Electron donation in Photosystem II

The electron transfer resulting from illumination and dark storage of PS II has been studied using EPR signals from several electron carriers. The recombination of D⁺ (Signal II) and Q⁻_A formed by illumination occurred during dark storage at 77 K and was used to deplete reaction centres of D⁺. The donor D was then shown to be oxidized in the dark by the S₂ state of the oxygen-evolving complex. A slow change which occurred during dark storage of PS II samples was detected using the power saturation characteristics of D. We interpret this effect on D to be an indirect result of a rearrangement of the manganese complex during long-term dark adaptation. A role for D in the stability, protection and perhaps initial manganese binding of the oxygen-evolving complex is suggested.     

Exciton interactions in synthetic porphyrin dimers

The Soret absorption spectra of six synthetic rigid porphyrin dimers whose crystal structures have been determined are simulated using simple exciton theory. The objective is to test the validity of the point dipole and associated approximations; the electronic interaction parameters are thus calculated using data obtained from the monomer spectra, with no adjustable parameters. Satisfactory agreement between theory and experiment is obtained for one class of dimers but not for a second. This poses a challenge for semiempirical electronic structure methods as to whether improvements over the point dipole calculations can be obtained.     

Allozyme diversity and introgression in the Galapagos Islands endemic Gossypium darwinii and its relationship to continental G. barbadense

Gossypium darwiniiWatt is a tetraploid cotton endemic to the Galapagos Islands. Opinion has been divided as to whether or not it deserves recognition at the specific rank, with some considering it a variety of its presumed progenitor, the widely distributed South American species G. barbadense L. A previous hypothesis states that much of the perceived intergradation between the two taxa arose as a consequence of introgression from G. barbadense following its introduction to the archipelago during the past several hundred years. We performed allozyme analysis on 58 accessions of G. darwinii from six islands, using 17 enzymes collectively encoded by 59 loci. Levels of variation were high for an island endemic, with a mean number of alleles per locus of 1.34 and an average panmictic heterozygosity of 0.062. Principal component analysis revealed clustering of accessions according to their island of origin, and a spatial pattern of island-clusters that approximates geographical relationships among islands. Genetic relationships of G. darwinii with G. barbadense and G. hirsutum L. were studied using previously generated allozyme data. Significant introgression of G. hirsutum alleles was detected; however morphological considerations support the hypothesis that much of G. darwinii's diversity stems from interspecific gene flow from G. barbadense, Evidence is presented suggesting that the occurrence of G. hirsutum alleles in G. darwinii derives not from direct hybridization, but from a mediated transfer through introduced, G. hirsutum-introgressed; G. barbadense. Gossypium darwinii and G. barbadense are nearly fixed for different alleles at four loci and each contains a large number of unique alleles. Notwithstanding the high interspecific Nei's genetic identity (0.949), the allozyme data support geographical and morphological evidence in suggesting that a specific rank for G. darwinii is warranted.     

Screening for prolonged incubation of HTLV-I infection in British and Jamaican relatives of British patients with tropical spastic paraparesis.

OBJECTIVE--To compare the prevalence of antibody to and proviral DNA of the retrovirus HTLV-I in relatives of 11 British patients with tropical spastic paraparesis who had migrated from Jamaica before they developed symptoms, and to examine factors possibly related to transmission of HTLV-I. DESIGN--Migrant, family study. Antibody state was determined by several methods and confirmed by western blotting; the polymerase chain reaction was used to detect proviral DNA. SETTING--Britain and Jamaica. SUBJECTS--All available first degree relatives: those born and still resident in Jamaica (group 1); those born in Jamaica who migrated to Britain (group 2); and index patients'' children who were born and resident in Britain (group 3). All had been breast fed and none had had blood transfusions. RESULTS--Of the 66 living relatives, 60 were traced. Seroprevalence among those born in Jamaica (irrespective of current residence) was 22% (10/46; 95% confidence limits 9 to 34%) compared with zero among British born offspring (0/14) and was higher in group 2 at 33% (7/21; 12 to 55%) than in group 1 at 12% (3/25; 0 to 25%). (Patients in group 1 had the greatest mean age.) Proviral DNA was not detected in any subject negative for HTLV-I antibody, making prolonged viral incubation in those negative for the antibody unlikely. CONCLUSION--In this sample factors related to place of birth and early residence were more important in transmission of HTLV-I than maternal or age effects. In areas with a low to moderate prevalence policies of preventing mothers who are carriers of the virus from breast feeding would be premature.     

Minitron II system for precise control of the plant growth environment

A transparent, cylindrical chamber system was developed to allow measurement of gas-exchange by small crop canopies in the undisturbed plant growth environment. The system is an elaboration of the Minitron system developed previously to compare growth of small plants in different environments within the same general growth area. The Minitron II system described herein accommodates hydroponic culture and separate control of atmospheric composition in individual chambers. Root and shoot environments are compartmented separately to accommodate atmospheres of different flow rate and/or gaseous composition. A series of 0-rings and tension-adjustable springs allow carbon dioxide in the flowing atmosphere to be analyzed without cross-contamination between chamber compartments or from external gas sources. Carbon dioxide has been maintained at set point +/- 9 g m-3 over a range of CO2 concentrations from 382 to 2725 g m-3 and with an atmosphere turnover rate of 136.7 cm3 s-1 by computer-assisted mass flow controllers. Each chamber has dimensions large enough (61 cm internal diameter, 0.151 m3 internal volume) to allow adequate replication of individual plants for statistical purposes (e.g., up to 36 equally-spaced plant holders). No significant variation in growth or photosynthetic rate of leaf lettuce occurred between chambers for a given set of environmental conditions. Gas-exchange rates in different chambers changed to a similar extent as CO2 concentration in the flowing atmosphere or chamber temperature were varied by the same amount. When coupled with appropriate control systems, Minitron II chambers can provide separate controlled environments for multiple small plants with adequate precision and at relatively low cost.     

Ritual production of environmental history among the Arawakan Wakuénai of Venezuela

This essay examines ritual and ceremonial activities among the Arawakspeaking Wakuénai of the Venezuelan Amazon as processes of constructing power relations in changing historical and ecological conditions. Ritual evocations of the vertical dimension of power relations between mythic ancestors and human descendants adapt local populations to conditions of relatively severe stress, such as epidemics and scarcity of fish in long wet seasons. Other rituals evoke the horizontal dimension of power relations between affinally-related groups as a way of expanding the local descent group in conditions of lowered stress. These two ways of exercising ritual power link human populations to specific natural habitats and provide flexibility needed to adjust to demographic and other historical changes. Through ritual performances, the Wakuénai transform the natural environment into a cultural landscape of socialized objects and, conversely, remember the history of political relations among peoples through spirit-naming of natural species, objects, places, and geographic landmarks.     

[3H]MK-801 Labels a Site on the N-Methyl-D-Aspartate Receptor Channel Complex in Rat Brain Membranes

The potent noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist [3H]MK-801 bound with nanomolar affinity to rat brain membranes in a reversible, saturable, and stereospecific manner. The affinity of [3H]MK-801 was considerably higher in 5 mM Tris-HCl (pH 7.4) than in previous studies using Krebs-Henseleit buffer. [3H]MK-801 labels a homogeneous population of sites in rat cerebral cortical membranes with KD of 6.3 nM and Bmax of 2.37 pmol/mg of protein. This binding was unevenly distributed among brain regions, with hippocampus greater than cortex greater than olfactory bulb = striatum greater than medulla-pons, and the cerebellum failing to show significant binding. Detailed pharmacological characterization indicated [3H]MK-801 binding to a site which was competitively and potently inhibited by known noncompetitive NMDA receptor antagonists, such as phencyclidine, thienylcyclohexylpiperidine (TCP), ketamine, N-allylnormetazocine (SKF 10,047), cyclazocine, and etoxadrol, a specificity similar to sites labelled by [3H]TCP. These sites were distinct from the high-affinity sites labelled by the sigma receptor ligand (+)-[3H]SKF 10,047. [3H]MK-801 binding was allosterically modulated by the endogenous NMDA receptor antagonist Mg2+ and by other active divalent cations. These data suggest that [3H]MK-801 labels a high-affinity site on the NMDA receptor channel complex, distinct from the NMDA recognition site, which is responsible for the blocking action of MK-801 and other noncompetitive NMDA receptor antagonists.     

Rabbit MHC. II. Sequence analysis of the R-DP alpha- and beta-genes

Molecular genetics studies have recently revealed complexity in the MHC class II region that has not been detected by previous serologic and genetic studies. In humans, three subregions, DP, DQ, and DR, of the class II genes as well as the DZ alpha and DO beta genes, have been extensively characterized. Although homologs of these human genes were identified in many species, their expressibility has not been well defined in species other than the mouse. We have previously cloned the rabbit homologs of the HLA-DP alpha and beta genes whose protein products had never been detected. The sequences of rabbit DP alpha 1 and DP beta genes are reported herein and they indicate that the rabbit DP genes encode functional alpha- and beta-chains. Unfavorable nucleotides surrounding the first AUG codon may, however, reduce the translational efficiency of the R-DP beta mRNA and explain the difficulty in generating serologic reagents specific for rabbit DP molecules. A complex mutation in the beta 1 domain of the R-DP beta gene was similar to the one found in the H-2A beta 1 gene of five strains of mice. The origin of this mutation is discussed.