Trimethyloxonium modification of single batrachotoxin-activated sodium channels in planar bilayers. Changes in unit conductance and in block by saxitoxin and calcium

Single batrachotoxin-activated sodium channels from rat brain were modified by trimethyloxonium (TMO) after incorporation in planar lipid bilayers. TMO modification eliminated saxitoxin (STX) sensitivity, reduced the single channel conductance by 37%, and reduced calcium block of inward sodium currents. These effects always occurred concomitantly, in an all-or-none fashion. Calcium and STX protected sodium channels from TMO modification with potencies similar to their affinities for block. Calcium inhibited STX binding to rat brain membrane vesicles and relieved toxin block of channels in bilayers, apparently by competing with STX for the toxin binding site. These results suggest that toxins, permeant cations, and blocking cations can interact with a common site on the sodium channel near the extracellular surface. It is likely that permeant cations transiently bind to this superficial site, as the first of several steps in passing inward through the channel.     

Age,smoking habits,heat stress,and their interactive effects with carbon monoxide and peroxyacetylnitrate on man's aerobic power

Metabolic, body temperature, and cardiorespiratory responses of 16 healthy middle-aged (40–57 years) men, 9 nonsmokers and 7 smokers, were obtained during tests of maximal aerobic power at ambient environmental temperatures of 25 ± 0.5 and 35 ± 0.5°C and 20% relative humidity under four conditions: (a) filtered air, FA; (b) 50 ppm carbon monoxide in filtered air, CO; (c) 0.27 ppm peroxyacetylnitrate in filtered air, PAN; and (d) a combination of all three mixtures, PANCO. There was no significant change in maximum aerobic power \(\left( {\dot VO2max} \right)\) related to the presence of air pollutants, although total working time was lowered in the 25°C environment while breathing CO. Older nonsmokers did have a decrement in \(\left( {\dot VO2max} \right)\) while breathing 50 ppm CO, while older smokers failed to show any change. This difference was related to the initial COHb levels of the smokers, who, when breathing this level of ambient CO, had only a 14% increase in COHb over their initial levels in contrast to the 200% increase in the nonsmokers. Smoking habits were the most influential factor affecting the cardiorespiratory responses of these older men to maximal exercise. Regardless of ambient conditions, smokers had a significantly lower (27%) aerobic power than nonsmokers, were breathing closer to their maximal breathing capacities throughout the walk, and had a higher respiratory exchange ratio. While the \(\left( {\dot VO2max} \right)\) of nonsmokers was only 6% less than that of younger nonsmoking males ( \(\bar x\) age = 25 years) working under similar conditions, the aerobic power of the older smokers was 26% lower than that of young smokers ( \(\bar x\) age = 24 years).     

Free-radical-induced mutation vs redox regulation: Costs and benefits of genes in organelles

The prokaryotic endosymbionts that became plastids and mitochondria contained genes destined for one of three fates. Genes required for free-living existence were lost. Most genes useful to the symbiosis were transferred to the nucleus of the host. Some genes, a small minority, were retained within the organelle. Here we suggest that a selective advantage of movement of genes to the nucleus is decreased mutation: plastids and mitochondria have high volume-specific rates of redox reactions, producing oxygen free radicals that chemically modify DNA. These mutations lead to synthesis of modified electron carriers that in turn generate more mutagenic free radicals—the “vicious circle” theory of aging. Transfer of genes to the nucleus is also advantageous in facilitating sexual recombination and DNA repair. For genes encoding certain key components of photosynthesis and respiration, direct control of gene expression by redox state of electron carriers may be required to minimize free radical production, providing a selective advantage of organelle location which outweighs that of location in the nucleus. A previous proposal for transfer of genes to the nucleus is an economy of resources in having a single genome and a single apparatus for gene expression, but this argument fails if any organellar gene is retained. A previous proposal for the retention of genes within organelles is that certain proteins are organelle-encoded because they cannot be imported, but there is now evidence against this view. Decreased free radical mutagenesis and increased sexual recombination upon transfer to the nucleus together with redox control of gene expression in organelles may now account for the slightly different gene distributions among nuclei, plastids, and mitochondria found in major eukaryote taxa. This analysis suggests a novel reason for uniparental inheritance of organelles and the evolution of anisogametic sex, and may also account for the occurrence of nitrogen fixation in symbionts rather than in nitrogen-fixing organelles. Correspondence to: J.F. Allen     

Relative apparent synapomorphy analysis (RASA). I: The statistical measurement of phylogenetic signal

We have developed a new approach to the measurement of phylogenetic signal in character state matrices called relative apparent synapomorphy analysis (RASA). RASA provides a deterministic, statistical measure of natural cladistic hierarchy (phylogenetic signal) in character state matrices. The method works by determining whether a measure of the rate of increase of cladistic similarity among pairs of taxa as a function of phenetic similarity is greater than a null equiprobable rate of increase. Our investigation of the utility and limitations of RASA using simulated and bacteriophage T7 data sets indicates that the method has numerous advantages over existing measures of signal. A first advantage is computational efficiency. A second advantage is that RASA employs known methods of statistical inference, providing measurable sensitivity and power. The performance of RASA is examined under various conditions of branching evolution as the number of characters, character states per character, and mutations per branch length are varied. RASA appears to provide an unbiased and reliable measure of phylogenetic signal, and the general approach promises to be useful in the development of new techniques that should increase the rigor and reliability of phylogenetic estimates.      

Spatial scale impacts microbial community composition and distribution within and across stream ecosystems in North and Central America

A mechanistic understanding of factors that structure spatiotemporal community composition is a major challenge in microbial ecology. Our study of microbial communities in the headwaters of three freshwater stream networks showed significant community changes at the small spatial scale of benthic habitats when compared to changes at mid- and large-spatial scales associated with stream order and catchment. Catchment (which included temperate and tropical catchments) had the strongest influence on community composition followed by habitat type (epipsammon or epilithon) and stream orders. Alpha diversity of benthic microbiomes resulted from interactions between catchment, habitat, and canopy. Epilithon contained relatively more Cyanobacteria and algae while Acidobacteria and Actinobacteria proportions were higher in epipsammic habitats. Turnover from replacement created ~60%–95% of beta diversity differences among habitats, stream orders, and catchments. Turnover within a habitat type generally decreased downstream indicating longitudinal linkages in stream networks while between habitat turnover also shaped benthic microbial community assembly. Our study suggests that factors influencing microbial community composition shift in dominance across spatial scales, with habitat dominating locally and catchment dominating globally.     

SOURCES OF INORGANIC CARBON FOR PHOTOSYNTHESIS BY THREE SPECIES OF MARINE DIATOM1

The utilization of inorganic carbon by three species of marine diatom, Skeletonema costatum (Grev.) Cleve. Ditylum brightwellii (West) Grun., and Chaetoceros calcitrans Paulsen was investigated using an inorganic carbon isotopic disequilibnum technique and inorganic carbon dose-response curves. Stable carbon isotope data of the diatoms are also presented. Observed rates of photosynthetic oxygen evolution were greater than could be accounted for by the theoretical rate of CO₂ supply from the uncatalyzed dehydration of HCO₃^? in the external medium, suggesting use of HCO₃^? as an inorganic carbon source. Data from the isotopic disequilibrium experiment demonstrate the use of both HCO₃^? and CO₂ for photosynthesis. Carbon isotope discrimination values support the use of HCO₃^? by the diatoms.     

The embryology and relationships ofPenaeaceae (Myrtales)

Two species ofPenaeaceae (Penaea mucronata andSaltera sarcocolla), a unique South African family ofMyrtales, were investigated embryologically.Penaeaceae clearly agrees with otherMyrtales in its basic embryological characteristics, and further is characterized by its highly specialized features: ephemeral endothecium, 16-nucleatePenaea-type embryo sac, and unique ovular form. A wider range of affinities of families includingPenaeaceae, Oliniaceae, Rhynchocalycaceae, Alzateaceae, andCrypteroniaceae sensu stricto, as well as a possible common divergence from an ancestral line leading toLythraceae and/orMelastomataceae, are discussed on embryological and other grounds.