Live cell dynamics of production, explosive release and killing activity of phage tail-like weapons for Pseudomonas kin exclusion.

Interference competition among bacteria requires a highly specialized, narrow-spectrum weaponry when targeting closely-related competitors while sparing individuals from the same clonal population. Here we investigated mechanisms by which environmentally important Pseudomonas bacteria with plant-beneficial activity perform kin interference competition. We show that killing between phylogenetically closely-related strains involves contractile phage tail-like devices called R-tailocins that puncture target cell membranes. Using live-cell imaging, we evidence that R-tailocins are produced at the cell center, transported to the cell poles and ejected by explosive cell lysis. This enables their dispersal over several tens of micrometers to reach targeted cells. We visualize R-tailocin-mediated competition dynamics between closely-related Pseudomonas strains at the single-cell level, both in non-induced condition and upon artificial induction. We document the fatal impact of cellular self-sacrifice coupled to deployment of phage tail-like weaponry in the microenvironment of kin bacterial competitors, emphasizing the necessity for microscale assessment of microbial competitions

Structure and Function of Negri Bodies

Replication and assembly of many viruses occur in viral factories which are specialized intracellular compartments formed during viral infection. For rabies virus, those viral factories are called Negri bodies (NBs). NBs are cytoplasmic inclusion bodies in which viral RNAs (mRNAs as well as genomic and antigenomic RNAs) are synthesized. NBs are spherical, they can fuse together, and can reversibly deform when encountering a physical barrier. All these characteristics are similar to those of eukaryotic membrane-less liquid organelles which contribute to the compartmentalization of the cell interior. Indeed, the liquid nature of NBs has been confirmed by FRAP experiments. The co-expression of rabies virus nucleoprotein N and phosphoprotein P is sufficient to induce the formation of cytoplasmic inclusions recapitulating NBs properties. Remarkably, P and N have features similar to those of cellular proteins involved in liquid organelles formation: N is an RNA-binding protein and P contains intrinsically disordered domains. An overview of the literature indicates that formation of liquid viral factories by phase separation is probably common among Mononegavirales. This allows specific recruitment and concentration of viral proteins. Finally, as virus-associated molecular patterns recognized by cellular sensors of RNA virus replication are probably essentially present in the viral factory, there should be a subtle interplay (which remains to be characterized) between those liquid structures and the cellular proteins which trigger the innate immune response

The Tree of Life

International audienceThe tree of life, representing the evolution and the relationships between all life-forms, has challenged scientists as soon as Darwin’s work became accepted. A fuel for imagination for a long time, it became more concrete after the molecular biology revolution and the application of mathematical tools to quantify sequence evolution. Despite tremendous advances fueled by the continuous progress in DNA sequencing, from PCR to metagenomics, and in algorithms for phylogenetic reconstruction, many fundamental questions remain still open in the tree of life topology. The biggest of them all would currently be the relationship between Archaea and Eukarya: while some authors argue that they are sister groups (the Woese tree), others state that the latter emerged from the former (the eocyte tree). The tree of life and its subsequent questions are definitely complex objects to comprehend. Evolution states that in order to fully understand life, one has to first know its history, and this mantra should apply here as well. We thus decided to focus in this chapter on the recent history of the tree of life, from its entry into phylogenetics in the 1970s to the recent identification of Asgard archaea and the controversies they have brought. We also briefly discuss the position of the viruses in the tree of life and how their analysis is helpful to understand their host evolution