The effects of date of planting on field establishment of serotinous cape Proteaceae

Season of fire have marked effects on the germination and establishment of serotinous shrubs of the family Proteaceae in fynbos vegetation. To investigate reasons for this, we simulated the effects of different fire seasons by planting seeds into cleared fynbos and then followed their progress. Four species of Proteaceae were planted monthly at four sites over two and a half years. Exclosures were used to exclude rodent seed predators. Germination was confined largely to the three winter months (June–Aug.). Seeds planted from January–June had higher germination than those planted in the second half of the year. Higher levels of regeneration noted after fires in the first half of the year, were previously hypothesised to be results of predation. However, we obtained similar results despite the exclusion of seed predators. Monthly minimum temperature was strongly correlated with germination percentage but monthly rainfall was not. Loss of seed viability may be important, in determining post-fire seedling densities. Differential seedling mortality of earlier and late germinants appears to be unimportant in determining establishment levels. Our results nevertheless support the current practice of restricting management fires in fynbos to the summer-autumn period.     

Single-stranded circular DNA generated from broad host range plasmid R1162 and its derivatives

Extraction of R1162 plasmid DNA with the alkaline lysis method yields considerable amounts of single-stranded circular plasmid DNA. Destabilization of plasmid DNA is stimulated by the R1162 mob region in cis. The formation of single-stranded circular DNA is initiated at a specific site on the plasmid, presumably the origin of transfer (oriT).     

Effects of Previous Pollination and Stylar Ethylene on Pollen Tube Growth in Petunia hybrida Styles

The effect of ethylene on the growth rate of pollen tubes in styles of Petunia hybrida was examined. Apart from its strong inhibition of pollination-induced ethylene synthesis, aminoethoxyvinylglycine, placed on the stigma, did not impede tube growth. The inhibitors of the action of ethylene, silver thiosulfate and 2,5-norbornadiene, were similarly ineffective. Application of the ethylene precursor, 1-amino-cyclopropane-1-carboxylic acid, onto the stigma at different intervals prior to pollination evoked synthesis of ethylene, but was without effect on tube growth. However, prepollination (by 24 hours) with Nicotiana tabacum pollen, significantly enhanced tube growth of Petunia pollen. This enhancement was not counteracted by the pretreatment of stigmas with aminoethoxy-vinylglycine. It is concluded that the ethylene associated with pollination is without effect on pollen tube growth in the style, but that other pollination-induced factors may lead to an acceleration of growth.     

Desiccation Tolerance of Papaver dubium L. Pollen during Its Development in the Anther: Possible Role of Phospholipid Composition and Sucrose Content

Developing pollen of Papaver dubium L. becomes functional and desiccation tolerant at approximately 2 to 1 days prior to anthesis, coincident with degradation of starch and a doubling of the amount of sucrose, the primary soluble carbohydrate present. When anthers were taken from flower buds at 3 days before anthesis, pollen dehisced upon exposure to the ambient air. This dried pollen did not fluoresce with the vital stain fluorescein diacetate, had increased leakage of K⁺, and did not swell properly in a germination medium. In contrast, pollen became functional and desiccation resistant when these young anthers were first incubated in a water-saturated atmosphere for 30 hours. Phospholipid composition revealed no major differences over the last 3 days of development. When this immature pollen was liberated mechanically and allowed to mature in humid air, starch degraded and sucrose content nearly doubled, and the grains became largely functional and dehydration tolerant. Large unilamellar vesicles were prepared from isolated phospholipids to study dehydration-induced fusion and leakage. When dried in the presence of increasing concentrations of sucrose, vesicle integrity was progressively retained. These data indicate that pollen maturation during the last 3 days of development occurred independently from the parent plant. Sucrose may play an essential role in the acquired tolerance to severe dehydration.     

Functional relationship among the gene clusters encoding F7(1), F7(2), F9, and F11 fimbriae of human uropathogenic Escherichia coli.

The P fimbrial gene clusters encoding the serologically different F7(1), F7(2), F9, and F11 fimbriae were compared functionally. The results show that these gene clusters are closely related.     

Kinetics of pH-dependent fusion between influenza virus and liposomes

The pH-dependent fusion between influenza virus and liposomes (large unilamellar vesicles) has been investigated as a model for the fusion step in the infectious entry of the virus into cells. Fusion was monitored continuously, with a fluorescence assay based on resonance energy transfer (RET) [Struck, D. K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099], which allows an accurate quantitation of the fusion process. Evidence is presented indicating that the dilution of the RET probes from the liposomal bilayer into the viral membrane is not due to transfer of individual lipid molecules. The initial rate and final extent of the fusion reaction increase dramatically with decreasing pH, fusion being virtually complete within 1 min at pH 4.5-5.0. From experiments in which the ratio of virus to liposomes was varied, it is concluded that virus-liposome fusion products continue to fuse with liposomes, but not with virus. Fusion is most efficient with liposomes consisting of negatively charged phospholipids, while phosphatidylcholine and sphingomyelin are inhibitory. The reaction is completely blocked by an antiserum against the virus and inhibited by pretreatment of the virus with trypsin. The effect of proteolytic pretreatment at pH 7.4 is enhanced after preincubation of the virus at pH 5.0, consistent with the occurrence of a low pH induced, irreversible, conformational change in the viral fusion protein, the hemagglutinin (HA), exposing trypsin cleavage sites. When, after initiation of the fusion reaction at pH 5.0, the pH is readjusted to neutral, the process is arrested instantaneously, indicating that the low pH induced conformational change in the HA protein, in itself, is not sufficient to trigger fusion activity.