The Entamoeba histolytica Dnmt2 Homolog (Ehmeth) Confers Resistance to Nitrosative Stress

Nitric oxide (NO) has antimicrobial properties against many pathogens due to its reactivity as an S-nitrosylating agent. It inhibits many of the key enzymes that are involved in the metabolism and virulence of the parasite Entamoeba histolytica through S-nitrosylation of essential cysteine residues. Very little information is available on the mechanism of resistance to NO by pathogens in general and by this parasite in particular. Here, we report that exposure of the parasites to S-nitrosoglutathione (GSNO), an NO donor molecule, strongly reduces their viability and protein synthesis. However, the deleterious effects of NO were significantly reduced in trophozoites overexpressing Ehmeth, the cytosine-5 methyltransferase of the Dnmt2 family. Since these trophozoites also exhibited high levels of tRNA^Asp methylation, the high levels suggested that Ehmeth-mediated tRNA^Asp methylation is part of the resistance mechanism to NO. We previously reported that enolase, another glycolytic enzyme, binds to Ehmeth and inhibits its activity. We observed that the amount of Ehmeth-enolase complex was significantly reduced in GSNO-treated E. histolytica, which explains the aforementioned increase of tRNA methylation. Specifically, we demonstrated via site-directed mutagenesis that cysteine residues 228 and 229 of Ehmeth are susceptible to S-nitrosylation and are crucial for Ehmeth binding to enolase and for Ehmeth-mediated resistance to NO. These results indicate that Ehmeth has a central role in the response of the parasite to NO, and they contribute to the growing evidence that NO is a regulator of epigenetic mechanisms.     

Elevation impacts the balance between growth and oxidative stress in coal tits

The short favorable period of time available for the growth in seasonal environments could constrain the resources allocation between growth and other life-history traits, and the short-term fitness benefits of increased growth rate may prevail over other functions. Accelerated growth rates have been associated with long-term deleterious consequences (e.g., decreased lifespan), and recently oxidative stress (the imbalance between pro-oxidants generation and antioxidant defenses) has been suggested as a mediator of these effects. Here, we examined the impact of elevation on growth rate and self-maintenance parameters (resting metabolism, oxidative damage, and antioxidant defenses) of coal tit chicks (Periparus ater). We predicted that the shorter favorable season at the higher-elevation site could lead to a reallocation of resources towards growth at the expense of self-maintenance processes. We found that chicks at high elevation grew significantly faster in terms of body mass and body size. Chicks from the high-elevation site presented higher resting metabolism, higher oxidative damage level, but similar antioxidant defenses, compared to low-elevation chicks. Interestingly, the chicks exhibiting the better antioxidant defenses at 7 days were also those with the highest resting metabolic rate, and the chicks that grew at the faster rate within the high-elevation site were those with the highest levels of oxidative damage on DNA. Our study supports the idea that increasing elevation leads to a higher growth rate in coal tit chicks, possibly in response to a shorter favorable season. In accordance with life-history theory, a bigger investment in growth was done at the expense of body maintenance, at least in terms of oxidative stress.     

Precursors of glutamic acid nitrogen in primary neuronal cultures: Studies with15N

We utilized gas chromatography-mass spectrometry to study the transfer of¹⁵N from [2-¹⁵N]glutamine, [¹⁵N]leucine, [¹⁵N]alanine, or¹⁵NH₄Cl to [¹⁵N]glutamate and [¹⁵N]aspartate in cultured cerebrocortical GABA-ergic neurons from the mouse. Initial rates of¹⁵N appearance (atom % excess) were somewhat higher with 2mM [2-¹⁵N]glutamine as a precursor than with 1mM [¹⁵N]leucine or 1mM [¹⁵N]alanine, but initial net formation (nmol [¹⁵N]glutamate/mg protein.min^–1) was roughly comparable with all precursors. At steady-state¹⁵N labeling was about two times greater with 2mM [2-¹⁵N]glutamine as precursor. The subsequent transfer of¹⁵N from glutamate to aspartate was extremely rapid, the labelling pattern of these two amino acid pools being virtually indistinguishable. We observed little reductive amination of 2-oxo-glutarate to yield [¹⁵N]glutamate in the presence of 0.3mM¹⁵NH₄Cl. Reductive amination through glutamate dehydrogenase was much more prominent at a concentration of 3.0mM¹⁵NH₄Cl. Glutamate formation via reductive amination was unaffected by inclusion of 1 mM 2-oxo-glutarate in the incubation medium. These results indicate that glutamate synthesis in cultured GABA-ergic neurons is derived not only from the glutaminase reaction, but also from transamination reactions in which both leucine and alamine are efficient N donors. Reductive amination of 2-oxo-glutarate in the glutamate dehydrogenase pathway plays a relatively minor role at lower concentrations of extracellular ammonia but becomes quite active at 3mM ammonia.     

MORPHOLOGICAL FLEXIBILITY OF COCCONEIS PLACENTULA (BACILLARIOPHYCEAE) NANOSTRUCTURE TO CHANGING SALINITY LEVELS1

Diatoms possess a silica frustule decorated with unique patterns of nanosize features. Here, we show for the first time from in situ samples that the size of the nanopores present at the surface of the diatom Cocconeis placentula Ehrenb. varies with fluctuating salinity levels. The observed reduction in nanopore size with decreasing salinity agrees with previous laboratory experiments. We also uniquely combined our observations with theoretical considerations to demonstrate that the decrease in the diffusive layer thickness is compensated for by the changes in pore size, which maintain a steady diffusive flux toward the diatom’s cell at different salinities. This process allows diatoms to absorb similar amount of nutrients whatever the salinity and as such to increase their ecological competitiveness in fluctuating environments. These results further suggest that the overall ecological success of diatoms, and their ability to react to environmental changes, may be controlled by the flexibility of the morphological characteristics of their frustules.     

Homoeologous Chromosome Sorting and Progression of Meiotic Recombination in Brassica napus: Ploidy Does Matter!

Meiotic recombination is the fundamental process that produces balanced gametes and generates diversity within species. For successful meiosis, crossovers must form between homologous chromosomes. This condition is more difficult to fulfill in allopolyploid species, which have more than two sets of related chromosomes (homoeologs). Here, we investigated the formation, progression, and completion of several key hallmarks of meiosis in Brassica napus (AACC), a young polyphyletic allotetraploid crop species with closely related homoeologous chromosomes. Altogether, our results demonstrate a precocious and efficient sorting of homologous versus homoeologous chromosomes during early prophase I in two representative B. napus accessions that otherwise show a genotypic difference in the progression of homologous recombination. More strikingly, our detailed comparison of meiosis in near isogenic allohaploid and euploid plants showed that the mechanism(s) promoting efficient chromosome sorting in euploids is adjusted to promote crossover formation between homoeologs in allohaploids. This suggests that, in contrast to other polyploid species, chromosome sorting is context dependent in B. napus.     

Cross-reactions between Staphylococcus epidermidis and 23 other bacterial species

By quantitative immunoelectrophoretic methods, 43 antigens were found in a mixture of sonicated preparations of four Staphylococcus epidermidis strains, using corresponding rabbit antiserum. Two of the antigens were identified as cell wall teichoic acid and a peptidoglycan antigen, respectively. Using this antigen/antibody reference system, cross-reactions between S. epidermidis antigens and antigens from other bacterial species were investigated. Fourteen of the S. epidermidis antigens cross-reacted with antigens from all S. aureus strains investigated. Only few cross-reactions were found between S. epidermidis and bacteria not belonging to the Micrococcaceae. The antigenic relatedness, expressed as a matching coefficient, seems promising for taxonomic work.