating current, phasic motoneurons showed one or two spikes per depolarization period irrespective of oscillating frequency ranging from 1 to 16 Hz. Tonic motoneurons showed larger numbers of spikes per stimulus period at lower frequencies. Nonspiking interneurons also showed phase-locked effects on the motoneuron spike activity. The effective frequency range over which injected oscillating current could modulate motoneuron spike activity was similar for tonic motoneurons and nonspiking interneurons.