Changes in lipid distribution and dynamics in degranulated rat liver rough endoplasmic reticulum due to the membrane attachment of polyribosomes

Binding of rat liver polyribosomes to homologous degranulated rough endoplasmic reticulum (dRER) labeled with 10-(pyren-1-yl)decanoic acid (PDA) was studied. As a consequence of the membrane association of polysomes, the excimer/monomer fluorescence intensity ratios (Ie/Im) decreased, thus indicating alterations in the dynamics and organization of lipids. These fluorescence changes were complete within approximately 1 min, in accordance with the tight binding of ribosomes to RER. In order to characterize the changes in membrane lipid dynamics in more detail, polysomes were covalently labeled with trinitrobenzenesulfonic acid so as to allow their use as F?rster-type resonance energy-transfer acceptors while utilizing PDA as a donor. Accordingly, assuming the binding of native and quencher-labeled ribosomes to the PDA-labeled membranes to be identical, we were able to discriminate fluorescence changes (a) in the proximity of the ribosome binding site from (b) those arising in the surrounding ribosome-free membrane and beyond the effective quenching radii of the TNP residues coupled to polysomes. Our data suggest that lipids in the polysome attachment site of dRER are less mobile than those in the remaining, ribosome-free membrane. In addition, there appears to be a relative enrichment of the PDA probe in the polyribosome membrane attachment sites.     

Generation of an Fsp1 (fibroblast‐specific protein 1)‐Flpo transgenic mouse strain

Recombination systems represent a major breakthrough in the field of genetic model engineering. The Flp recombinases (Flp, Flpe, and Flpo) bind and cleave DNA Frt sites. We created a transgenic mouse strain ([Fsp1‐Flpo]) expressing the Flpo recombinase in fibroblasts. This strain was obtained by random insertion inside mouse zygotes after pronuclear injection. Flpo expression was placed under the control of the promoter of Fsp1 (fibroblast‐specific protein 1) gene, whose expression starts after gastrulation at Day 8.5 in cells of mesenchymal origin. We verified the correct expression and function of the Flpo enzyme by several ex vivo and in vivo approaches. The [Fsp1‐Flpo] strain represents a genuine tool to further target the recombination of transgenes with Frt sites specifically in cells of mesenchymal origin or with a fibroblastic phenotype.     

Patterns of change in zooplankton community structures and the selective feeding of Bleak, Alburnus alburnus (L.) in the Serre Ponçon dam between 1980 and 1996

The Serre Ponçon reservoir, in the southern French alps, filled when the dam was completed in 1960. It has meso-eutrophic characteristics. Two hydrobiological studies carried out in 1980 and in 1996 showed interspecific variation in zooplankton and fish communities. A strong development of B. longirostris, D. brachyurum and C. pulchella, characteristic of eutrophic environments, was apparent in 1996, while the population of Daphnia longispina, which was dominant in 1980, had declined. At the same time, strong growth of the bleak Alburnus alburnus, was observed. Research into prey selection over several seasons showed that prey size and density were often a determining factor for capture, e.g., in C. pulchella, D. longispina and C.v. vicinus. However, some small sized rare species such as B. longirostris were also much sought after.     

Cytosolic Access of Mycobacterium tuberculosis: Critical Impact of Phagosomal Acidification Control and Demonstration of Occurrence In Vivo

Mycobacterium tuberculosis (Mtb) uses efficient strategies to evade the eradication by professional phagocytes, involving—as recently confirmed—escape from phagosomal confinement. While Mtb determinants, such as the ESX-1 type VII secretion system, that contribute to this phenomenon are known, the host cell factors governing this important biological process are yet unexplored. Using a newly developed flow-cytometric approach for Mtb, we show that macrophages expressing the phagosomal bivalent cation transporter Nramp-1, are much less susceptible to phagosomal rupture. Together with results from the use of the phagosome acidification inhibitor bafilomycin, we demonstrate that restriction of phagosomal acidification is a prerequisite for mycobacterial phagosomal rupture and cytosolic contact. Using different in vivo approaches including an enrichment and screen for tracking rare infected phagocytes carrying the CD45.1 hematopoietic allelic marker, we here provide first and unique evidence of M. tuberculosis-mediated phagosomal rupture in mouse spleen and lungs and in numerous phagocyte types. Our results, linking the ability of restriction of phagosome acidification to cytosolic access, provide an important conceptual advance for our knowledge on host processes targeted by Mtb evasion strategies.     

UvrD controls the access of recombination proteins to blocked replication forks

Blocked replication forks often need to be processed by recombination proteins prior to replication restart. In Escherichia coli, the UvrD repair helicase was recently shown to act at inactivated replication forks, where it counteracts a deleterious action of RecA. Using two mutants affected for different subunits of the polymerase III holoenzyme (Pol IIIh), we show here that the anti-RecA action of UvrD at blocked forks reflects two different activities of this enzyme. A defective UvrD mutant is able to antagonize RecA in cells affected for the Pol IIIh catalytic subunit DnaE. In this mutant, RecA action at blocked forks specifically requires the protein RarA (MgsA). We propose that UvrD prevents RecA binding, possibly by counteracting RarA. In contrast, at forks affected for the Pol IIIh clamp (DnaN), RarA is not required for RecA binding and the ATPase function of UvrD is essential to counteract RecA, supporting the idea that UvrD removes RecA from DNA. UvrD action on RecA is conserved in evolution as it can be performed in E. coli by the UvrD homologue from Bacillus subtilis, PcrA.     

Identification of the missing trans-acting enoyl reductase required for phthiocerol dimycocerosate and phenolglycolipid biosynthesis in Mycobacterium tuberculosis

Phthiocerol dimycocerosates (DIM) and phenolglycolipids (PGL) are functionally important surface-exposed lipids of Mycobacterium tuberculosis. Their biosynthesis involves the products of several genes clustered in a 70-kb region of the M. tuberculosis chromosome. Among these products is PpsD, one of the modular type I polyketide synthases responsible for the synthesis of the lipid core common to DIM and PGL. Bioinformatic analyses have suggested that this protein lacks a functional enoyl reductase activity domain required for the synthesis of these lipids. We have identified a gene, Rv2953, that putatively encodes an enoyl reductase. Mutation in Rv2953 prevents conventional DIM formation and leads to the accumulation of a novel DIM-like product. This product is unsaturated between C-4 and C-5 of phthiocerol. Consistently, complementation of the mutant with a functional pks15/1 gene from Mycobacterium bovis BCG resulted in the accumulation of an unsaturated PGL-like substance. When an intact Rv2953 gene was reintroduced into the mutant strain, the phenotype reverted to the wild type. These findings indicate that Rv2953 encodes a trans-acting enoyl reductase that acts with PpsD in phthiocerol and phenolphthiocerol biosynthesis.