Methanolysis and aminolysis of N-acetylmuramic acid lactones: Evidence for the retention of the d-gluco configuration

Methanolysis of benzyl α-glycosides of N-acetylmuramic acid lactones with HO-6 free (2) and substituted (4, 7, 10, and 12) is catalysed by small amounts of silica gel to give, exclusively, the corresponding methyl esters with HO-4 unsubstituted (3, 5, 8, 11, 13); opening of the lactone ring proceeds with retention of the d-gluco configuration and can be followed by ¹H-n.m.r. spectroscopy. Condensation of 2 with 2-methyl-(3,4,6-tri-O-acetyl-1,2-dideoxy-α-d-glucopyrano)-[2,1-d]-2-oxazoline (15) gave the β-(1→6)-linked disaccharide lactone 16 which, on methanolysis, yielded the disaccharide methyl ester 17, also obtained by condensation of 3 and 15. In the presence of imidazole, the lactones 2 and 4 underwent aminolysis with amino acid and peptide esters as nucleophiles to give the N-acetylmuramoylamide derivatives 19–24. The structures of methanolysis and aminolysis products were established by ¹H-n.m.r. spectroscopy and independent syntheses.     

Mechanics and Dynamics of X-Chromosome Pairing at X Inactivation

At the onset of X-chromosome inactivation, the vital process whereby female mammalian cells equalize X products with respect to males, the X chromosomes are colocalized along their Xic (X-inactivation center) regions. The mechanism inducing recognition and pairing of the X's remains, though, elusive. Starting from recent discoveries on the molecular factors and on the DNA sequences (the so-called “pairing sites”) involved, we dissect the mechanical basis of Xic colocalization by using a statistical physics model. We show that soluble DNA-specific binding molecules, such as those experimentally identified, can be indeed sufficient to induce the spontaneous colocalization of the homologous chromosomes but only when their concentration, or chemical affinity, rises above a threshold value as a consequence of a thermodynamic phase transition. We derive the likelihood of pairing and its probability distribution. Chromosome dynamics has two stages: an initial independent Brownian diffusion followed, after a characteristic time scale, by recognition and pairing. Finally, we investigate the effects of DNA deletion/insertions in the region of pairing sites and compare model predictions to available experimental data.     

Mitochondrial ferredoxin is required for heme A synthesis in Saccharomyces cerevisiae.

Heme A is a prosthetic group of all eukaryotic and some prokaryotic cytochrome oxidases. This heme differs from heme B (protoheme) at two carbon positions of the porphyrin ring. The synthesis of heme A begins with farnesylation of the vinyl group at carbon C-2 of heme B. The heme O product of this reaction is then converted to heme A by a further oxidation of a methyl to a formyl group on C-8. In a previous study (Barros, M. H., Carlson, C. G., Glerum, D. M., and Tzagoloff, A. (2001) FEBS Lett. 492, 133-138) we proposed that the formyl group is formed by an initial hydroxylation of the C-8 methyl by a three-component monooxygenase consisting of Cox15p, ferredoxin, and ferredoxin reductase. In the present study three lines of evidence confirm a requirement of ferredoxin in heme A synthesis. 1) Temperature-conditional yah1 mutants grown under restrictive conditions display a decrease in heme A relative to heme B. 2) The incorporation of radioactive delta-aminolevulinic acid into heme A is reduced in yah1 ts but not in the wild type after the shift to the restrictive temperature; and 3) the overexpression of Cox15p in cytochrome oxidase mutants that accumulate heme O leads to an increased mitochondrial concentration of heme A. The increase in heme A is greater in mutants that overexpress Cox15p and ferredoxin. These results are consistent with a requirement of ferredoxin and indirectly of ferredoxin reductase in hydroxylation of heme O.     

Integrating Sustainable Hunting in Biodiversity Protection in Central Africa: Hot Spots,Weak Spots,and Strong Spots

Wild animals are a primary source of protein (bushmeat) for people living in or near tropical forests. Ideally, the effect of bushmeat harvests should be monitored closely by making regular estimates of offtake rate and size of stock available for exploitation. However, in practice, this is possible in very few situations because it requires both of these aspects to be readily measurable, and even in the best case, entails very considerable time and effort. As alternative, in this study, we use high-resolution, environmental favorability models for terrestrial mammals (N = 165) in Central Africa to map areas of high species richness (hot spots) and hunting susceptibility. Favorability models distinguish localities with environmental conditions that favor the species'' existence from those with detrimental characteristics for its presence. We develop an index for assessing Potential Hunting Sustainability (PHS) of each species based on their ecological characteristics (population density, habitat breadth, rarity and vulnerability), weighted according to restrictive and permissive assumptions of how species'' characteristics are combined. Species are classified into five main hunting sustainability classes using fuzzy logic. Using the accumulated favorability values of all species, and their PHS values, we finally identify weak spots, defined as high diversity regions of especial hunting vulnerability for wildlife, as well as strong spots, defined as high diversity areas of high hunting sustainability potential. Our study uses relatively simple models that employ easily obtainable data of a species'' ecological characteristics to assess the impacts of hunting in tropical regions. It provides information for management by charting the geography of where species are more or less likely to be at risk of extinction from hunting.