Download Accurate Cell Division in E. Coli: How Does a Bacterium Know

How does the ParABC system
segregate low copy number
plasmids in bacteria?
Martin Howard
Dept of Systems Biology
John Innes Centre
Norwich, UK
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)
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)
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)
MinD in
Filamentous
Cells
Raskin & de Boer: PNAS (1999)
• Induce filamentous cells by
deleting FtsZ
• Clear evidence for
characteristic wavelength
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
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
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)
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
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?
Par and Min Oscillations Arise
From Different Mechanisms
• Position of plasmid foci seem to
depend on the number of plasmids
Niki et al: Mol. Microbiol. (2007)
• 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
Ebersbach et al:
Mol. Microbiol. (2006)
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
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