Raindrop Momentum Triggers Growth of Leaf-Associated Populations of Pseudomonas syringae on Field-Grown Snap Bean Plants

Observational and microclimate modification experiments were conducted under field conditions to determine the role of the physical environment in effecting large increases in phyllosphere population sizes of Pseudomonas syringae pv. syringae, the causal agent of bacterial brown spot disease of snap bean (Phaseolus vulgaris L.). Comparisons of daily changes in population sizes of P. syringae on three plantings of snap bean cultivar Cascade and one of cultivar Eagle with weather conditions indicated a strong association of rainfalls with periods of 1 to 3 days in duration during which increases in bacterial population sizes were greater than 10-fold and up to 1,000-fold. The effects of rain on populations of P. syringae were explored further by modifying the microclimate of bean plants in the field with polyethylene shelters to shield plants from rain and fine-mesh inert screens to modify the momentum of raindrops. After each of three separate intense rains, the greater-than-10-fold increases in population sizes of P. syringae observed on plants exposed to the rains did not occur on plants in the shelters or under the screens. The screens decreased the velocity and, thus, the momentum of raindrops but not the volume or quality of rainwater that fell on plants under the screens. Thus, the absence of increases in population sizes of P. syringae on plants under the screens suggests that raindrop momentum plays a role in the growth-triggering effect of intense rains on populations of P. syringae on bean plants under field conditions.     

The interaction of small domains between the subunits of phosphatidylinositol 3-kinase determines enzyme activity.

Previous studies have suggested that the two subunits of phosphatidylinositol (PI) 3-kinase, p85 and p110, function as localizing and catalytic subunits, respectively. Using recombinant p85 and p110 molecules, we have reconstituted the specific interaction between the two subunits of mouse PI 3-kinase in cells and in vitro. We have previously shown that the region between the two Src homology 2 (SH2) domains of p85 is able to form a functional complex with the 110-kDa subunit in vivo. In this report, we identify the corresponding domain in p110 which directs the binding to p85. We demonstrate that the interactive domains in p85 and p110 are less than 103 and 124 amino acids, respectively, in size. We also show that the association of p85 and p110 mediated by these domains is critical for PI 3-kinase activity. Surprisingly, a complex between a 102-amino-acid segment of p85 and the full-length p110 molecule is catalytically active, whereas p110 alone has no activity. In addition to the catalytic domain in the carboxy-terminal region, 123 amino acids at the amino terminus of p110 were required for catalytic activity and were sufficient for the interaction with p85. These results indicate that the 85-kDa subunit, previously thought to have only a linking role in localizing the p110 catalytic subunit, is an important component of the catalytic complex.     

Conformation of (2→1)-β-d-fructan in aqueous solution

The conformation and dilute solution properties of (2→1)-β-d-fructan in aqueous solution were studied by gel permeation chromatography, low-angle laser light-scattering photometry, viscometry, small-angle X-ray scattering and electron microscopy. Fractions covering a broad range of weight-average molecular weights (M_w) from 1.49 × 10⁴ to 5.29 × 10⁶ were obtained from a native sample by ultrasonic degradation and fractional precipitation. For M_w < 4 × 10⁴, the intrinsic viscosity [η] varies with M_w^0.71, indicating that the fructan chain behaves as a random coil expanded by an excluded-volume effect in this molecular weight region. For M_w > 10⁵, [η] exhibits an unusually weak dependence on M_w and finally becomes almost independent of molecular weight. This behaviour is interpreted in terms of a globular conformation of the high-molecular-weight fructan molecules. Small-angle X-ray-scattering measurements and electron microscopic observations support this interpretation of the values of [η] observed.     

Characterization of spinach leaf α-l-arabinofuranosidases and β-galactosidases and their synergistic action on an endogenous arabinogalactan-protein

Two β-galaclosidases (β-Galase-I and -II, EC 3.2.1.23) and two α-l -arabinofuranosidases (α-l -Arafase-I and -II. EC 3.2.1.55). were purified from mesophyll tissues of spinach (Spinacia oleracea L.), using chromatography on DEAE-cellulose, lactose-conjugated Sepharose CL-4B, and Sephadex G-100, or on hydroxylapatite and Sephadex G-150. The apparent molecular mass (M_r) of β-Galase-I and -II, respectively, were estimated to be 38 000 and 58 000 on SDS-PAGE and 64 000 and 60 000 on gel-permeation chromatography, indicating that the former was a dimeric protein. The isoelectric points of β-Galase-I and -II were 6.9 and 5.2, respectively. Both enzymes hydrolyzed maximally p-nitrophenyl (PNP) β-galactoside at pH 4.3, and were activated about 2-fold in the presence of BSA (100 μg ml^?1). The activity of both enzymes was inhibited strongly by heavy metal ions and p-chloromercuriberszoate (p-CMB). d -Galactono-(1→4)-lactone and d -galactal served as potent competitive inhibitors for the enzymes. β-Galase-I and -II could be distinguished from each other in their relative rates and kinetic properties in the hydrolysis of aryl β-galactosides as well as of lactose and galacto-oligosaccharides. In particular. β-Galase-I exhibited a preferential exowise cleavage of β-1,6-galactotriose and β-1.3-galactan. α-l -Arafase-l (M_r 118000) and -II (M, 68 000) were optimally active on PNP α-l -arabinofuranoside at pH 4.8 and gave K_m values of 1.2 and 2.2 mM. respectively. l -Arabino-(1 → 4)-lactone. Ag⁺, and SDS acted as inhibitors for the isozymes. α-l Arafase-I was characterized by its activity to hydrolyze PNP β-d -xylopyranoside besides PNP α-l -arabinofuranoside. inhibition by d -xylose and d -glucono-(1 → 5)-lactone. and less sensitivity to Hg²⁺. Cu²⁺, and p-CMB. Sugar beet arabinan was hydrolyzed rapidly by α-l Arafase-II at one-half the rate for PNP α-l arabinofuranoside, while the polysaccharide was less susceptible to α-l Arafase-I. A spinach leaf arabinogalactan-protein was practically resistant to the action of β-Galases, but its susceptibility to the enzymes increased remarkably after prior hydrolysis with α-l Arafase-Il.     

Correlation of betacyanin synthesis with cell division in cell suspension cultures of Phytolacca americana

In suspension cultures of Phytolacca americana , betacyanin accumulation was reduced when cell division was inhibited by treatment with various inhibitors of DNA synthesis or anti-microtubule drugs. Aphidicolin (APC), an inhibitor of DNA synthesis, reduced the incorporation of radioactivity from labeled tyrosine into betacyanin, but the incorporation of radioactivity from labeled 3,4-dihydroxyphenylalanine (DOPA) into betacyanin was not affected by similar treatments. Propyzamide, another anti-microtubule drug, reduced incorporation of radioactivity from tyrosine and DOPA into betacyanin. However, the rate of incorporation from DOPA was higher than that from tyrosine. The results suggest that inhibition of betacyanin accumulation in Phytolacca americana cells by APC and propyzamide is due to suppression of the reaction converting tyrosine to DOPA, which may be closely related to cell division.     

Identification and characterization of a gene cluster involved in nitrate transport in the cyanobacterium Synechococcus sp. PCC7942

Summary The nrtA gene, which has been proposed to be involved in nitrate transport of Synechococcus sp. PCC7942 (Anacystis nidulans R2), was mapped at 3.9 kb upstream of the nitrate reductase gene, narB. Three closely linked genes (designated nrtB, nrtC, and nrtD), which encode proteins of 279, 659, and 274 amino acids, respectively, were found between the nrtA and narB genes. NrtB is a hydrophobic protein having structural similarity to the integral membrane components of bacterial transport systems that are dependent on periplasmic substrate-binding proteins. The N-terminal portion of NrtC (amino acid residues 1–254) and NrtD are 58% identical to each other in their amino acid sequences, and resemble the ATP-binding components of binding protein-dependent transport systems. The C-terminal portion of NrtC is 30% identical to NrtA. Mutants constructed by interrupting each of nrtB and nrtC were unable to grow on nitrate, and the nrtD mutant required high concentration of nitrate for growth. The rate of nitrate-dependent O₂ evolution (photosynthetic O₂ evolution coupled to nitrate reduction) in wild-type cells measured in the presence of l-methionine d,l-sulfoximine and glycolaldehyde showed a dual-phase relationship with nitrate concentration. It followed saturation kinetics up to 10 mM nitrate (the concentration required for half-saturation = 1 M), and the reaction rate then increased above the saturation level of the first phase as the nitrate concentration increased. The high-affinity phase of nitrate-dependent O₂ evolution was absent in the nrtD mutant. The results suggest that there are two independent mechanisms of nitrate uptake and that the nrtB-nrtC-nrtD cluster encodes a high-affinity nitrate transport system.     

Functional analysis of matrix proteins expressed from cloned genes of measles virus variants that cause subacute sclerosing panencephalitis reveals a common defect in nucleocapsid binding.

We have developed an in vitro nucleocapsid-binding assay for studying the function of the matrix (M) protein of measles virus (MV) (A. Hirano, A. H. Wang, A. F. Gombart, and T. C. Wong, Proc. Natl. Acad. Sci. USA, 89:8745-8749, 1992). In this communication we show that the M proteins of three MV strains that cause acute infection (Nagahata, Edmonston, and YN) bind efficiently to the viral nucleocapsids whereas the M proteins of four MV strains isolated from patients with subacute sclerosing panencephalitis (SSPE) (Biken, IP-3, Niigata, and Yamagata) fail to bind to the viral nucleocapsids. MV Biken (an SSPE-related virus) produces variant M sequences which encode two antigenically distinct forms of M protein. A serine-versus-leucine difference is responsible for the antigenic variation. MV IP-3 (an SSPE-related virus) also produces variant M sequences, some of which have been postulated to encode a functional M protein responsible for the production of an infectious revertant virus. However, the variant M proteins of Biken and IP-3 strains show no nucleocapsid-binding activity. These results demonstrate that the nucleocapsid-binding function is conserved in the M proteins of MV strains that cause acute infection and that the M proteins of MV strains that cause SSPE exhibit a common defect in this function. Analysis of chimeric M proteins indicates that mutations in the amino-terminal, carboxy-proximal, or carboxy-terminal region of the M protein all abrogate nucleocapsid binding, suggesting that the M protein conformation is important for interaction with the viral nucleocapsid.     

Conformational maturation of measles virus nucleocapsid protein.

We have obtained a polyclonal antiserum, N-BE, against the denatured, amino-terminal half of the measles virus (MV) nucleocapsid (N) protein and a monoclonal antibody (MAb), N46, which recognizes a conformation-dependent epitope in the same region. Amino acid residues 23 to 239 were required and sufficient for the formation of the conformational epitope. Using these antibodies, we show that the N protein of MV is synthesized as a relatively unfolded protein which first appears in the free-protein pool. This nascent N protein undergoes a conformational change into a more folded mature form. This change does not require the participation of other viral proteins or genomic RNA. The mature N protein does not accumulate in the free-protein pool but is quickly and selectively incorporated into the viral nucleocapsids. The mature N protein is a target for interaction with the phosphoprotein (P protein) of MV. This interaction interferes with the recognition of the N protein by the N46 MAb. This suggests that the association with the P protein may mask the binding site for the N46 MAb or that it induces a conformational change in the N protein.