Doctoral Dissertations

Date of Award

5-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Jaan Mannik

Committee Members

Anthony Mezzacappa, Soren Sorensen, Steve Abel, Maxim Lavrentovich

Abstract

During the first stage of cell division in bacteria, FtsZ filaments form a ring-like assembly, the Z-ring, which encircles the cell’s middle. This filament scaffold recruits a set of enzymes that synthesize the new cell wall, which divides the cell into two daughters. This dissertation addresses two questions related to the formation of the Z-ring. First, what molecular mechanism triggers Z-ring formation? Second, what are the intermediary structures through which the Z-ring assembly proceeds? The experiments are carried out in Escherichia coli (E.coli) bacteria, whose FtsZ molecules are fluorescently labelled. Using quantitative analysis of high temporal and spatial resolution time-lapse images, we show that the Z-ring formation at the midcell is preceded by a period when transient membrane-linked FtsZ assemblies form throughout the cell body. These transient assemblies result from the attachment of short FtsZ filaments to the inner membrane from the cytosol. We estimate these filaments to be less than 20 monomers long. Once attached filaments become more stable, but despite this stabilization they show treadmilling and periods of rapid growth and shrinkage. At the time of Z-ring formation one of the midcell assemblies is able to grow at the expense of the other transient assemblies indicating that FtsZ is a limiting factor for Z-ring assembly. The latter conclusion is further confirmed by our measurements of FtsZ concentration during the E. coli cell cycle. In slow growth conditions, we find a 4-fold upregulation of the FtsZ synthesis rate at the time of Z-ring formation. Altogether, our work shows that dynamic FtsZ filaments sample the cell surface for the possible location of the Z-ring and that there is a cell cycle control over the timing of the Z-ring formation, albeit not stringent.

Comments

One section of this work has been published in the journal of "Molecular Microbiology" and another was published in the journal of "Current Biology". Material from the "Current Biology" publication was also presented in a platform session during the 2019 Biophysical Society meeting.

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