Structure and floristic composition of the lowland rain forest of Los Tuxtlas,Mexico

Physiognomy, structure and floristic composition of one hectare of lowland tropical rain forest was studied in detail at Los Tuxtlas, Mexico. Physiognomically, the Los Tuxtlas forest should be classified as lowland tropical high evergreen rain forest. The forest showed a closed canopy at 30–35 m. Of all woody, non-climbing species with a DBH1.0 cm 89.4% (94.5% of all individuals) were evergreen, 25.4% (59.5% of the individuals) had compound leaves, and over 80% of species (and individuals) had leaves in the notophyll and mesophyll size classes. The forest structure was characterized by a low density (2976 individuals with a DBH1.0 cm, 346 individuals with a DBH10.0 cm, per ha, excluding vines) with an average basal area (38.1 m², DBH1.0 cm, 34.9 m², DBH10.0 cm, per ha, excluding vines). This was attributed to the relative maturity of the forest on the study plot. The study plot contained 234 species (11 208 individuals with a height 0.5 m), of which 55.1% (34.8% of individuals) were trees, 9.4% (6.8%) shrubs, 3.4% (44.3%) palms, 20.1% (5.2%) vines, 6.8% (8.7%) herbs and 5.1% (0.3%) of unknown lifeform. Furthermore, 58 species of epiphytes and hemi-epiphytes were found. Diversity of trees, shrubs and palms with a DBH1.0 cm was calculated as Shannon-Wiener index (4.65), Equitability index (0.65), and Simpson index (0.10). The dominance-diversity curve showed a lognormal form, characteristic for tropical rain forest. The community structure was characterized by a relative dominance of Astrocaryum mexicanum in the understorey, Pseudolmedia oxyphyllaria in the middle storeys, and Nectandra ambigens in the canopy. Species population structures of 31 species showed three characteristic patterns, differentiated by recruitment: continuously high, discontinuously high, and continuously low recruitment. Height/diameter and crown cover/diameter diagrams suggested a very gradual shift from height growth to crown growth during tree development. Forest turnover was calculated as 138 years. Compared to other tropical rain forests the Los Tuxtlas forest had 1. similar leaf physiognomical characteristics, 2. a lower diversity, 3. a lower density, 4. an average basal area, and 5. a slow canopy turnover.     

In vivo inhibition of nitrogenase by hydroxylamine in Rhodospirillaceae Role of nitric oxide

Rhodobacter capsulatus strains E1F1 and B10 and Rhodobacter sphaeroides DSM 158 did not use hydroxylamine as nitrogen source for growth but metabolized it mainly through the glutamine synthetase reaction. Hydroxylamine had a high toxicity for cells growing either under phototrophic or dark-aerobic conditions. l-methionine-d,l-sulfoximine partially inhibited hydroxylamine uptake and increased the inhibition time of nitrogenase activity by this nitrogen compound. Nitric oxide was also a powerful inhibitor of nitrogenase in intact cells of R. capsulatus. Since low amounts of NO were produced from hydroxylamine, short-term inhibition of nitrogenase in the presence of this compound could be mediated in vivo by nitric oxide.Abbreviations GS glutamine synthetase - MSX l-methionine-d,l-sulfoximine - MTA mixed alkyltrimethylammonium bromide     

Urinary excretion of digoxin-like immunoreactive factor and arginine-vasopressin in rats after several osmotic loads

Urinary digoxin-like immunoreactive factor (DLIF), arginine-vasopressin (AVP) and other urinary parameters were investigated under normal conditions and after the i.p. injection of the following solutions: distilled water, isotonic and hypertonic NaCl, NaHCO3, KCl and urea, at a rate of 3 ml/100 g body weight. The measurement of digoxin-like immunoreactivity by two different radioimmunoassays showed that DLIF was stimulated by all volume loads regardless of the presence or absence of osmolar compounds. This dissociation between DLIF and urinary sodium excretion suggests that DLIF may not constitute the natriuretic hormone. Moreover, a dissociation between DLIF and AVP excretion also were found, which speaks against the hypothesis of a common mechanism of stimulation for both substances.     

The peptide antibiotic microcin B17 induces double-strand cleavage of DNA mediated by E. coli DNA gyrase.

Microcin B17 (MccB17) is a bactericidal peptide antibiotic which inhibits DNA replication. Two Escherichia coli MccB17 resistant mutants were isolated and the mutations were shown to map to 83 min of the genetic map. Cloning of the mutations and Tn5 insertional analysis demonstrated that they were located inside gyrB. The approximate location of the mutations within gyrB was determined by constructing hybrid genes, as a previous step to sequencing. Both mutations were shown to consist of a single AT----GC transition at position 2251 of the gene, which produces a Trp751----Arg substitution in the amino acid sequence of the GyrB polypeptide. The inhibitory effect of MccB17 on replicative cell-free extracts was assayed. In this in vitro system, interaction of MccB17 with a component of the extracts induced double-strand cleavage of plasmid DNA. In vivo treatment with MccB17 also induced a well-defined cleavage pattern on chromosomal DNA. These effects were not observed with a MccB17-resistant, gyrB mutant. Altogether, our results indicate that MccB17 blocks DNA gyrase by trapping an enzyme-DNA cleavable complex. Thus, the mode of action of this peptide antibiotic resembles that of quinolones and a variety of antitumour drugs currently used in cancer chemotherapy. MccB17 is the first peptide shown to inhibit a type II DNA topoisomerase.     

Genetic control of the mitochondrial form of superoxide dismutase in hexaploid wheat

Extracts of mature grains of a large number of aneuploid derivatives of Triticum aestivum cv. Chinese Spring and of the members of five wheat-alien chromosome addition series were subjected to isoelectric focusing in polyacrylamide gels in order to study the genetic control of superoxide dismutase (SOD). Evidence was obtained that homologous structural genes for the mitochondrial form of SOD are located in the long arms of the homoeologous group 2 chromosomes of Chinese Spring and in chromosome 2R of Secale cereale cv. Imperial. The SOD gene loci located in chromosomes 2A, 2B, 2D, and 2R were designated Sod-A1, Sod-B1, Sod-D1, and Sod-R1, respectively. Chromosome-arm pairing data indicate that 2DL is not homoeologous to either 2AS or 2BL. The results of this study suggest, however, that 2BL is partially homoeologous to both 2AL and 2DL.Technical article No. 21074 of the Texas Agricultural Experiment Station. This work was supported by USDA Grant 83-CRCR-1-1322 to GEH.     

Balance between anionic and cationic extracellular peroxidase activities in Sedum album leaves after ozone exposure. Analysis by high-performance liquid chromatography

Peroxidase activity was assayed with different electron donors (guaiacol, ascorbate, syringaldazine) in the intercellular fluid of Sedum album L. leaves after ozone exposure. Anionic and cationic peroxidases were separated and purified by high performance ion-exchange and gel permeation chromatography. Both isoperoxidases were tested as regards their molecular weight and apparent kinetic constants with different substrates. Ascorbate peroxidase activity was rapidly stimulated after ozone exposure, whereas syringaldazine peroxidase activity reached its maximum 24 h later. Increases in ascorbate and syringaldazine peroxidase activities occurred simultaneously with increases in cationic and anionic peroxidase activities, respectively. Apparent K_m values indicate a high affinity of cationic peroxidases for ascorbate and of anionic peroxidases for syringaldazine. The metabolic role of this balance between cationic and anionic peroxidases after ozone exposure is discussed.     

An auxiliary protein for DNA polymerase-delta from fetal calf thymus

An auxiliary protein which affects the ability of calf thymus DNA polymerase-delta to utilize template/primers containing long stretches of single-stranded template has been purified to homogeneity from the same tissue. The auxiliary protein coelutes with DNA polymerase-delta on DEAE-cellulose and phenyl-agarose chromatography but is separated from the polymerase on phosphocellulose chromatography. The physical and functional properties of the auxiliary protein strongly resemble those of the beta subunit of Escherichia coli DNA polymerase III holoenzyme. A molecular weight of 75,000 has been calculated from a sedimentation coefficient of 5.0 s and a Stokes radius of 36.5 A. A single band of 37,000 daltons is seen on sodium dodecyl sulfate gel electrophoresis, suggesting that the protein exists as a dimer of identical subunits. The purified protein has no detectable DNA polymerase, primase, ATPase, or nuclease activity. The ability of DNA polymerase-delta to replicate gapped duplex DNA is relatively unaffected by the presence of the auxiliary protein, however, it is required to replicate templates with low primer/template ratios, e.g. poly(dA)/oligo(dT) (20:1), primed M13 DNA, and denatured calf thymus DNA. The auxiliary protein is specific for DNA polymerase-delta; it has no effect on the activity of calf thymus DNA polymerase-alpha or the Klenow fragment of E. coli DNA polymerase I with primed homopolymer templates. Although the auxiliary protein does not bind to either single-stranded or double-stranded DNA, it does increase the binding of DNA polymerase-delta to poly(dA)/oligo(dT), suggesting that the auxiliary protein interacts with the polymerase in the presence of template/primer, stabilizing the polymerase-template/primer complex.     

Catabolism of myo-Inositol to Precursors Utilized for De Novo Glycerolipid Biosynthesis

Systemic injection of [2-3H]myo-inositol into frogs resulted in the incorporation of more than half of the label into glycerolipid classes other than phosphoinositides in retinal rod outer segment membranes. Following methanolysis and differential extraction of isolated lipid classes, radioactivity was recovered primarily in the aqueous phase. After phospholipase C hydrolysis of the total membrane lipids, 97% of the radioactivity was extractable with organic solvents, and 70% of the label in lipids was in 1,2-diglycerides. These results indicate that the label was incorporated primarily into the glyceryl moiety of the membrane glycerolipids. Intraocular injection of frog eyes or in vitro incubation of frog retinas with [2-3H]myo-inositol resulted in the incorporation of radioactivity almost exclusively into phosphoinositides in rod outer segment membranes. Incubation of retinas with [U-14C]glucuronic acid did not result in the formation of labeled retinal lipids. These results suggest that myo-inositol can be catabolized systemically to precursors utilized for glycerolipid biosynthesis in the retina.