ParMRC es la maquinaria mitótica conocida mas simple

Structures of actin-like ParM filaments show architecture of plasmid-segregating spindles

Active segregation of Escherichia coli low-copy-number plasmid R1 involves formation of a bipolar spindle made of left-handed double-helical actin-like ParM filaments^{1, 2, 3, 4, 5, 6}. ParR links the filaments with centromeric parC plasmid DNA, while facilitating the addition of subunits to ParM filaments^{3, 7, 8, 9}. Growing ParMRC spindles push sister plasmids to the cell poles^{9, 10}. Here, using modern electron cryomicroscopy methods, we investigate the structures and arrangements of ParM filaments in vitro and in cells, revealing at near-atomic resolution how subunits and filaments come together to produce the simplest known mitotic machinery. To understand the mechanism of dynamic instability, we determine structures of ParM filaments in different nucleotide states. The structure of filaments bound to the ATP analogue AMPPNP is determined at 4.3 Å resolution and refined. The ParM filament structure shows strong longitudinal interfaces and weaker lateral interactions. Also using electron cryomicroscopy, we reconstruct ParM doublets forming antiparallel spindles. Finally, with whole-cell electron cryotomography, we show that doublets are abundant in bacterial cells containing low-copy-number plasmids with the ParMRC locus, leading to an asynchronous model of R1 plasmid segregation.


Fig. a, A mutant of ParM that hydrolyses ATP more slowly (D170A) was overexpressed in E. coli cells. Tomographic slices show large bundles of ParM blocking cell division. This experiment was performed two times. b, The ParMRC operon driven from high‐copy‐number plasmid pDD19. Tomographic slice showing an example of observed doublets. c, Tomographic slice for a medium‐copy‐number plasmid (pKG321). d, Tomographic slice for a low‐copy‐number plasmid, emulating the native low‐copy‐number R1 plasmids (pKG491, ‘mini‐R1’ replicon) in E. coli (see Supplementary Videos 5 and 6 to view entire tomograms). Each experiment with different copy‐number plasmids was performed once. e, Schematic depicting proposed asynchronous plasmid DNA segregation. Bipolar ParM spindles are seeded when replication has produced two parC centromeric regions, still in close proximity. Each seeds one unipolar ParM filament, which then come together in an antiparallel fashion to form the segregating bipolar spindle. Non‐productive unipolar filaments or spindles that lack plasmid attachment will be destroyed through the dynamic instability of ParM. This is in contrast to earlier ideas in which all sister plasmids would be segregated through one bundle of filaments, containing double the number of unipolar filaments as the copy number of the plasmid in the cell¹⁹.

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Bacterias que se comportan como cristales vivos

Fast-Moving Bacteria Self-Organize into Active Two-Dimensional Crystals of Rotating Cells

We investigate a new form of collective dynamics displayed by Thiovulum majus, one of the fastest-swimming bacteria known. Cells spontaneously organize on a surface into a visually striking two-dimensional hexagonal lattice of rotating cells. As each constituent cell rotates its flagella, it creates a tornadolike flow that pulls neighboring cells towards and around it. As cells rotate against their neighbors, they exert forces on one another, causing the crystal to rotate and cells to reorganize. We show how these dynamics arise from hydrodynamic and steric interactions between cells. We derive the equations of motion for a crystal, show that this model explains several aspects of the observed dynamics, and discuss the stability of these active crystals.


Fig. 1. A large bacterial crystal in dark-field illumination. The bright glow of individual cells results from light scattering off intercellular sulfur globules. The illumination of cells differs because the concentration of sulfur globules varies between cells. The scale bar is 10  μm.

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The future of the postdoc

There is a growing number of postdocs and few places in academia for them to go. But change could be on the way.



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