Joined: May 2002
Yet another kind of bacterial motility:
Mol Microbiol 2003 Feb;47(3):671-97
The bacterial linear motor of Spiroplasma melliferum BC3: from single molecules to swimming cells.
Spiroplasma melliferum BC3 are wall-less bacteria with internal cytoskeletons. Spiroplasma, Mycoplasma and Acholeplasma belong to the Mollicutes, which represent the smallest, simplest and minimal free-living and self-replicating forms of life. The Mollicutes are motile and chemotactic. Spiroplasma cells are, in addition, helical in shape. Based on data merging, obtained by video dark-field light microscopy of live, swimming helical Spiroplasma cells and by cryoelectron microscopy, unravelling the subcellular structure and molecular organization of the cytoskeleton, we propose a functional model in which the cytoskeleton also acts as a bacterial linear motor enabling and controlling both dynamic helicity and swimming. The cytoskeleton is a flat, monolayered ribbon constructed from seven contractile fibrils (generators) capable of changing their length differentially in a co-ordinated manner. The individual, flat, paired fibrils can be viewed as chains of tetramers approximately 100 A in diameter composed of 59 kDa monomers. The cytoskeletal ribbon is attached to the inner surface of the cell membrane (but is not an integral part of it) and follows the shortest helical line on the coiled cellular tube. We show that Spiroplasma cells can be regarded, at least in some states, as near-perfect dynamic helical tubes. Thus, the analysis of experimental data is reduced to a geometrical problem. The proposed model is based on simple structural elements and functional assumptions: rigid circular rings are threaded on a flexible, helical centreline. The rings maintain their circularity and normality to the centreline at all helical states. An array of peripheral, equidistant axial lines forms a regular cylindrical grid (membrane), by crossing the lines bounding the rings. The axial and peripheral spacing correspond to the tetramer diameter and fibril width (100 A) respectively. Based on electron microscopy data, we assign seven of the axial grid lines in the model to function as contractile generators. The generators are clustered along the shortest helical paths on the cellular coil. In the model, the shortest generator coincides with the shortest helical line. The rest, progressively longer, six generators follow to the right or to the left of the shortest generator in order to generate the maximal range of lengths. A rubbery membrane is stretched over (or represented by) the three-dimensional grid to form a continuous tube. Co-ordinated, differential length changes of the generators induce the membranal cylinder to coil and uncoil reversibly. The switch of helical sense requires equalization of the generators' length, forming a straight cylindrical tube with straight generators. The helical parameters of the cell population, obtained by light microscopy, constitute several subpopulations related, most probably, to cell size and age. The range of molecular dimensions in the active cytoskeleton inferred from light microscopy and modelling agrees with data obtained by direct measurements of subunit images on electron micrographs, scanning transmission electron microscopy (STEM) and diffraction analysis of isolated ribbons. Swimming motility and chemotactic responses are carried out by one or a combination of the following: (i) reciprocating helical extension and compression ('breathing'; (ii) propagation of a deformation (kink) along the helical path; (iii) propagation of a reversal of the helical sense along the cell body; and (iv) irregular flexing and twitching, which is functionally equivalent to standard bacterial tumbling. Here, we analyse in detail only the first case (from which all the rest are derived), including switching of the helical sense.
Omigod! Yes another nonflagellar swimming system!! That designer sure was a busybody!
My cursory thoughts:
1) This page on spiroplasma:
...says that sprioplasmas are related to gram-positive bacteria, and have no cell wall, have a cytoskeleton and membranes with cholesterol. This all seems to go along fairly well with Cavalier-Smith's proposed scheme for the evolution of eukaryotes as something like:
gram-negative --> gram-positive --> early divergence from archaeabacteria --> eukaryotes
...even if this doesn't pan out it is an interesting bit of (vague) convergence.
2) I don't really even understand how the spiroplasma swimming works so speculating on evolution is pointless, however it sounds rather more like what Lynn Margulis thought spirochetes worked like when she proposed the spirochete-->eukaryotic cilium hypothesis. I doubt that spiroplasma-->cilium has any prospects either but it is interesting.
3) Even so, the more ways there are to swim, the more it seems that Dembski has drawn the target around the arrow with the flagellum.
It was suggested that this is vaguely like spirochetes, but not really IMO.
|Periplasmic flagella of spirochaetes|
Perhaps the most unusual case of bacterial flagellation is
that of the spirochaetes. Here flagella are located in the
periplasm between the outer membrane sheath and cell
cylinder, subterminally attached to one end of the cell
cylinder (Fig. 3). The number of periplasmic flagella and
whether the flagella overlap at the centre of the cell varies among species (Li et al., 2000a). The flagella function by rotating within the periplasmic space. Unlike some other bacteria in which flagellation depends on environmental changes, the spirochaete periplasmic flagella are expressed throughout the cell’s life-cycle and are believed to have vital skeletal and motility functions (Li et al., 2000b; Motaleb et al., 2000). Due to their continuous presence, the complex regulatory controls observed for motility gene expression in many bacteria seem to be absent in at least certain spirochaetes.
Titanospirillum velox: A huge, speedy, sulfur-storing spirillum from Ebro Delta microbial mats
paper online at http://www.pnas.org/cgi/content/full/96/20/11584