Mechanism of α Factor Biosynthesis in Saccharomyces cerevisiae

The biosynthesis of alpha factor, a mating-type-specific regulatory oligopeptide which is secreted by Saccharomyces cerevisiae cells of alpha mating type, was studied. In batch cultures only small amounts of the peptide were synthesized during the exponential growth phase. During the stationary phase, alpha factor was produced at a constant rate and accumulated in the culture medium. Inhibition of translation in wild-type cells by cycloheximide, or in mutant strains under conditions which blocked protein or ribonucleic acid (RNA) synthesis completely inhibited the production of alpha factor. These results indicate that the factor is produced by ribosomal translation of a specific messenger RNA and not by an extraribosomal mechanism of peptide synthesis.     

Genetic structure of natural populations ofDryas iulia (Lepidoptera: Nymphalidae) revealed by enzyme polymorphism and mitochondrial DNA (mtDNA) restriction fragment length polymorphism (RFLP)

Dryas iulia appears to have undergone a mode of evolution different from that of other members of its subfamily (Heliconiinae). While other species constitute highly subdivided and inbred populations, those ofD. iulia are thought to be large and uniform. Analyzing six samples from Southern Brazil (state of Rio Grande do Sul) in relation to three enzyme systems (EST, LAP, and PGM) and their mtDNA RFLP patterns, we found that they are very similar at the molecular level. TheF statistics for enzyme polymorphism data revealed that inbreeding makes a great contribution to the population homozygosity, sinceF _IS equals 0.1322 andF _ST equals 0.0023. Since the chi-square test showed thatF _ST is not significant, we conclude that all localities belong to the same population. The mtDNA differentiation was about 12 times greater than for nuclear genes;F _ST was equivalent to 0.0265. We suggest that this difference is due to a higher dispersal of males, in relation to females.     

The intrinsic electrophysiological characteristics of fly lobula plate tangential cells: I. Passive membrane properties

The passive membrane properties of the tangential cells in the fly lobula plate (CH, HS, and VS cells, Fig. 1) were determined by combining compartmental modeling and current injection experiments. As a prerequisite, we built a digital base of the cells by 3D-reconstructing individual tangential cells from cobalt-stained material including both CH cells (VCH and DCH cells), all three HS cells (HSN, HSE, and HSS cells) and most members of the VS cell family (Figs. 2, 3). In a first series of experiments, hyperpolarizing and depolarizing currents were injected to determine steady-state I-V curves (Fig. 4). At potentials more negative than resting, a linear relationship holds, whereas at potentials more positive than resting, an outward rectification is observed. Therefore, in all subsequent experiments, when a sinusoidal current of variable frequency was injected, a negative DC current was superimposed to keep the neurons in a hyperpolarized state. The resulting amplitude and phase spectra revealed an average steady-state input resistance of 4 to 5 M and a cut-off frequency between 40 and 80 Hz (Fig. 5). To determine the passive membrane parameters R _m (specific membrane resistance), R _i (specific internal resistivity), and C _m (specific membrane capacitance), the experiments were repeated in computer simulations on compartmental models of the cells (Fig. 6). Good fits between experimental and simulation data were obtained for the following values: R _m = 2.5 kcm², R _i = 60 cm, and C _m = 1.5 F/cm² for CH cells; R _m = 2.0 kcm², R _i = 40 cm, and C _m = 0.9 F/cm² for HS cells; R _m = 2.0 kcm², R _i = 40 cm, and C _m = 0.8 F/cm² for VS cells. An error analysis of the fitting procedure revealed an area of confidence in the R _m -R _i plane within which the R _m -R _i value pairs are still compatible with the experimental data given the statistical fluctuations inherent in the experiments (Figs. 7, 8). We also investigated whether there exist characteristic differences between different members of the same cell class and how much the exact placement of the electrode (within ±100 m along the axon) influences the result of the simulation (Fig. 9). The membrane parameters were further examined by injection of a hyperpolarizing current pulse (Fig. 10). The resulting compartmental models (Fig. 11) based on the passive membrane parameters determined in this way form the basis of forthcoming studies on dendritic integration and signal propagation in the fly tangential cells (Haag et al., 1997; Haag and Borst, 1997).     

Implications of a non-lamellar lipid phase for the tight junction stability. Part II: Reversible modulation of transepithelial resistance in high and low resistance MDCK-cells by basic amino acids, Ca2+, protamine and protons.

The transepithelial resistance of confluent epithelial cell monolayers was monitored to investigate the influence of basic amino acids, Ca2+, protamine and protons on tight junction electrical resistance. In an accompanying paper we investigated the effect of these substances on the lamellar/hexagonal II phase transition in reconstituted phospholipid membranes containing phosphatidylserine and phosphatidylethanolamine. We conclude that the permeability of tight junctions may be described by a lipid phase equilibrium where the lamellar phase corresponds to an open state and the hexagonal lipid phase to the closed state of the cell contact. This dynamic lipid model is well suited to describe the morphological as well as functional properties of the tight junctions.     

P2X7-mediated ATP secretion is accompanied by depletion of cytosolic ATP

Purinergic Signalling - ATP and its metabolites are important extracellular signal transmitters acting on purinergic P2 and P1 receptors. Most cells can actively secrete ATP in response to a...     

Compensatory vs. pseudocompensatory evolution in molecular and developmental interactions

The evolution of molecules, developmental circuits, and new species are all characterized by the accumulation of incompatibilities between ancestors and descendants. When specific interactions between components are necessary at any of these levels, this requires compensatory coevolution. Theoretical treatments of compensatory evolution that only consider the endpoints predict that it should be rare because intermediate states are deleterious. However, empirical data suggest that compensatory evolution is common at all levels of molecular interaction. A general solution to this paradox is provided by plausible neutral or nearly neutral intermediates that possess informational redundancy. These intermediates provide an evolutionary path between coadapted allelic combinations. Although they allow incompatible end points to evolve, at no point was a deleterious mutation ever in need of compensation. As a result, what appears to be compensatory evolution may often actually be “pseudocompensatory.” Both theoretical and empirical studies indicate that pseudocompensation can speed the evolution of intergenic incompatibility, especially when driven by adaptation. However, under strong stabilizing selection the rate of pseudocompensatory evolution is still significant. Important examples of this process at work discussed here include the evolution of rRNA secondary structures, intra- and inter-protein interactions, and developmental genetic pathways. Future empirical work in this area should focus on comparing the details of intra- and intergenic interactions in closely related organisms.     

Genetic load in sexual and asexual diploids: segregation, dominance and genetic drift

In diploid organisms, sexual reproduction rearranges allelic combinations between loci (recombination) as well as within loci (segregation). Several studies have analyzed the effect of segregation on the genetic load due to recurrent deleterious mutations, but considered infinite populations, thus neglecting the effects of genetic drift. Here, we use single-locus models to explore the combined effects of segregation, selection, and drift. We find that, for partly recessive deleterious alleles, segregation affects both the deterministic component of the change in allele frequencies and the stochastic component due to drift. As a result, we find that the mutation load may be far greater in asexuals than in sexuals in finite and/or subdivided populations. In finite populations, this effect arises primarily because, in the absence of segregation, heterozygotes may reach high frequencies due to drift, while homozygotes are still efficiently selected against; this is not possible with segregation, as matings between heterozygotes constantly produce new homozygotes. If deleterious alleles are partly, but not fully recessive, this causes an excess load in asexuals at intermediate population sizes. In subdivided populations without extinction, drift mostly occurs locally, which reduces the efficiency of selection in both sexuals and asexuals, but does not lead to global fixation. Yet, local drift is stronger in asexuals than in sexuals, leading to a higher mutation load in asexuals. In metapopulations with turnover, global drift becomes again important, leading to similar results as in finite, unstructured populations. Overall, the mutation load that arises through the absence of segregation in asexuals may greatly exceed previous predictions that ignored genetic drift.