Insight into centromere-binding properties of ParB proteins: a secondary binding motif is essential for bacterial genome maintenance

ParB proteins are one of the three essential components of partition systems that actively segregate bacterial chromosomes and plasmids. In binding to centromere sequences, ParB assembles as nucleoprotein structures called partition complexes. These assemblies are the substrates for the partitioning process that ensures DNA molecules are segregated to both sides of the cell. We recently identified the sopC centromere nucleotides required for binding to the ParB homologue of plasmid F, SopB. This analysis also suggested a role in sopC binding for an arginine residue, R219, located outside the helix-turn-helix (HTH) DNA-binding motif previously shown to be the only determinant for sopC-specific binding. Here, we demonstrated that the R219 residue is critical for SopB binding to sopC during partition. Mutating R219 to alanine or lysine abolished partition by preventing partition complex assembly. Thus, specificity of SopB binding relies on two distinct motifs, an HTH and an arginine residue, which define a split DNA-binding domain larger than previously thought. Bioinformatic analysis over a broad range of chromosomal ParBs generalized our findings with the identification of a non-HTH positively charged residue essential for partition and centromere binding, present in a newly identified highly conserved motif. We propose that ParB proteins possess two DNA-binding motifs that form an extended centromere-binding domain, providing high specificity.     

Chromosome Synapsis Alleviates Mek1-Dependent Suppression of Meiotic DNA Repair

Faithful meiotic chromosome segregation and fertility require meiotic recombination between homologous chromosomes rather than the equally available sister chromatid, a bias that in Saccharomyces cerevisiae depends on the meiotic kinase, Mek1. Mek1 is thought to mediate repair template bias by specifically suppressing sister-directed repair. Instead, we found that when Mek1 persists on closely paired (synapsed) homologues, DNA repair is severely delayed, suggesting that Mek1 suppresses any proximal repair template. Accordingly, Mek1 is excluded from synapsed homologues in wild-type cells. Exclusion requires the AAA⁺-ATPase Pch2 and is directly coupled to synaptonemal complex assembly. Stage-specific depletion experiments further demonstrate that DNA repair in the context of synapsed homologues requires Rad54, a repair factor inhibited by Mek1. These data indicate that the sister template is distinguished from the homologue primarily by its closer proximity to inhibitory Mek1 activity. We propose that once pairing or synapsis juxtaposes homologues, exclusion of Mek1 is necessary to avoid suppression of all templates and accelerate repair progression.     

Perception of Affordance during Short-Term Exposure to Weightlessness in Parabolic Flight

We investigated the role of the visual eye-height (VEH) in the perception of affordance during short-term exposure to weightlessness. Sixteen participants were tested during parabolic flight (0g) and on the ground (1g). Participants looked at a laptop showing a room in which a doorway-like aperture was presented. They were asked to adjust the opening of the virtual doorway until it was perceived to be just wide enough to pass through (i.e., the critical aperture). We manipulated VEH by raising the level of the floor in the visual room by 25 cm. The results showed effects of VEH and of gravity on the perceived critical aperture. When VEH was reduced (i.e., when the floor was raised), the critical aperture diminished, suggesting that widths relative to the body were perceived to be larger. The critical aperture was also lower in 0g, for a given VEH, suggesting that participants perceived apertures to be wider or themselves to be smaller in weightlessness, as compared to normal gravity. However, weightlessness also had an effect on the subjective level of the eyes projected into the visual scene. Thus, setting the critical aperture as a fixed percentage of the subjective visual eye-height remains a viable hypothesis to explain how human observers judge visual scenes in terms of potential for action or “affordances”.     

A randomized study of combined 5-fluorouracil and plasma perfusion over protein A-Sepharose in human advanced colorectal carcinoma

To evaluate the advantage with regard to toxicity, response rate, time to progression and survival of combination chemoimmunotherapy over single-agent chemotherapy in patients with metastatic colorectal carcinoma (CRC), 30 patients were randomized to receive a combination of 5-fluorouracil (5-FU) by continuous i.v. infusion and plasma perfusion (PP) over protein A-Sepharose (group A), or a combination of 5-FU and PP over sepharose (group B) or 5-FU alone (group C). 5-FU was given at 1,000 mg/m²/d on days 1-5 of a 4-weekly cycle until progression. Patients of groups A and B received bi-weekly on-line PPs until disease progression or for a maximum of 19 treatments. PP was well tolerated and no severe or life-threatening toxicity was observed. The response rates were 10% for the group A (1 PR), 0% for the group B and 20% for the group C (1 CR + 1 PR). The times to tumor progression for patients in groups A and C were 22 months, 12 and 11 months, respectively and the median survival times were 17 months, 10 months and 9 months. Although the time to progression and survival tended to be higher in patients treated with protein A. PP, these differences were not statistically significant. This is the first report of a randomized trial showing some therapeutic advantage in combining protein A. PP with 5-FU in CRC patients. Further randomized studies are required to demonstrate the real true value of this chemoimmunotherapeutic approach.This investigation was partially supported, for the protein A equipment, by Pharmacia Fine Chemicals (M. Manach, Paris, France and A. Hörst, Uppsala, Sweden).     

Evolution of Sirenian Pachyosteosclerosis,a Model-case for the Study of Bone Structure in Aquatic Tetrapods

Osteosclerosis, or inner bone compaction, and pachyostosis, or outer hyperplasy of bone cortices (swollen bones), are typical features of tetrapods secondarily adapted to life in water. These peculiarities are spectacularly exemplified by the ribs of extant and extinct Sirenia. Sea cows are thus the best model for studying this kind of bone structural specializations. In order to document how these features differentiated during sirenian evolution, the ribs of 15 species, from the most basal form (Pezosiren portelli) up to extant taxa, were studied, and compared to those of other mammalian species from both morphometric and histological points of view. Pachyostosis was the first of these two specializations to occur, by the middle of the Eocene, and is a basal feature of the Sirenia. However, it subsequently regressed in some taxa that do not exhibit hyperplasic rib cortices. Osteosclerosis was only incipient in P. portelli. Its full development occurred later, by the end of the Eocene. These two structural specializations of bone are variably pronounced in extinct and extant sirenians, and relatively independent from each other, although frequently associated. They are possibly due to similar heterochronic mechanisms bearing on the timing of osteoblast activity. These results are discussed with respect to the functional constraints of locomotion in water.     

Modelling Marine Predator Habitat Using the Abundance of Its Pelagic Prey in the Tropical South-Western Pacific

Ecosystems - Understanding the ecological mechanisms underpinning distribution patterns is vital in managing populations of mobile marine species. This study is a first step towards an integrated...     

Overlapping enhancer/promoter and transcriptional termination signals in the lentiviral long terminal repeat

We have studied the effects associated with two single amino acid substitution mutations in HIV-1 capsid (CA), the E98A and E187G. Both amino acids are well conserved among all major HIV-1 subtypes. HIV-1 infectivity is critically dependent on proper CA cone formation and mutations in CA are lethal when they inhibit CA assembly by destabilizing the intra and/or inter molecular CA contacts, which ultimately abrogate viral replication. Glu98, which is located on a surface of a flexible cyclophilin A binding loop is not involved in any intra-molecular contacts with other CA residues. In contrast, Glu187 has extensive intra-molecular contacts with eight other CA residues. Additionally, Glu187 has been shown to form a salt-bridge with Arg18 of another N -terminal CA monomer in a N-C dimer. However, despite proper virus release, glycoprotein incorporation and Gag processing, electron microscopy analysis revealed that, in contrast to the E187G mutant, only the E98A particles had aberrant core morphology that resulted in loss of infectivity.     

Trans-generational Immune Priming Protects the Eggs Only against Gram-Positive Bacteria in the Mealworm Beetle

In many vertebrates and invertebrates, offspring whose mothers have been exposed to pathogens can exhibit increased levels of immune activity and/or increased survival to infection. Such phenomena, called “Trans-generational immune priming” (TGIP) are expected to provide immune protection to the offspring. As the offspring and their mother may share the same environment, and consequently similar microbial threats, we expect the immune molecules present in the progeny to be specific to the microbes that immune challenged the mother. We provide evidence in the mealworm beetle Tenebrio molitor that the antimicrobial activity found in the eggs is only active against Gram-positive bacteria, even when females were exposed to Gram-negative bacteria or fungi. Fungi were weak inducers of TGIP while we obtained similar levels of anti-Gram-positive activity using different bacteria for the maternal challenge. Furthermore, we have identified an antibacterial peptide from the defensin family, the tenecin 1, which spectrum of activity is exclusively directed toward Gram-positive bacteria as potential contributor to this antimicrobial activity. We conclude that maternal transfer of antimicrobial activity in the eggs of T. molitor might have evolved from persistent Gram-positive bacterial pathogens between insect generations.     

Draft Genome Sequences of Actinomyces timonensis Strain 7400942T and Its Prophage

A draft genome sequence of Actinomyces timonensis, an anaerobic bacterium isolated from a human clinical osteoarticular sample, is described here. CRISPR-associated proteins, insertion sequence, and toxin-antitoxin loci were found on the genome. A new virus or provirus, AT-1, was characterized.