Download 22. Myxobacterial Differentiation

BIMM 122 Lecture Notes #22
Dr. Milton Saier
Myxobacterial Differentiation
Comparison of Motility in Myxobacteria and E. coli
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Motility:
Involves:
Cells are:
Velocity:
Chemotaxis genes:
Agent recognized:
Purpose:
Regulation:
Myxobacteria
gliding (2d)/(slime)
pili (type IV) (retract)
flexible (bend)
5/min
frz; methylation is slow (1 hr)
surface proteins
colony cohesion
by developmental programs
E. coli
swimming (3d)/(solution)
flagelli (rotate)
rigid (never bend)
5,000 /min
che; methylation is fast (1-10 min)
small (chemo)effectors
nutrient acquisition
by nutrient source
Phenotypes of the program: gliding  rippling  aggregation  fruiting  sporulation 
germination
Classes of Mutants Defective for Myxobacterial Development
There are 5 complementation groups of genes producing products that control development.
These genes are: a- b- c- d- and e-sg (sg = signal). These signals control gene expression during
development.
Example of a hierarchical order
Asg signals before Csg as revealed by the following:
All Asg-independent genes are Csg-independent.
All Csg-dependent genes are Asg-dependent.
Some Asg-dependent genes are Csg-independent.
Asg is a group of extracellular amino acids and peptides (which signal), generated by proteolysis.
Csg is a cell surface protein which can be released from the cell by detergent or mild protease
treatment.
(1) csg mutants do not exhibit any of the differentiated cellular phenotypic characteristics
noted above.
(2) Neither motility nor csg mutants sporulate.
(3) Purified Csg added to these mutant cells restores rippling, aggregation, fruiting and
sporulation.
(4) If one centrifuges either csg mutants or motility mutants by themselves, they do not
differentiate, but if both are mixed together and centrifuged, they do differentiate.
Thus, cellcell contacts promote differentiation.
In a csg mutant:
0.6 nM Csg restores rippling (but not aggregation or fruiting).
0.8 nM Csg restores mound formation (but not fruiting).
1.0 nM Csg restores fruiting (but not full sporulation).
1.2 nM Csg restores full sporulation, and all differentiation genes are turned on.
Each level of Csg noted above corresponds to the level required for induction of the genes
relevant to that differentiated process. Thus we observe strong positive cooperativity, and one
morphogenetic cell-surface signaling molecule controls at least four stages of differentiation.
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Parallels Between Myxococcus xanthus (a Prokaryote) and Dictyostelium discoideum (a
Eukaryote) Developmental Programs
Cell surface C-signal coordinates differentiation in Myxobacteria (a “slime” bacterium).
Pulses of cyclic AMP coordinate differentiation in Dictyostelium (a cellular “slime” mold).
These chemoattractants are stimulants of differentiation that first promote aggregation and then
promote later steps in differentiation.
1. Both align the cells and mediate rippling, aggregation, fruiting, sporulation and the control of
gene expression.
2. Mutants that can’t produce the stimulant cannot differentiate, but they regain all activities by
exogenous addition of the stimulant. However, Dictyostelium requires pulses of cyclic AMP
while Myxobacteria uses uniform concentrations of Csg. Cyclic AMP is a small, freely diffusing
molecule, while Csg is a cell surface macromolecule.
3. Production of both extracellular developmental molecular signals is regulated by positively
cooperative feedback loops that require cell movement.
4. Both may provide spatial cues in cellular aggregates.
Differences:
1. a. Dictyostelium feeds as dispersed amoebae.
 recruitment requires a freely diffusible signal.
b. The length of the amoeboid cell allows orientation in a gradient, so they can (and do)
respond to spatial gradients.
2. a. Myxobacteria feed in packs and move slowly.
 they can use cell surface signals rather than small diffusible molecules. The use of cell
surface macromolecules as attractants helps to keep them together. This is advantageous
because they feed in “wolf packs” as “micropredators”.
b. The small cell size does not allow orientation in response to a freely diffusible spatial
gradient.
 they use memory and temporal gradients as do all tactic bacteria.
Note: In animals, short range, spatially restricted cellular interactions are mediated by cell
surface proteins which direct differentiation (i.e., mesodermal induction in vertebrate embryos;
antigen presentation in antibody producing cells, etc.). In this respect regulation resembles that
in the Myxobacteria.
References:
Avadhani M, Geyer R, White DC, Shimkets LJ. Lysophosphatidylethanolamine is a substrate for
the short-chain alcohol dehydrogenase SocA from Myxococcus xanthus. J Bacteriol.
2006 Dec;188(24):8543-50.
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Berleman JE, Kirby JR. Deciphering the hunting strategy of a bacterial wolfpack. FEMS
Microbiol Rev. 2009 Sep;33(5):942-57.
Kaiser D (2001) Building a multicellular organism. Annu Rev Genet 35:103-23
Mignot T (2007) The elusive engine in Myxococcus xanthus gliding motility. Cell Mol Life Sci
64:2733-45
Mittal S, Kroos L. A combination of unusual transcription factors binds cooperatively to control
Myxococcus xanthus developmental gene expression. Proc Natl Acad Sci U S A. 2009
Feb 10;106(6):1965-70.
Velicer GJ, Vos M. Sociobiology of the myxobacteria. Annu Rev Microbiol. 2009;63:599-623.
Weijer CJ (2004) Dictyostelium morphogenesis. Curr Opin Genet Dev 14:392-8