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For a typical small molecule e. Bacteria that do induce currents for their benefit e. Another postulated function of primitive motility, swimming for the sake of running into more molecules, also does not work: Thus, if diffusion of molecules into the cell is the only matter of concern, a bacterium will do just as well by sitting still as it will by stirring or swimming.
The reason bacteria swim is not to increase diffusion but to find locations with a higher local concentration of nutrient molecules Purcell, ; Berg, ; Vogel, However, a typical value for a is 0. Thus, fundamental physical considerations make the hypothesized stirring filament an unlikely intermediate.
Similarly, Rizzotti implies that the F0-c subunit is homologous with the flagellar motor proteins MotAB, but sequence homology has instead been discovered homology between MotAB and a phylogenetically widespread family of proteins that couple protonmotive force to diverse membrane transport processes.
The homologies could be explained by invoking additional independent cooption events, but this would require a rather more complex scenario than that presented by Rizzotti.
Therefore several of the ideas proposed here have been previously raised in informal debates about flagellar evolution. Millerand Musgrave review this aspect of the debate in detail, and Musgrave proposes a model that is similar in outline to that presented here, although his account is more general.
Phylogenetic context and assumed starting organism The paradigm for prokaryote phylogeny, if there is one, is the universal rRNA tree. This shows a number of widely separated bacterial lineages, with archaea and eukaryotes separated from them all by a very long branch.
This tree is unrooted, and many possible rootings have been proposed in the literature. As these are the most remote and difficult phylogenetic events it is possible to study, and as there is by definition no outgroup to life in general, the debate can be expected to continue for some time.
For current purposes the most important point is that flagella are widespread across the bacterial phylogenetic tree, with losses in various taxa and no clearly primitive nonflagellate taxa. It is therefore assumed that flagella evolved near the base of the bacterial tree.
Rizzotti and others e. However, the very general consideration that most of the bacterial phyla are gram negative, including the many different taxa that come out as basal on different analyses, weighs against this hypothesis.
Therefore, we shall side with Cavalier-Smith, who argues that the last common ancestor was gram-negative. He has put forward the most detailed model for the origin of bacteria and the double membrane Cavalier-Smith, a, a. The model thus begins with a generic double-membraned, gram-negative bacterium.
The present model will begin with a reasonably complex bacterium, already possessing the general secretory pathway and type II secretion system, as well as signal transduction, a peptidoglycan cell wall, and F1F0-ATP synthetase.
Cavalier-Smith a hypothesizes that chlorobacteria may be the most basal offshoot of the tree and be primitively nonflagellate. An export system plus a mechanism to cross the outer membrane forms a secretion system. Bacteria make use of a number of distinct secretion systems, reviewed as a group elsewhere Hueck, ; Thanassi and Hultgren, a; van Wely et al.
Six major well-characterized secretion systems Figure 4aFigure 5 are reviewed by Thanassi and Hultgren a. It is likely that systems will be added to the list in time.In the last two decades, the widespread application of genetic and genomic approaches has revealed a bacterial world astonishing in its ubiquity and diversity.
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And on that dread day, the Ineffable One will summon the artificers and makers of graven images, and He will command them to give life to their creations, and failing, they and their creations will be dedicated to the flames.
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Abstract: The bacterial flagellum is a complex molecular system with multiple components required for functional motility. Such systems are sometimes proposed as puzzles for evolutionary theory on the assumption that selection would have no function to act on until all components are in place.
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