how does the parabc system segregate low copy number plasmids in bacteria? martin howarddept of...
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How does the ParABC system segregate low copy number plasmids in bacteria?
Martin Howard Dept of Systems BiologyJohn Innes CentreNorwich, UK
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Patterning in Bacteria• Traditional view of bacterial organization:
“randomly filled bag”
• But this isn’t true at all!
• Many processes where proteins are precisely localized in space and time
• Several key examples have emerged, including:
Chemotaxis (polar localization of chemotactic receptors)
Min system (how to divide accurately)
• How is precise positioning achieved?• Self-organization: Howard & Kruse, J. Cell Biol. 168 533 (2005)
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MinCDE Oscillations
• MinE stimulates coherent pole to pole oscillations of MinCDE
• Centre of cell marked by minimum MinC/MinD concentration
MinD Oscillations
Hale, Meinhardt, de Boer:
EMBO J. 20 1563 (2001)
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MinCDE Oscillations
• Formation of oscillating MinE ring structure
Fu, Shih, Zhang, Rothfield: PNAS 98 980 (2001)
• Centre of cell marked by minimum MinC/MinD concentration
MinE Oscillations
Hale, Meinhardt, de Boer:
EMBO J. 20 1563 (2001)
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MinD in Filamentous Cells
Raskin & de Boer: PNAS (1999)
• Induce filamentous cells by deleting FtsZ
• Clear evidence for characteristic wavelength
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Modelling the Min System
• Many mathematical models of the Min system
Howard et al: Phys. Rev. Lett. (2001) Meinhardt et al: PNAS (2001) Kruse: Biophys J. (2002)
• All have common basis: oscillations result from a dynamical instability resulting from intrinsic interactions of Min proteins
Howard & Kruse: JCB (2005) Kruse, Howard & Margolin: Mol. Microbiol. (2007)
• Models can explain characteristic wavelength of Min patterning important in identifying underlying mechanism
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Introduction to Plasmids
• DNA not on main chromosomes that can replicate independently
• Sometimes present at very low copy numbers
• Causes a problem at cell division: how to ensure plasmid are transmitted reliably to both daughter cells?
Importance:
• Encode important functions e.g. antibiotic resistance or virulence
• Method of segregation prior to cell division is a primitive ancestor of mitotic apparatus in eukaryotes
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Par Dynamics and Plasmid Segregation
• How are low copy number plasmids segregated in bacteria?
• Often through oscillatory dynamics of ParABC system
• Without oscillations plasmids are not segregated
• Somehow Par oscillations generate force that moves plasmids!
• What is the mechanism behind the oscillations?
ParA dynamics in E. coli plasmid pB171
Ebersbach & Gerdes: Mol. Microbiol. (2004)
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The ParABC System
• ParA: ATPase that binds nonspecifically to DNA
polymerizes on nucleoid surface
• ParB: protein that binds to DNA at parC regions
together form “partition complex”
• ParB interacts with ParA functioning as “adaptor” between ParA and parC
• Dynamics of plasmids/Par proteins occurs along nucleoid surface
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Par and Min: Similarities
• ParA/MinD both form polymers
• ParA/MinD are both ATPases
• ParA/MinD both undergo spatiotemporal oscillations, though in difference locations (nucleoid vs membrane)
• ParA oscillations require ParB and parC centromere-like site
• MinD oscillations require MinE
• ParB/MinE stimulate ATPase activity of ParA/MinD
• Are Par dynamics also driven by reaction-diffusion dynamic instability? As in Adachi et al: JMB (2006)
• Is there a characteristic wavelength for Par oscillations?
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Ebersbach et al: Mol. Microbiol. (2006)
• Position of plasmid foci seem to depend on the number of plasmids
• For Min-like dynamic instability, number of foci would depend on the length of the nucleoid
• Mechanism of oscillation could be fundamentally different in Par vs Min
Par and Min Oscillations Arise From Different Mechanisms
Niki et al: Mol. Microbiol. (2007)
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Pushing or Pulling?• Do ParA filaments push (similar to ParM) or pull?
• No evidence that plasmids directly nucleate ParA
• How are plasmids close together separated?
• More likely to be by pulling
Plasmid: red ParA: green
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Conclusions• Many similarities between Min and Par
• Nevertheless, new mechanism may be needed for Par dynamics
• Pulling force generated by depolymerisation of ParA filaments by ParB/parC