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.     

A yeast protein analogous to Escherichia coli RecA protein whose cellular level is enhanced after UV irradiation

Summary In Saccharomyces cerevisiae, a protein was recognized by polyclonal antibodies raised against homogeneous Escherichia coli K12 RecA protein. The cellular level of the yeast protein called RecAsc (molecular weight 44 kDa, pI 6.3), was transiently enhanced after UV irradiation. Protease inhibitors were required to minimize degradation of the RecAsc protein during cell lysis. The RecAsc protein exhibited similar basal levels and similar kinetics of increase after UV irradiation in DNA-repair proficient (RAD ⁺) strains carrying mitochondrial DNA or not (rho ⁰). This was also true for the following DNA-repair deficient (rad ^-) strains: rad2-6 rad6-1 rad52-1, a triple mutant blocked in three major repair pathways; rad6-, a mutant containing an integrative deletion in a gene playing a central role in mutagenesis; pso2-1, a mutant that exhibits a reduced rate of mutagenesis and recombination after exposure to DNA cross-linking agents.     

Reproductive strategy of a facultatively paedomorphic salamander Ambystoma talpoideum

Summary The reproductive strategy of the salamander Ambystoma talpoideum was studied on an age-specific basis in five breeding populations using ponds subjected to different annual probabilities of drying. Ambystoma talpoideum is facultatively paedomorphic in semi-permanent ponds but sexually mature individuals occur only as terrestrial morphs in temporary ponds. Larvae of paedomorphs and terrestrial morphs mature at the same age but different body sizes. Body mass and snout-vent length increased with age in both morphs. One-year old terrestrial morphs had larger snoutvent lengths than 1-year-old paedomorphs but were smaller in body mass. Clutch mass, number of ova, and diameter of ova increased with snout-vent length. After removing the effects of snout-vent length with an analysis of covariance, only the diameter of ova and the resultant clutch mass increased with age. One-year-old terrestrial morphs produced more ova than 1-year-old paedomorphs but paedomorphs have the potential to reproduce earlier than terrestrial morphs. Parental investment, as shown by the relationship between clutch size and egg size, increased with age. This suggested that as potential energy increased with age, A. talpoideum produced more eggs and larger eggs. Rather than optimizing egg size, individuals may produce a range of egg sizes that theoretically fare better in spatially and temporally varying environments.     

Temporal dynamics of estuarine phytoplankton: A case study of San Francisco Bay

Detailed surveys throughout San Francisco Bay over an annual cycle (1980) show that seasonal variations of phytoplankton biomass, community composition, and productivity can differ markedly among estuarine habitat types. For example, in the river-dominated northern reach (Suisun Bay) phytoplankton seasonality is characterized by a prolonged summer bloom of netplanktonic diatoms that results from the accumulation of suspended particulates at the convergence of nontidal currents (i.e. where residence time is long). Here turbidity is persistently high such that phytoplankton growth and productivity are severely limited by light availability, the phytoplankton population turns over slowly, and biological processes appear to be less important mechanisms of temporal change than physical processes associated with freshwater inflow and turbulent mixing. The South Bay, in contrast, is a lagoon-type estuary less directly coupled to the influence of river discharge. Residence time is long (months) in this estuary, turbidity is lower and estimated rates of population growth are high (up to 1–2 doublings d^–1), but the rapid production of phytoplankton biomass is presumably balanced by grazing losses to benthic herbivores. Exceptions occur for brief intervals (days to weeks) during spring when the water column stratifies so that algae retained in the surface layer are uncoupled from benthic grazing, and phytoplankton blooms develop. The degree of stratification varies over the neap-spring tidal cycle, so the South Bay represents an estuary where (1) biological processes (growth, grazing) and a physical process (vertical mixing) interact to cause temporal variability of phytoplankton biomass, and (2) temporal variability is highly dynamic because of the short-term variability of tides. Other mechanisms of temporal variability in estuarine phytoplankton include: zooplankton grazing, exchanges of microalgae between the sediment and water column, and horizontal dispersion which transports phytoplankton from regions of high productivity (shallows) to regions of low productivity (deep channels).Multi-year records of phytoplankton biomass show that large deviations from the typical annual cycles observed in 1980 can occur, and that interannual variability is driven by variability of annual precipitation and river discharge. Here, too, the nature of this variability differs among estuary types. Blooms occur only in the northern reach when river discharge falls within a narrow range, and the summer biomass increase was absent during years of extreme drought (1977) or years of exceptionally high discharge (1982). In South Bay, however, there is a direct relationship between phytoplankton biomass and river discharge. As discharge increases so does the buoyancy input required for density stratification, and wet years are characterized by persistent and intense spring blooms.     

The mechanism and specificity of iron transport in Rhodotorula pilimanae probed by synthetic analogs of rhodotorulic acid

The yeast Rhodotorula pilimanae produces the dihydroxamate siderophore rhodotorulic acid (RA) in prodigious amounts when starved for iron. Synthetic dihydroxamate analogs of RA have been prepared in which the diketopiperazine ring of RA is replaced by a simple chain of n methylene groups. It is found that R. pilimanae is able to accumulate iron using these achiral complexes, as well as from simple monohydroxamate analogs, at rates comparable to those of RA. While the Fe2RA3 complex does not enter the cell, there is a receptor system whose geometric requirements for siderophore recognition have been probed using analogs. In contrast to mono- or dihydroxamate ligands, the trihydroxamate siderophores such as ferrioxamine B are completely ineffective at delivering iron to R. pilimanae. This is ascribed to the greater stability of these complexes, which blocks release of the Fe(III) in a ligand exchange process that is required for uptake. To explore whether this ligand exchange involves redox catalysis, Ga(III) was substituted for Fe(III). The gallium was taken up at rates near those of iron and were also energy-dependent, as determined by metabolic inhibition with KCN.     

Cellular systems for the study of the biosynthesis of polyamines and ethylene,as well as of virus multiplication

Leaves of Chinese cabbage from healthy plants or from those infected with turnip yellow mosaic virus yield protoplasts which convert methionine to protein, S-adenosylmethionine, decarboxylated S-adenosylmethionine, spermidine, spermine and 1-aminocyclopropane-1-carboxylate. The enzyme spermidine synthase is entirely cytosolic and has been purified extensively. An inhibitor of this enzyme, dicyclohexylamine, blocks spermidine synthesis in intact protoplasts, and in so doing stimulates spermine synthesis. Aminoethoxyvinylglycine blocks the conversion of S-adenosylmethionine to 1-aminocyclopropane-1-carboxylate, the precursor to ethylene, in protoplasts. This inhibitor markedly stimulates the synthesis of both spermidine and spermine. Essentially all the protoplasts obtained from new leaves of plants infected 7 days earlier are infected. On incubation, such protoplasts convert exogenous methionine to viral protein and viral spermidine whose specific radioactivity is twice that of total cell spermidine. Exogeneous spermidine is also converted to cell putrescine and viral spermidine and spermine. Normal and virus-infected cells are being studied for their content of phenolic acid amides of the polyamines.     

The strontium-induced calcium-release process and its implication in contractility of skeletal muscle of Rana ridibunda

The electrical and mechanical activities of isolated frog muscle fibres have been recorded simultaneously under conditions (chloride-free saline containing 78.5 mM strontium acetate substituting for NaCl and CaCl2) that allow the development of a tubular strontium permeability. Under voltage-clamp conditions a large part of the contraction is due to the slow inward ISr since both are inhibited by Ni (10 mM). The remaining component of contraction, which seems to be potential-dependent, is not abolished by tetracaine (40 microM) which blocks the current-dependent component. A cumulative effect of strontium, which is not observed in the presence of Ni, leads to a 60-80% reduction in contractility for an estimated [Sr]i near 3 X 10(-4) M while the ending of the contraction observed when Sr is replaced by Ba is never obtained. In contrast no cumulative effect is observed when Ca substitutes for Sr. The first evoked inward current following a caffeine contracture fails to elicit a contraction, but in Ringer 78.5 Sr, contractility is progressively restored by successive depolarizations up to an amplitude which corresponds to 25-40% of the maximum activity. In the presence of Ca instead of Sr, the restoration of contractility reaches 100%. This recovery does not occur when the inward current is blocked by Ni. After strontium loading, a calcium entry fails immediately and reversibly to induce a mechanical response while barium ions induce a progressive and irreversible block of contractility. These results suggest that the strontium entry during successive depolarizations leads to a progressive replacement of intrareticular calcium by strontium. When all the calcium ions have been substituted for by strontium ions, the contractile apparatus remains capable of being activated by intrareticular strontium.     

Determinants in the perceptions of visual blight

Research on visual blight has been predominantly qualitative and has pointed out salient attributes of blight: that it is durable, visually demeaning, and somehow aesthetically depressing. Data from a national sample (n=3005) are used in this study to measure the effect of demographic variation on the perception of visual blight. The perception of blight shows marked and regular variation with socioeconomic status, marital status, and religiosity.Harris data for this study was purchased with funds from the Shell Foundation. The author wishes to acknowledge Robert Q. Hanham, whose assistance proved invaluable.     

Analysis of Dysgenesis-Induced Lethal Mutations on the X Chromosome of a Q Strain of DROSOPHILA MELANOGASTER

The Q strain known as v6 was tested for its ability to induce X-linked lethal mutations in male and female hybrids from crosses with M strains in the P-M system of hybrid dysgenesis. All measurements of the mutation rate were made on the X chromosome derived from the v6 strain. The lethal rate for young hybrid males from the cross M female X v6 male was 1.11% per chromosome. For older males, it was only 0.44%, suggesting that there is less mutational or more repair activity in the germ cells of the older males or that mutant cells are selectively eliminated as the hybrid males age. The lethal rate for hybrid females from comparable crosses was approximately the same for both ages that were tested. However, it was substantially less than the rate for the hybrid males--only 0.26% per chromosome. Genetically identical hybrid females from reciprocal crosses also showed a low mutation rate, 0.13% per chromosome. Again, there was no difference between young and old flies. Mapping experiments established that most of the lethal mutations that were recovered from the male and female hybrids were located in two regions on the X chromosome, one between bands 14B13 and 15A9 , the other between bands 19A1 and 20A , which encompasses the maroonlike locus. More refined mapping of the lethals in the maroonlike region demonstrated that the vast majority of these affected a single gene located in band 19C4 . Cytological analysis of the lethal chromosomes revealed that several carried rearrangements, including inversions, duplications and deficiencies. Chromosome breakage occurred primarily in bands 14D1 -3 and 18F- 20A , and most of the breaks in the latter segment were located in 19C . However, rearrangements involving 19C and mutations of the gene in 19C4 were mutually exclusive events. In situ hybridization of a P element probe to the chromosomes of v6 demonstrated that P elements reside at a minimum of five sites on the X chromosome. These P element sites correspond to the mutational and breakage hot spots on that chromosome. The combined genetic and cytological data imply that most of the X-linked lethal mutations that occur in M X v6 hybrids are due to local P element action. Consideration of these and other data suggest that v6 is a weak P strain in the P-M system of hybrid dysgenesis and that other Q strains might also be regarded in this way.