An endogenous rate of time preference, the Penrose effect, and dynamic optimality of environmental quality.

In the present paper, the endogenous theory of time preference is extended to analyze those processes of capital accumulation and changes in environmental quality that are dynamically optimum with respect to the intertemporal preference ordering of the representative individual of the society in question. The analysis is carried out within the conceptual framework of the dynamic analysis of environmental quality, as has been developed by a number of economists for specific cases of the fisheries and forestry commons. The duality principles on intertemporal preference ordering and capital accumulation are extended to the situation where processes of capital accumulation are subject to the Penrose effect, which exhibit the marginal decrease in the effect of investment in private and social overhead capital upon the rate at which capital is accumulated. The dynamically optimum time-path of economic activities is characterized by the proportionality of two systems of imputed, or efficient, prices, one associated with the given intertemporal ordering and another associated with processes of accumulation of private and social overhead capital. It is particularly shown that the dynamically optimality of the processes of capital accumulation involving both private and social overhead capital is characterized by the conditions that are identical with those involving private capital, with the role of social overhead capital only indirectly exhibited.

Intrasarcomere [Ca2+] gradients in ventricular myocytes revealed by high speed digital imaging microscopy.

Cardiac muscle contraction is triggered by a small and brief Ca2+ entry across the t-tubular membranes, which is believed to be locally amplified by release of Ca2+ from the adjacent junctional sarcoplasmic reticulum (SR). As Ca2+ diffusion is thought to be markedly attenuated in cells, it has been predicted that significant intrasarcomeric [Ca2+] gradients should exist during activation. To directly test for this, we measured [Ca2+] distribution in single cardiac myocytes using fluorescent [Ca2+] indicators and high speed, three-dimensional digital imaging microscopy and image deconvolution techniques. Steep cytosolic [Ca2+] gradients from the t-tubule region to the center of the sarcomere developed during the first 15 ms of systole. The steepness of these [Ca2+] gradients varied with treatments that altered Ca2+ release from internal stores. Electron probe microanalysis revealed a loss of Ca2+ from the junctional SR and an accumulation, principally in the A-band during activation. We propose that the prolonged existence of [Ca2+] gradients within the sarcomere reflects the relatively long period of Ca2+ release from the SR, the localization of Ca2+ binding sites and Ca2+ sinks remote from sites of release, and diffusion limitations within the sarcomere. The large [Ca2+] transient near the t-tubular/ junctional SR membranes is postulated to explain numerous features of excitation-contraction coupling in cardiac muscle.

Complexity of the erythroid transcription factor NF-E2 as revealed by gene targeting of the mouse p18 NF-E2 locus.

High-level globin expression in erythroid precursor cells depends on the integrity of NF-E2 recognition sites, transcription factor AP-1-like protein-binding motifs, located in the upstream regulatory regions of the alpha- and beta-globin loci. The NF-E2 transcription factor, which recognizes these sites, is a heterodimer consisting of (i) p45 NF-E2 (the larger subunit), a hematopoietic-restricted basic leucine zipper protein, and (ii) a widely expressed basic leucine zipper factor, p18 NF-E2, the smaller subunit. p18 NF-E2 protein shares extensive homology with the maf protooncogene family. To determine an in vivo role for p18 NF-E2 protein we disrupted the p18 NF-E2-encoding gene by homologous recombination in murine embryonic stem cells and generated p18 NF-E2-/- mice. These mice are indistinguishable from littermates throughout all phases of development and remain healthy in adulthood. Despite the absence of expressed p18 NF-E2, DNA-binding activity with the properties of the NF-E2 heterodimer is present in fetal liver erythroid cells of p18 NF-E2-/- mice. We speculate that another member of the maf basic leucine zipper family substitutes for the p18 subunit in a complex with p45 NF-E2. Thus, p18 NF-E2 per se appears to be dispensable in vivo.

Structure of the N intermediate of bacteriorhodopsin revealed by x-ray diffraction.

X-ray diffraction experiments revealed the structure of the N photointermediate of bacteriorhodopsin. Since the retinal Schiff base is reprotonated from Asp-96 during the M to N transition in the photocycle, and Asp-96 is reprotonated during the lifetime of the N intermediate, or immediately after, N is a key intermediate for understanding the light-driven proton pump. The N intermediate accumulates in large amounts during continuous illumination of the F171C mutant at pH 7 and 5 degrees Celsius. Small but significant changes of the structure were detected in the x-ray diffraction profile under these conditions. The changes were reversible and reproducible. The difference Fourier map indicates that the major change occurs near helix F. The observed diffraction changes between N and the original state were essentially identical to the diffraction changes reported for the M intermediate of the D96N mutant of bacteriorhodopsin. Thus, we find that the protein conformations of the M and N intermediates of the photocycle are essentially the same, in spite of the fact that in M the Schiff base is unprotonated and in N it is protonated. The observed structural change near helix F will increase access of the Schiff base and Asp-96 to the cytoplasmic surface and facilitate the proton transfer events that begin with the decay of the M state.

Population dynamics of flaviviruses revealed by molecular phylogenies.

The phylogeny of 123 complete envelope gene sequences was reconstructed in order to understand the evolution of tick- and mosquito-borne flaviviruses. An analysis of phylogenetic tree structure reveals a continual and asymmetric branching process in the tick-borne flaviviruses, compared with an explosive radiation in the last 200 years in viruses transmitted by mosquitoes. The distinction between these two viral groups probably reflects differences in modes of dispersal, propagation, and changes in the size of host populations. The most serious implication of this work is that growing human populations are being exposed to an expanding range of increasingly diverse viral strains.

Structure of the sodium channel pore revealed by serial cysteine mutagenesis.

The pores of voltage-gated cation channels are formed by four intramembrane segments that impart selectivity and conductance. Remarkably little is known about the higher order structure of these critical pore-lining or P segments. Serial cysteine mutagenesis reveals a pattern of side-chain accessibility that contradicts currently favored structural models based on alpha-helices or beta-strands. Like the active sites of many enzymes of known structure, the sodium channel pore consists of irregular loop regions.

Contrasting histories of avian and mammalian Mhc genes revealed by class II B sequences from songbirds.

To explore the evolutionary dynamics of genes in the major histocompatibility complex (Mhc) in nonmammalian vertebrates, we have amplified complete sequences of the polymorphic second (beta1) and third (beta2) exons of class II beta chain genes of songbirds. The pattern of nucleotide substitution in the antigen-binding site of sequences cloned from three behaviorally and phylogenetically divergent songbirds [scrub jays Aphelocoma coerulescens), red-winged blackbirds (Agelaius phoeniceus), and house finches (Carpodacus mexicanus) reveals that class II B genes of songbirds are subject to the same types of diversifying forces as those observed at mammalian class II loci. By contrast, the tree of avian class II B genes reveals that orthologous relationships have not been retained as in placental mammals and that, unlike class II genes in mammals, genes in songbirds and chickens have had very recent common ancestors within their respective groups. Thus, whereas the selective forces diversifying class II B genes of birds are likely similar to those in mammals, their long-term evolutionary dynamics appear to be characterized by much higher rates of concerted evolution.

Molecular heterogeneity of progenitors and radial migration in the developing cerebral cortex revealed by transgene expression.

We have analyzed the developmental pattern of beta-galactosidase (beta-gal) expression in the cerebral cortex of the beta 2nZ3'1 transgenic mouse line, which was generated using regulatory elements of the beta 2-microglobulin gene and shows ectopic expression in nervous tissue. From embryonic day 10 onward, beta-gal was expressed in the medial and dorsal cortices, including the hippocampal region, whereas lateral cortical areas were devoid of labeling. During the period of cortical neurogenesis (embryonic days 11-17), beta-gal was expressed by selective precursors in the proliferative ventricular zone of the neocortex and hippocampus, as well as by a number of migrating and postmigratory neurons arranged into narrow radial stripes above the labeled progenitors. Thus, the transgene labels a subset of cortical progenitors and their progeny. Postnatally, radial clusters of beta-gal-positive neurons were discernible until postpartum day 10. At this age, the clusters were 250 to 500 microns wide, composed of neurons spanning all the cortical layers and exhibiting several neuronal phenotypes. These data suggest molecular heterogeneity of cortical progenitors and of the cohorts of postmitotic neurons originating from them, which implies intrinsic molecular mosaicism in both cortical progenitors and developing neurons. Furthermore, the data show that neurons committed to the expression of the transgene migrate along very narrow, radial stripes.

Object-related activity revealed by functional magnetic resonance imaging in human occipital cortex.

The stages of integration leading from local feature analysis to object recognition were explored in human visual cortex by using the technique of functional magnetic resonance imaging. Here we report evidence for object-related activation. Such activation was located at the lateral-posterior aspect of the occipital lobe, just abutting the posterior aspect of the motion-sensitive area MT/V5, in a region termed the lateral occipital complex (LO). LO showed preferential activation to images of objects, compared to a wide range of texture patterns. This activation was not caused by a global difference in the Fourier spatial frequency content of objects versus texture images, since object images produced enhanced LO activation compared to textures matched in power spectra but randomized in phase. The preferential activation to objects also could not be explained by different patterns of eye movements: similar levels of activation were observed when subjects fixated on the objects and when they scanned the objects with their eyes. Additional manipulations such as spatial frequency filtering and a 4-fold change in visual size did not affect LO activation. These results suggest that the enhanced responses to objects were not a manifestation of low-level visual processing. A striking demonstration that activity in LO is uniquely correlated to object detectability was produced by the "Lincoln" illusion, in which blurring of objects digitized into large blocks paradoxically increases their recognizability. Such blurring led to significant enhancement of LO activation. Despite the preferential activation to objects, LO did not seem to be involved in the final, "semantic," stages of the recognition process. Thus, objects varying widely in their recognizability (e.g., famous faces, common objects, and unfamiliar three-dimensional abstract sculptures) activated it to a similar degree. These results are thus evidence for an intermediate link in the chain of processing stages leading to object recognition in human visual cortex.

Features of MotA proton channel structure revealed by tryptophan-scanning mutagenesis.

The MotA protein of Escherichia coli is a component of the flagellar motors that functions in transmembrane proton conduction. Here, we report several features of MotA structure revealed by use of a mutagenesis-based approach. Single tryptophan residues were introduced at many positions within the four hydrophobic segments of MotA, and the effects on function were measured. Function was disrupted according to a periodic pattern that implies that the membrane-spanning segments are alpha-helices and that identifies the lipid-facing parts of each helix. The results support a hypothesis for MotA structure and mechanism in which water molecules form most of the proton-conducting pathway. The success of this approach in studying MotA suggests that it could be useful in structure-function studies of other integral membrane proteins.

Relatedness threshold for the production of female sexuals in colonies of a polygynous ant, Myrmica tahoensis, as revealed by microsatellite DNA analysis.

The genetic relationships of colony members in the ant Myrmica tahoensis were determined on the basis of highly polymorphic microsatellite DNA loci. These analyses show that colonies fall into one of two classes. In roughly half of the sampled colonies, workers and female offspring appear to be full sisters. The remaining colonies contain offspring produced by two or more queens. Colonies that produce female sexuals are always composed of highly related females, while colonies that produce males often show low levels of nestmate relatedness. These results support theoretical predictions that workers should skew sex allocation in response to relatedness asymmetries found within colonies. The existence of a relatedness threshold below which female sexuals are not produced suggests a possible mechanism for worker perception of relatedness. Two results indicate that workers use genetic cues, not queen number, in making sex-allocation decisions. (i) The number of queens in a colony was not significantly correlated with either the level of relatedness asymmetry or the sex ratio. (ii) Sex-ratio shifts consistent with a genetically based mechanism of relatedness assessment were seen in an experiment involving transfers of larvae among unrelated nests. Thus workers appear to make sex-allocation decisions on the basis of larval cues and appear to be able to adjust sex ratios long after egg laying.