Prokaryotic Cell Structure and function (Part II)

1. Prokaryotic Cell Structure and function (Part II) BIO3124 Lecture #3 (II)

2. Flagella and Motility • 15-20 um long appendages • extended through CW and anchored to CM • Provide motility • differently distributed on each cell type

3. Patterns of Flagella Distribution • monotrichous – one flagellum usually polar (ie. flagellum at one end of cell) • amphitrichous – one flagellum at each end of cell • lophotrichous(tuft): cluster of flagella at one or both ends • peritrichous – spread over entire surface of cell

4. Flagellar Ultrastructure Gram positive Gram negative

5. Animation: Bacterial flagellum rotation mechanism

6. The filament • extends from cell surface to the tip • hollow, rigid cylinder • composed of the protein flagellin • some procaryotes have a sheath around filament, eg. Spirochetes

7. Flagellum Synthesis • an example of self-assembly • complex process involving many genes and gene products • new molecules of flagellin are transported through the hollow filament • growth is from tip, not base

8. Chemotaxis is the movement of a bacterium in response to chemical gradients. Attractants cause CCW rotation. - Flagella bundle together. - Push cell forward - “Run” Repellents cause CW rotation. - Flagellar bundle falls apart. - “Tumble” = Bacterium briefly stops, then changes direction Chemotaxis

9. The alternating runs and tumbles cause a “random walk.” - Receptors detect attractant concentrations. - Sugars, amino acids - Attractant concentration increases and prolongs run. - This is termed a “biased random walk.” - Causes a net movement of bacteria toward attractants (or away from repellents) Chemotaxis

10. CCW and CW rotation of flagella

11. Chemotaxis: molecular events • Regulated by two-component signaling Major proteins • MCPs:Methyl-accepting chemotaxis proteins • clustered at cell poles bind chemoattractants, receptor sensor and kinase (CheA/CheW), phosphorayte CheY • CheY-P, aresponse regulator, increase the tumble frequency • Other regulatory proteins • CheR & CheB: reversible methylation or demethylation of MCPs desensitizes or sensitizes MCPs • CheZ, dephosphorylation of CheY-P

13. Chemotaxis

14. The Nucleoid • Single loop of double-stranded DNA • Attached to cell envelope • No membrane separates DNA from cytoplasm • Replicates once for each cell division • Compacted via supercoiling by topoisomerases I and II

15. Plasmids • usually small, closed circular DNA molecules • exist and replicate independently of chromosome • have relatively few genes present • genes on plasmids are not essential to host but may confer selective advantage (e.g., drug resistance) • classification of plasmids based on mode of existence and function eg. R-factors, F-plasmids and metabolic plasmids

16. Cell division, or cell fission, requires highly coordinated growth and expansion of all the cell’s parts. Unlike eukaryotes, prokaryotes synthesize RNA and proteins continually while the cell’s DNA undergoes replication. Bacterial DNA replication is coordinated with the cell wall expansion and ultimately the separation of the two daughter cells. Cell Division

17. In prokaryotes, a circular chromosome begins to replicate at its origin, or orisite. Two replication forks are generated, which proceed outward in both directions. - At each fork, DNA is synthesized by DNA polymerase with the help of accessory proteins (the replisome). As the termination site is replicated, the two forks separate from the DNA. DNA Replication

18. Cell Division (Fission) • Cell elongates as it grows • Adds new wall at cell equator • DNA replicates to make 2 chromosomes • DNA replicates bidirectionally • Can begin next replication before cell divides • Cell undergoes septation • Usually at equator • Each daughter has same shape

19. Cell Division (Binary Fission)

20. Coordination of leading and lagging strands

21. Cytokinesis: Role of Cytoskeletal Proteins • process not well understood • protein MreB • similar to eucaryotic actin • determination of cell shape and movement of chromosomes to opposite cell poles • protein FtsZ, • similar to eucaryotic tubulin • Z ring formation • MinCD protein • inhibitor of FtsZ multimerization • Oscillates between cell poles • Localizes the Z ring to the equatorial plane FtsZ

22. Divisome • Cytokinesis & chromosome replication coordinately regulated • Fts proteins form divisome • FtsA, ZipA: anchor Z ring to cytoplasmic membrane • FtsK: coordinates septation with chromosome partitioning • Others: FtsI,L,N,Q,B,W and AmiC involved in PG synthesis

23. Gene Expression • RNA Polymerase transcribes DNA to mRNA • Ribosome translates RNA to Protein • Processes occur simultaneously - This is aided by the signal recognition particle (SRP), which binds to the growing peptide.

24. Special strutures • Cyanobacteria have thylakoids • Extensively folded inner membrane • Contain chlorophyll • Ancestors of chloroplasts • Carboxysomes fix carbon • Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase),use energy to make sugar • Other bacterial photosynthetic pigments • Purple membranes containing Bacteriorhodopsin among Halobacteria • Phycobilisome proteins collect light energy

25. Organic inclusion bodies • Intracellular deposits of material • Glycogen (sugar) for energy • Parahydroxy butyrate (PHB), fatty acid polymer for energy Carboxysomes, lipid energy-storage granules • Gas vacuoles • found in cyanobacteria and some other aquatic procaryotes, provide buoyancy • aggregates of hollow cylindrical structures called gas vesicles • Function:floatation to regulate O2 and light intensity

26. Inorganic inclusion bodies • Polyphosphate granules • also called volutin granules or metachromatic granules • linear polymers of phosphates, stored and used in DNA synthesis • sulfur granules: periplasmic or cytoplasmic, accumulated by sulfur bacteria Iridescent sulfur granules • Magnetosomes • contain iron in the form of magnetite (Fe3O4) • used to orient magnetotactic bacteria in magnetic fields • Reviews: Arash, Schuler