#bacterial cell division

Microbial Physiology

In bacterial chromosome segregation, ParA is responsible for:

In tripartite ParABS chromosome segregation ParA role dynamic scaffolding providing motive force. ParA proteins family Walker A ATPase dimerize upon ATP binding forming DNA binding competent form coating bacterial chromosome as cargo independent cloud. Upon ATP hydrolysis to ADP affinity for DNA drops, detachment occurs. ParB parS partition complex stimulates ATPase converting ParA ATP to ADP, so region behind moving complex depleted ParA, while front retains high concentration. Newly synthesized ParA ATP rebinds nucleoid generating gradient that directs chemophoretic movement of ParB parS origin complex toward higher ParA concentration toward cell pole. In some models ParA forms retracting filaments that pull chromosomes apart analogous to mitotic spindle. ParA responsible forming filaments that pull chromosomes apart, not binding origin directly which is ParB function, not breaking peptidoglycan nor attaching chromosomes to ribosomes. This ATP driven segregation ensures timely separation replicated oriC regions before septum closure, complemented by Topoisomerase IV decatenation and FtsK DNA translocase clearing trap at septum.

Ref: Lutkenhaus, Annu Rev Biochem, 2020, ParA filament formation pulling chromosomes apart during segregation.

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Microbial Physiology

What is the primary function of ParA and ParB proteins in bacterial cells?

Stable inheritance of chromosome and low copy plasmids cannot rely on random diffusion given small cell volume, requires active segregation machinery. ParA and ParB constitute conserved partition system. ParB centromere binding protein binds parS sequence palindromic heptad repeats near origin or on plasmid forming large nucleoprotein complex via spreading oligomerization stimulated by CTP binding, also recruits SMC condensin compacting DNA. ParA Walker ATPase dimeric ATP bound associates non specifically with nucleoid DNA forming gradient or filamentous cloud. ParB triggers ParA ATPase releasing ParA from DNA, creating depletion zone behind ParB parS complex, complex moves up gradient chasing higher ParA concentration toward poles via diffusion ratchet mechanism. Results plasmids chromosomes segregated to opposite halves ensuring each daughter receives genome. Function segregation of plasmids chromosomes not controlling Z-ring placement which done by Min system and nucleoid occlusion, not protein degradation nor ribosome regulation. Mutations cause high frequency anucleate cells demonstrating essentiality for viability in many species.

Ref: Baxter & Funnell, J Bacteriol, 2014, ParA ParB plasmid and chromosome segregation system.

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Microbial Physiology

The Min system prevents Z-ring formation at the bacterial poles by:

Accurate division requires Z ring forms only at midcell between segregated chromosomes preventing minicell formation anucleate wasted progeny. Min system provides positional information via reaction diffusion oscillation. MinD ATPase dimer binds membrane, recruits MinC FtsZ inhibitor blocking polymerization, MinE stimulates MinD ATPase causing release, complex cycles pole to pole with period about 1 minute resulting concentration minima at center where FtsZ can assemble. Second safeguard nucleoid occlusion proteins SlmA in Escherichia coli and Noc in Bacillus bind DNA and directly inhibit FtsZ assembly over chromosome, ensuring septum forms only after chromosome segregation complete. Thus Min system prevents Z ring formation at bacterial poles by preventing FtsZ polymerization at incorrect sites rather than degrading divisome proteins or inhibiting MreB which controls elongation, nor activating crescentin. Mutants lacking MinCDE display polar Z rings producing minicells small spherical chromosome less particles, phenotype used to discover system and model Turing pattern formation in vivo.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Min system prevents polar Z-ring via FtsZ inhibition.

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Microbial Physiology

Which protein anchors the Z-ring to the bacterial plasma membrane?

Because FtsZ polymers lack lipid binding domains they require membrane tethering proteins to assemble productive contractile structure. In Escherichia coli two proteins provide anchoring: FtsA and ZipA. FtsA is actin homolog belonging to actin Hsp70 superfamily containing ATP binding site and C terminal amphipathic helix inserting into inner membrane, interacting with conserved C terminal peptide DPAFLRK of FtsZ via pocket. FtsA also forms minirings on membrane recruiting downstream divisome FtsN. ZipA is bitopic membrane protein with N terminal transmembrane anchor and periplasmic domain containing FtsZ binding region, stabilizing bundling though not essential at low temperature. FtsZ tethered can exert inward force as filaments bend upon GTP hydrolysis pulling membrane via anchored ends. FtsK is DNA translocase not anchor, FtsN late recruitment activation factor. Depletion FtsA leads to spirals delocalized Z, division block filamentous phenotype. Understanding anchoring illustrates how prokaryotic cytoskeleton generates force without myosin, via polymerization coupled membrane attachment informing design of synthetic divisomes in artificial cells.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 20: FtsA anchors Z-ring to membrane.

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Microbial Physiology

During the log phase, bacterial cells:

Logarithmic phase also termed exponential phase reflects balanced growth where all cellular constituents increase in constant proportion and cells divide at maximal specific rate mu max set by genotype and environment. Biomass, protein, RNA, DNA and numbers double at exact constant generation time. DNA replication precisely matches division rate, ribosome content peaks up to 70 percent dry weight, peptidoglycan precursor synthesis via Mur enzymes operates maximally, and transcription of glycolysis and TCA cycle remains high while alarmone ppGpp stays low. Plotting log colony forming units versus time yields straight line slope related to mu. Physiologically cells are most uniform, possess thinnest walls, are most sensitive to beta-lactams targeting active wall synthesis and to aminoglycosides requiring active translation, and produce minimal secondary metabolites. They do not sporulate or lyse; those events dominate stationary and death phases where mass increase decouples from division. This uniform high activity distinguishes log phase and explains its use for preparation of industrial inocula, starter cultures, and for studies requiring homogeneous actively growing cells.

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 4: Log Phase and Exponential Growth Dynamics.

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