Significant impact of divalent metal ions on the fidelity,sugar selectivity,and drug incorporation efficiency of human PrimPol

Human PrimPol is a recently discovered bifunctional enzyme that displays DNA template-directed primase and polymerase activities. PrimPol has been implicated in nuclear and mitochondrial DNA replication fork progression and restart as well as DNA lesion bypass. Published evidence suggests that PrimPol is a Mn²⁺-dependent enzyme as it shows significantly improved primase and polymerase activities when binding Mn²⁺, rather than Mg²⁺, as a divalent metal ion cofactor. Consistently, our fluorescence anisotropy assays determined that PrimPol binds to a primer/template DNA substrate with affinities of 29 and 979 nM in the presence of Mn²⁺ and Mg²⁺, respectively. Our pre-steady-state kinetic analysis revealed that PrimPol incorporates correct dNTPs with 100-fold higher efficiency with Mn²⁺ than with Mg²⁺. Notably, the substitution fidelity of PrimPol in the presence of Mn²⁺ was determined to be in the range of 3.4 × 10⁻² to 3.8 × 10⁻¹, indicating that PrimPol is an error-prone polymerase. Furthermore, we kinetically determined the sugar selectivity of PrimPol to be 57–1800 with Mn²⁺ and 150–4500 with Mg²⁺, and found that PrimPol was able to incorporate the triphosphates of two anticancer drugs (cytarabine and gemcitabine), but not two antiviral drugs (emtricitabine and lamivudine).     

Neural inhibition sharpens auditory spatial selectivity of bat inferior collicular neurons

This study examines the role of neural inhibition in auditory spatial selectivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus, using a two-tone inhibition paradigm. Two-tone inhibition decreases auditory spatial response areas but increases the slopes of directional sensitivity curves of inferior collicular neurons. Inferior collicular neurons have either directionally-selective or hemifield directional sensitivity curves. A directionally-selective curve always has a peak which is at least 50% larger than the minimum. A hemifield directional sensitivity curve rises from an ipsilateral angle by more than 50% and either reaches a plateau or declines by less than 50% over a range of contralateral angles. Two-tone inhibition does not change directionally-selective curves but changes most hemifield directional sensitivity curves into directionally-selective curves. Auditory spatial selectivity determined both with and without two-tone inhibition increases with increasing best-excitatory frequency. Sharpening of auditory spatial selectivity by two-tone inhibition is larger for neurons with smaller differences between excitatory and inhibitory best frequencies. The effect of two-tone inhibition on auditory spatial selectivity increases with increasing inhibitory tone intensity but decreases with increasing intertone interval. The implications of these findings in bat echolocation are discussed. Accepted: 18 January 2000     

Alga consumption of four dominant planktonic crustaceans in Lake Balaton (Hungary)

Between 1981–83 the gut contents ofDaphnia galeata, D. cucullata, Eudiaptomus gracilis, andCyclops vicinus were examined with light and scanning electron microscope to obtain information on the feeding of these species in Lake Balaton. The twoDaphnia species feed mainly on abioseston, and it is assumed that their primary nutrient source was organic matter adsorbed onto the surfaces of the abioseston granules plus bacteria and detritus.E. gracilis feeds on algae, showing a preference for green algae and diatoms.C. vicinus is also a prodigious consumer of algae in Lake Balaton, utilizing the whole size spectrum of phytoplankton. Concerning the trophic relationships between phytoplankton and zooplankton in Lake Balaton, that between diatoms and bothE. gracilis andC. vicinus is the most conspicouos. Convincing evidence for an extensive utilization of blue-green algae was not found. Though there is no firm evidence yet, it is likely that theDaphnia are dependent on organic matter adsorbed on the abioseston.     

The host range and selectivity of a parasitic plant: Rhinanthus minor L.

Summary Rhinanthus minor (Yellow-rattle) is a widespread hemiparasitic plant of grassland habitats throughout Britain. Association analysis of the dune vegetation at Holme-next-the-Sea in eastern England revealed only two potential host plants through positive association. In contrast direct examination of the root systems revealed haustorial connections with 20 host species. The number of species parasitized by one plant ranged from one to seven. Data from another four sites in Britain and one in central Europe indicate that the natural host range of R. minor encompasses at least 50 species from 18 families with 22% in the Leguminosae and 30% in the Gramineae. Comparison of the number of haustorial connections made to each species with the abundance of roots in the soil shows that R. minor is a highly selective parasite, but that the selectivity is not consistent between populations or between plants from different parts of the same population. The reasons for host selectivity are discussed.     

Prey selectivity inEuseius tularensis [Acari: Phytoseiidae]

Differences in responsiveness to potential prey and pollen were tested in the facultative predator,Euseius tularensis Congdon. Following isolation without food for 16 h, adult female mites were observed with one of the following potential foods: pollen ofMalephora crocea (Jacquin) Schwantes,Scirtothrips citri (Moulton) 2nd instar larvae,Tetranychus pacificus McGregor nymphs and ♂,Panonychus citri (McGregor) nymphs and ♂,E. hibisci (Chant) larvae, andE. tularensis larvae. Responses were compared using the proportion of encounters that resulted in consumption. This proportion was highest when pollen was encountered, followed byS. citri andT. pacificus. Consumption was much lower in response toP. citri and congeneric larvae. WhenS. citri larvae were encountered a 2nd time, 1 to 2 h after the initial capture and consumption, responsiveness increased to a level equal to the response observed with pollen. These results suggest thatE. tularensis is a more effective biological control agent of citrus thrips (S. citri) than of citrus red mite (P. citri).     

Structure-activity relationships of cyclic β-casomorphin-5 analogues

Cyclic analogues of the β-casein-derived opioid peptide β-casomorphin-5 (H-Tyr-Pro-Phe-Pro-Gly-OH) were prepared through substitution of the Pro² residue with various ,ω-diamino acid residues (lysine, ornithine, 2,4-diaminobutyric acid) and cyclization of the ω-amino group to the C-terminal carboxyl function. Compounds of this type, with D-configuration at the 2-position residue, showed high opioid receptor affinity with some preference for μ receptors over δ receptors, high potency in the guinea pig ileum assay and considerable activity in the mouse vas deferens assay. Configurational inversion at the 4-position in these cyclic analogues resulted in enhanced affinity for both μ and δ receptors, whereas N-methylation of the Phe³ residue produced a potency decrease.     

Neural coding of speech sound in the telencephalic auditory area of the mynah bird

Summary The responses of neurons in field L in the auditory neostriatum of the mynah bird, Gracula religiosa, were recorded during presentation of intact or manipulated mimic voices. A typical mimic voice konnichiwa elicited responses in most of the neurons. Neurons in the input layer (L2) of field L showed many peaks on peristimulus time histograms while those in other layers (L1 and L3) exhibited only one or two peaks. Several neurons in L1 and L3 responded only to the affricative consonant /t/ in the intact mimic voices. They did not respond to the affricative consonant in the isolated segment or to the one in the playbacked voice in reverse. Forty-five percent of the neurons (33/ 73) decreased in firing rates at the affricative consonant in the isolated segment compared with in the intact voice. Some of these neurons, in which neither the affricative consonant in the isolated segment nor bursts of noise alone elicited responses, exhibited clear phasic responses to /t/ in the case when bursts of noise with particular central frequencies preceded the affricative consonant. The responsiveness of these neurons appears to receive temporal facilitation. These results suggest that these neurons code the temporal relationship of speech sound.Abbreviations HVc hyperstriatum ventrale, pars caudale - TFN temporally facilitated neuron - TSN temporally suppressed neuron     

Wide-field motion-sensitive neurons tuned to horizontal movement in the honeybee,Apis mellifera

This paper describes the morphology and response characteristics of two types of paired descending neurons (DNs) (classified as DNVII₁ and DNIV₁) and two lobula neurons (HR1 and HP1) in the honeybee, Apis mellifera.

Phagotrophic phytoflagellates in microbial food webs

Phagotrophy by pigmented flagellates is known from the literature but has recently been rediscovered in the context of microbial food webs. Particle ingestion rates were found to be equivalent for pigmented and nonpigmented microflagellates in both field and laboratory studies. Ingestion rates of the chrysophytes Ochromonas danica, O. minuta, and Poterioochromonas malhamensis, the dinoflagellate Peridinium inconspicuum, and the cryptophytes Cryptomonas ovata and C. erosa were compared with those of two nonpigmented Monas species using 0.57 μm polystyrene beads as a food source. Ingestion rates were 0.31 to 3.17 beads/cell/h and filtration rates were 10⁻⁷ to 10⁻⁸ ml/cell/h with no detectable difference between pigmented and nonpigmented forms. Ingestion rates in unpigmented Monas species showed a linear increase with increasing particle concentration from 1.9 × 10⁶ to 1.6 × 10⁷ beads/ml. Light and DOC levels in the range of those encountered by phytoflagellates in the field also influenced laboratory measurements of bead ingestion by Poterioochromonas malhamensis. Ingestion rates decreased and photosynthesis increased over the natural PAR light range from 0 to 1800 microeinsteins/s/m². At 40 microeinsteins/s/m² maximum ingestion rates and high rates of photosynthesis occurred simultaneously. Ingestion rates decreased above 4 mgC/l supplied as glucose. DOC levels commonly occurring in Lake Oglethorpe range from 3.5 to 10.0 mgC/l. These studies suggest that mixotrophy, the trophic utilization of particulate food and dissolved organic matter as well as photosynthetically fixed organic matter, is a balanced process that can be regulated by environmental conditions. In field studies during a chrysophyte bloom, phytoflagellate grazing exceeded heterotrophic microflagellate grazing and constituted up to 55% of the bactivory of all microflagellates, ciliates, rotifers, and crustaceans combined. Neither bacterial abundance, light nor temperature were good predicters of grazing rates for the phagotrophic phytoflagellate association as a whole during this unstratified period. Phagotrophs are often most abundant at the metalimnetic plate during stratification.     

Channels formed by colicin E1 in planar lipid bilayers are large and exhibit pH-dependent ion selectivity

Summary The E1 subgroup (E1, A, Ib, etc.) of antibacterial toxins called colicins are known to form voltage-dependent channels in planar lipid bilayers. The genes for colicins E1, A and Ib have been cloned and sequenced, making these channels interesting models for the widespread phenomenon of voltage dependence in cellular channels. In this paper we investigate ion selectivity and channel size—properties relevant to model building. Our major finding is that the colicin E1 channel is large, having a diameter ofat least 8 Å at its narrowest point. We established this from measurements of reversal potentials for gradients formed by salts of large cations or large anions. In so doing, we exploited the fact that the colicin channel is permeable to both cations and anions, and its relative selectivity to them is a functions and anions, and its relative selectivity to them is a function of pH. The channel is anion selective (Cl^– over K⁺) in neutral membranes, and the degree of selectivity is highly dependent on pH. In negatively charged membranes, it becomes cation selective at pH's higher than about 5. Experiments with pH gradients cross the membrane suggest that titratable groups both within the channel lumen and near the channel ends affect the selectivity. Individual E1 channels have more than one open conductance state, all displaying comparable ion selectivity. Colicins A and Ib also exhibit pH-dependent ion selectivity, and appear to have even larger lumens than E1.