Dithionite-Induced Fluorescence Quenching does not Reflect Reductive Activation in Spinach Chloroplasts*

Dithionite is found to cause substantial quenching of chlorophyll fluorescence in intact spinach chloroplasts. In a light-on induction curve, the I-P fluorescence rise is selectively suppressed, with a half-maximal effect at 1.7 × 10^?5 M dithionite. Quenching analysis by the saturation pulse method reveals that the dithionite-induced quenching is photochemical, in analogy to the effect of a Hill reagent. The quenching is suppressed by catalase, KCN and by O₂ removal before dithionite addition. These observations, in combination with recent findings on H₂O₂-induced photochemical quenching (Neubauer and Schreiber, 1989), lead to the conclusion that H₂O₂ is formed from reduction products of O₂ by dithionite and that it is this H₂O₂ which, via the action of the ascorbate peroxidase, sustains Hill activity. The results are discussed with reference to the concept of reductive activation of the ferredoxin: NADP oxidoreductase put forward by Satoh (Satoh, 1982). It is concluded that such activation is unlikely to play an essential role in spinach chloroplasts. This conclusion is supported by the observation of suppression of the I-P fluorescence rise by oxidized NADP in dark-adapted spinach chloroplasts.     

THE DEVELOPMENT OF THE ACHENE OF POLYGONUM PENSYLVANICUM: EMBRYO,ENDOSPERM, AND PERICARP

A study was made of the ontogeny of the achene of Polygonum pensylvanicum L. from fertilization to maturity. The proembryo is classified as the Polygonum Variation, Asterad Type. Cotyledons are initiated three days after anthesis, and by the fifth day procambium is present in the embryo axis. At approximately seven days after anthesis, the embryo begins to curve and occupy a marginal position in the ovary. By ten days the first foliage leaf primordium is initiated at the stem apex of the embryo. At maturity the embryo consists of two cotyledons, a plumule composed of the stem apex and one leaf primordium, and a hypocotyl with a well-developed radicle. Endosperm nuclei begin to divide before the first division of the zygote. Cell wall formation begins in the endosperm at the micropylar end of the embryo sac and proceeds toward the chalazal region. By the fifth day the endosperm is completely cellular, except for a basal projection; and a peripheral meristem has been established. At approximately ten days the peripheral meristem ceases periclinal cell division and becomes the aleurone. At the time of fertilization the ovary wall has its full complement of cell layers. The walls of the outermost cells elongate and become convoluted. Subsequent thickening and lignification of these cell walls produce the hard epicarp of the mature achene.     

Isolation of a cDNA clone encoding S-adenosylmethionine decarboxylase. Expression of the gene in mitogen-activated lymphocytes

S-Adenosylmethionine decarboxylase was purified from bovine liver and digested with endopeptidase Lys-C; the resulting peptides were chromatographically separated. Peptides containing either methionine or tryptophan were subjected to sequence analysis. An oligonucleotide mixture of 48 sequences, which was 17 nucleotides in length, was synthesized based on one of these peptide sequences. This synthetic oligonucleotide mixture was labeled and used to screen a bovine cDNA library in phage lambda gt11. A clone was identified which contained a 1350-nucleotide insert. This insert contained nucleotide sequences coding for amino acid sequences of two of the peptides that were analyzed, thus proving that this cDNA clone codes for S-adenosylmethionine decarboxylase. A subcloned fragment from the coding region of the cDNA was used as a probe to analyze the expression of this gene in mitogen-activated lymphocytes. Northern blots revealed two message species of 2.4 and 3.6 kilobases in length. Both mRNAs were coordinately expressed and were present in polysomes. The levels of these mRNAs increased approximately 4-fold by 9 h after activation of the cells. The magnitude of the increase in these messages is to be compared with an 8- to 10-fold increase in the rate of synthesis of the protein. The apparent increase in translational efficiency of this message upon lymphocyte activation was confirmed by analyzing polysomes from these cells. In resting lymphocytes, the average size of polysomes containing mRNA coding for S-adenosylmethionine decarboxylase was 1.4 ribosomes per mRNA, and this value increased to 2.7 in stimulated cells. Thus, it appears that the increase in translational efficiency of this mRNA arises from an elevated rate of translational initiation, leading to more ribosomes per polysome encoding this particular message. This is not a general effect on the expression of all proteins, since there is no change in the translational efficiency of cytoplasmic actin upon activation of lymphocytes.     

Phenotype variability in a lysogen carrying a heat inducible lambda prophage

Summary In this preliminary report two novel regulatory states of E. coli K12 recA ^–( int6 cI857) are described in which the lytic reproduction is suppressed in the absence of immunity. Occasionally, cells can shift from one state to another.     

The mosaic disease of mullein,Verbascum thapsiforme Schrader,occurring in Czechoslovakia

Field infected mulleins (Verbascum thapsiforme) exhibit dark green mosaic spots or blotches in leaves, stunting and slight reduction in leaf size. The disease inducessome reduction in flower formation depending on the stage, at which infection has taken place. Authors succeeded in mechanical transmission of the virus to fourteen out of fourty-four species tested including seven scrophulariaceous species,Chenopodium quinoa, C. giganteum, C. murale and C. polyspermum. The tests indicated that the thermal inactivation point lies between 51° and 55° C. Measurement of virus particles indicated the modal length 616 nm. Possibility of infection by further viruses is discussed.     

Clinical cardiac magnetic resonance spectroscopy - present state and future directions

MR spectroscopy opens a window to the non-invasive evaluation of various aspects of cardiac metabolism. Experimentally, the method has extensively been used since 1970's. ³¹P-MR allows the registration of cardiac high-energy phosphate metabolism to non-invasively estimate the energetic state of the heart: ATP, phosphocreatine, inorganic phosphate, monophosphate esters and intracellular pH can all be quantitated. In conjunction with extracellular shift reagents such as [DyTTHA]^3- or [TmDOTP]^5-5-, ²³Na- and ³⁹K-MR allow the measurement of intra- and extra-cellular cation pools. ¹H-MR spectroscopy allows the detection of a large number of metabolites such as, e.g. creatine, lactate, or carnitine.Human cardiac spectrocsopy has so far been confined to the³¹ P nucleus. Localization techniques (DRESS, ISIS, 3D-CSI etc.) are required to confine the acquired signal to the heart region. Relative quantification is straightforward (phosphocreatine/ATP ratio), absolute quantification (mM) is under development. Cardiac³¹ P-MR spectroscopy has research application in at least three clinical areas: (1) Coronary artery disease: A biochemical stress test for non-invasive ischemia detection (decrease of phosphocreatine with exercise) and viability assessment via quantification of ATP may become feasible. (2) Heart failure: The phosphocreatine /ATP ratio may provide an independent index for grading of heart failure, allow to monitor the longterm effects of different forms of drug therapy on cardiac energy metabolism in heart failure, and may also hold prognostic information on survival. (3) Valve disease: It is possible that the decrease of phosphocreatine/ATP can be used to guide the timing for the valve replacement.At the present time, no routine clinical applications can be defined for the use of human cardiac spectroscopy in patients with cardiac disease. However, the technique holds great potential for the future as a non-invasive approach to cardiac metabolism, and in coming years routine applications may become reality.