Xenostegia,a new genus of Convolvulaceae

Stigma anatomy, pollen morphology, cotyledon structure and other traits suggest that two species formerly considered members ofMerremia fall outside an acceptable range of variation for that genus. A new genus,Xenostegia, is proposed for these two species.     

A nonsense mutation in bacteriophage P1 eliminates the synthesis of a protein required for normal plasmid maintenance

A mutant of bacteriophage P1 that is defective in plasmid maintenance was isolated. P1 seg-101 carries an amber mutation in a region previously implicated in the control of plasmid maintenance. By use of a host bearing a temperature-sensitive suppressor, the dependence of P1 maintenance on the seg-101⁺ protein product was established. The rates of segregation of cured cells under various conditions suggest a role for the seg-101⁺ product in the partition of plasmids to daughter cells rather than in the replication of the plasmid. This hypothesis is supported by the observation that P1 seg-101 can drive host chromosomal DNA replication when integrated into the chromosome of a dnaA host under conditions that are nonpermissive for both the seg-101 and dnaA alleles.     

New species of the Australian endemic wasp genus Notosigalphus van Achterberg and Austin (Hymenoptera: Braconidae) from Flinders Island, Tasmania

A new species of the braconid genus Notosigalphus is described from Flinders Island, Tasmania ( N. sophae Iqbal and Austin sp. n). This is the third species recorded of this rare Australian endemic genus, and the first member of the subfamily Sigalphinae to be recorded elsewhere than on the mainland. A key to separate species of Notosigalphus is presented, as is a discussion of relationships, distribution and biology of the subfamily in Australia.     

Giardia sp. Cysts and Infectious Cryptosporidium parvum Oocysts in the Feces of Migratory Canada Geese (Branta canadensis)

Fecal droppings of migratory Canada geese, Branta canadensis, collected from nine sites near the Chesapeake Bay (Maryland), were examined for the presence of Cryptosporidium parvum and Giardia spp. Cryptosporidium sp. oocysts were found in feces at seven of nine sites, and Giardia cysts were found at all nine sites. The oocysts from three sites were infectious for mice and molecularly identified as the zoonotic genotype of Cryptosporidium parvum. Waterfowl can disseminate infectious C. parvum oocysts in the environment.     

Structure and partial amino acid sequence of calf thymus DNA topoisomerase II: comparison with other type II enzymes.

The partial amino acid sequence of p140 calf thymus DNA topoisomerase II was determined by analysis of cyanogen bromide peptides. Five peptides were aligned and shared extensive homology with sequences derived from cDNA clones for the human topoisomerase II isoenzyme forms. Less homology was seen with the Drosophila, yeast and bacterial type II enzymes. Calf and human enzymes shared epitopes allowing isolation of a cDNA clone to human topoisomerase II isoenzyme alpha. Our results indicate that calf thymus p140 topoisomerase II is an active N-terminal proteolytic fragment of the native p180 enzyme and demonstrate that mammalian type II enzymes exhibit close sequence similarity.     

Physical studies of DNA premelting equilibria in duplexes with and without homo dA.dT tracts: correlations with DNA bending

We have employed a variety of physical methods to study the equilibrium melting and temperature-dependent conformational dynamics of dA.dT tracts in fractionated synthetic DNA polymers and in well-defined fragments of kinetoplast DNA (kDNA). Using circular dichroism (CD), we have detected a temperature-dependent, "premelting" event in poly(dA).poly(dT) which exhibits a midpoint near 37 degrees C. Significantly, we also detect this CD "premelting" behavior in a fragment of kDNA. By contrast, we do not observe this "premelting" behavior in the temperature-dependent CD spectra of poly[d(AT)].poly[d(AT)], poly(dG).poly(dC), poly[d(GC)].poly[d(GC)], or calf thymus DNA. Thus, poly(dA).poly(dT) and kDNA exhibit a common CD-detected "premelting" event which is absent in the other duplex systems studied in this work. Furthermore, we find that the anomalous electrophoretic retardation of the kDNA fragments we have investigated disappears at temperatures above approximately 37 degrees C. We also observe that the rotational dynamics of poly(dA).poly(dT) and kDNA as assessed by singlet depletion anisotropy decay (SDAD) and electric birefringence decay (EBD) also display a discontinuity near 37 degrees C, which is not observed for the other duplex systems studied. Thus, in the aggregate, our static and dynamic measurements suggest that the homo dA.dT sequence element [common to both poly(dA).poly(dT) and kDNA] is capable of a temperature-dependent equilibrium between at least two helical states in a temperature range well below that required to induce global melting of the host duplex. We suggest that this "preglobal" melting event may correspond to the thermally induced "disruption" of "bent" DNA.     

Electrostatic properties of fiber cell membranes from the frog lens.

The electrostatic properties of lens fiber cell membranes have been investigated by recording the electrophoretic mobility of membrane vesicles formed from isolated fiber cells. The vesicles appear to be sealed and have external surfaces that are representative of the extracellular surface of fiber cells. The average mobility of a vesicle in normal Ringer's solution was 0.9 microns/s per v/cm, which gives a zeta potential of -9 mV, a value similar to that reported for other cells (McLaughlin, S. 1989. Annu. Rev. Biophys. Biophys. Chem. 18:113-136.). There was no significant difference in the mobility of vesicles formed from peripheral, middle cortical, or nuclear fiber cells. Vesicle surface changes were titrated using Ca and Mg and each had a pK of approximately 2, which is similar to that for the most common phospholipids. We also titrated these charges with varying pH and found the most significant changes in mobility at pH values between 5 and 6. The majority of lipids found in biological membranes are not titratable in this pH range, so the pH effect is probably through a membrane protein charged group. These experimental data in conjunction with the previously measured extracellular voltage gradient (Mathias, R. T., and J. L. Rae. 1985. Am. J. Physiol. 249:C181-C190) imply that electroosmosis can generate a fluid velocity of approximately 0.6 mm/h, directed from the aqueous or vitreous toward the center of the lens, along intercellular clefts.     

Genetic analysis of lipopolysaccharide core biosynthesis by Escherichia coli K-12: insertion mutagenesis of the rfa locus.

Tn10 insertions were selected on the basis of resistance to the lipopolysaccharide (LPS)-specific bacteriophage U3. The majority of these were located in a 2-kilobase region within the rfa locus, a gene cluster of about 18 kb that contains genes for LPS core biosynthesis. The rfa::Tn10 insertions all exhibited a deep rough phenotype that included hypersensitivity to hydrophobic antibiotics, a reduction in major outer membrane proteins, and production of truncated LPS. These mutations were complemented by a Clarke-Carbon plasmid known to complement rfa mutations of Salmonella typhimurium, and analysis of the insert from this plasmid showed that it contained genes for at least six polypeptides which appear to be arranged in the form of a complex operon. Defects in two of these genes were specifically implicated as the cause of the deep rough phenotype. One of these appeared to be rfaG, which encodes a function required for attachment of the first glucose residue to the heptose region of the core. The other gene did not appear to be directly involved in determination of the sugar composition of the core. We speculate that the product of this gene is involved in the attachment of phosphate or phosphorylethanolamine to the core and that it is the lack of one of these substituents which results in the deep rough phenotype.