Regulatory T Cells Condition Lymphatic Endothelia for Enhanced Transendothelial Migration

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Can transposable element copy number distribution parameters be estimated from natural populations of Drosophila melanogaster?

The copy frequency distribution of a transposable element family in a Drosophila melanogaster natural population is generally characterised by the values of the Charlesworths' model parameters α and β (Charlesworth & Charlesworth, 1983). The estimation of these parameters is made using the observed distribution of the occupied sites in a population sample. Several results have been interpreted as due either to the influence of stochastic factors or to deterministic factors (transposition, excision, selection…). The accuracy of this method was tested by estimations performed on samples from simulated populations. The results show that with the sample size usually used for natural population studies, the confidence intervals are too large to reasonably deduce either the element copy number distribution or the values of transposition and excision rate and selective coefficients.     

Dynamic Light Regulation of Photosynthesis (A Review)

Regulatory reactions providing the photosynthetic apparatus with the ability to respond to variations of irradiance by changes in activities of the light and the dark stages of photosynthesis within a time range of seconds and minutes are considered in the review. At the light stage, such reactions are related to the changes in both distribution of light energy between two photosystems and probability of nonphotochemical dissipation of absorbed quanta in PSI and PSII. These regulatory reactions operate in a negative feedback mode, thus avoiding overreduction of electron transport chain and minimizing the probability of generation of reactive oxygen species. The crucial role in preventing the generation of reactive oxygen species belongs to dynamic regulation of electron transport activity despite the presence of complex system of their detoxification in chloroplasts. In dark reactions of Calvin cycle, the regulatory responses involve a positive feedback principle being related to redox regulation of activities of several enzymes, which is sensitive to the reduction status of PSI acceptor side. The complex of regulatory reactions based on negative and positive feedback principles provides prolonged functioning of a chloroplast and high stability of photosynthetic activity under various light conditions.     

Wound phloem in transition to bundle phloem in primary roots ofPisum sativum L.

Summary 48 hours after interrupting the root stele ofPisum, wound phloem initiated (proximally or distally to the wound) to reconnect the vascular stumps was found to contain some nucleate wound-sieve elements. At the elongating end of an incomplete wound-sieve tube these elements exhibit a sequence of ultrastructural changes as known from protophloem-sieve tubes. Elongation occurs by the addition of newly divided (wound-) sieve-element/companion-cell complexes. In order to dedifferentiate and assume a new specialization formerly quiescent stelar or cortical cells require at least one (mostly more) preliminary division. Companion cells are consequently obligatory sister cells to wound-sieve elements.By reconstruction using serial sections it could be shown that wound-sieve tubes elongate bidirectionally, starting in an early activated procambial cell of the stele. The elongation is directed by the existence of plasmodesmata, preferably when lying in primary pit fields, and by the plane of preceding divisions. Thus, the developing wound-sieve tube can deviate from the damaged bundle and radiate into the cortex as soon as the plane of the preceding divisions is favourable. In the opposite direction, elongating wound-sieve tubes run parallel to pre-existing phloem traces, thus broading their base at the bundle for the deviating part of the wound-sieve tube. Frequently an individual wound-sieve tube is supplemented at the bundle by a further wound-sieve tube which is partly running parallel to it. Both sieve tubes are interlinked with sieve plates by three-poled sieve elements.Ultrastructurally, the developmental changes of nucleate wound-sieve elements follow the known pattern. In spite of its contrasting origin and odd shape a mature wound-sieve element eventually has the same contents as regular sieve elements: sieve-element plastids, mitochondria, stacked ER and small amounts of P-protein within an electronlucent cytoplasm.     

Survey of chemical speciation of trace elements using synchrotron radiation

Information concerning the chemical state of trace elements in biological systems generally has not been available. Such information for toxic elements and metals in metalloproteins could prove extremely valuable in the elucidation of their metabolism and other biological processes. The shielding of core electrons by binding electrons affect the energy required for creating inner-shell holes. Furthermore, the molecular binding and symmetry of the local environment of an atom affect the absorption spectrum in the neighborhood of the absorption edge. X-ray absorption near-edge structure (XANES) using synchrotron radiation excitation can be used to provide chemical speciation information for trace elements at concentrations as low as 10 ppm. The structure and position of the absorption curve in the region of an edge can yield vital data about the local structure and oxidation state of the trace element in question. Data are most easily interpreted by comparing the observed edge structure and position with those of model compounds of the element covering the entire range of possible oxidation states. Examples of such analyses will be reviewed.