Structure of the KcsA potassium channel from Streptomyces lividans: a site-directed spin labeling study of the second transmembrane segment.

KcsA is a prokaryotic potassium channel. The present study employs cysteine scanning mutagenesis and site-directed spin labeling to investigate the structure of the second transmembrane segment (residues 82-120) in functional tetrameric channels reconstituted in lipid bilayers. Spin-spin interactions are observed between nitroxide side chains at symmetry-related sites close to the 4-fold axis of symmetry. To aid in quantitative analysis of these interactions, a new diamagnetic analogue of the nitroxide side chain is used to prepare magnetically dilute samples with constant structure. Using constraints imposed by the spin-spin interactions, a packing model for this segment is deduced that is in excellent agreement with the recently reported crystal structure [Doyle, D., et al. (1998) Science 280, 69-77]. The relatively immobilized state of the nitroxide side chains suggests that the channel is rigid on the electron paramagnetic resonance time scale. Moreover, the poor sulfhydryl reactivity of the cysteine at many locations indicates that the channel is not subject to the low-frequency fluctuations that permit reaction of buried cysteines. At sites expected to be located in the pore, the accessibility of the side chains to collision with O(2) or nickel(II) ethylenediaminediacetate is low. This inaccessibility, together with the generally low mobility of the side chains throughout the sequence, makes it difficult to detect the presence of the pore based on these measurements. However, the presence of a solvated pore can be directly demonstrated using a polarity parameter deduced from the EPR spectra recorded at low temperature. These measurements also reveal the presence of a polarity gradient in the phospholipid bilayer.     

The effect of sperm preparation and co-incubation time on in vitro fertilization of Bos indicus oocytes.

The objective of the present study was to evaluate the effect of various methods of sperm selection and various sperm-oocyte co-incubation times on in vitro fertilization (IVF) of zebu (Bos indicus) oocytes. Frozen semen from one ejaculate of a single bull was used for all treatments and replicates. After thawed, sperm was subjected to one of the three treatments: 45 and 90% discontinuous Percoll gradient, swim-up and washing by centrifugation. In all treatments, the spermatozoa were incubated with in vitro matured oocytes for 3, 6, 12 and 18h. After co-incubation oocytes were transferred to the culture medium and culture for 44h, when the cleavage was evaluated. The uncleavaged oocytes were fixed and stained to determine penetration, pronucleus formation and polyspermy. The sperm selection method did not influence (P<0.05) polyspermy, pronucleus formation, penetration and cleavage rates. No interaction between method of selection and sperm-oocyte co-incubation time was observed (P>0.05). However, sperm-oocyte co-incubation time affected fertilization. The lower penetration (26.5%) and cleavage rates (13.1%) were obtained at 3-h period. The penetration and cleavage percentages increased (P<0.05) progressively at 6h (63.3 and 54.4%) and 12h (77.6 and 67.6%). No differences (P>0.05) were observed between 12 and 18h of incubation for penetration and cleavage rates. The incidence of polyspermy and pronucleus formation was similar (P>0.05) for all time points. It is concluded that the methods used in this study for sperm selection do not affect fertilization; therefore, they all can be used for bovine IVF. In addition, regardless the method used better fertilization results were obtained when sperm and oocytes were co-incubated for 12h, and the prolongation of that time for up to 18h had no detrimental effect on fertilization.     

Investigating the biology of consciousness.

The fact that consciousness is a private, first-person phenomenon makes it more difficult to study than other cognitive phenomena that, although being equally private, also have characteristic behavioural signatures. Nonetheless, by combining cognitive and neurobiological methods, it is possible to approach consciousness, to describe its cognitive nature, its behavioural correlates, its possible evolutionary origin and functional role; last but not least, it is possible to investigate its neuroanatomical and neurophysiological underpinnings. In this brief essay I distinguish between two kinds of consciousness: core consciousness and extended consciousness. Core consciousness corresponds to the transient process that is incessantly generated relative to any object with which an organism interacts, and during which a transient core self and transient sense of knowing are automatically generated. Core consciousness requires neither language nor working memory, and needs only a brief short-term memory. Extended consciousness is a more complex process. It depends on the gradual build-up of an autobiographical self, a set of conceptual memories pertaining to both past and anticipated experiences of an individual, and it requires conventional memory. Extended consciousness is enhanced by language.     

The Growth Response of the Stems of Genetically Modified Tobacco Plants (Nicotiana tabacum'Samsun') to Flexural Stimulation

Genetically modified tobacco plants (Nicotiana tabacum‘Samsun’)with antisense cinnamyl alcohol dehydrogenase DNA, produce secondaryxylem of a reduced tensile stiffness. These plants were grownalongside control plants. The stems of the plants were flexedor protected from flexing over a period of several weeks. Thetensile moduli and second moments of areas of the differenttissues inside the stems were measured and used to calculatethe bending stiffness of the plants. In tobacco, the cylinderof xylem was found to be the most important tissue in determiningthe bending stiffness of the plants. The thickness of the xylemtissue cylinder increased when plants were subjected to flexuralstimulation. This increased the bending stiffness of the stems.The response to mechanical stimulation was found to be correlatedwith tissue strain and the genetically modified plants wereable to exactly compensate for the reduced modulus of theirxylem tissue by increasing the thickness of the xylem tissuecylinder more than in control plants.Copyright 1999 Annals ofBotany Company. Tobacco plants, stem bending, xylem tissue, second moment of area, thigmomorphogenesis, mechanical strain.